Base Current vs Emitter Base voltageDependence of transistor current gain on operating conditionsIn a...
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Base Current vs Emitter Base voltage
Dependence of transistor current gain on operating conditionsIn a BJ-Transistor, why do we need to bias the junctions?How does a change in base current affect collector current once the transistor is biasedBipolar Junction Transistor Base-Emitter voltageWhy can current flow through the reverse biased base-collector junction (N-P junction) in a BJT with a forward biased base-emitter junction?Transistors why does increasing base current increase collector current?Transistor power dissipation, current, and voltageWhy do electrons cross the collector-base junction and does current flow between collector and emitter?Input stage of a current mirror circuitAre there cases where a transistor acts bilateral?Why is a voltage across emitter and base required in a bipolar transistor?
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$begingroup$
Why is it universally stated in so many places that the Base Current Ib controls the collector current Ic in a Bipolar Junction Transistor when I guess it is pretty obvious that its the Emitter Base forward bias voltage that will be responsible for the change in the base current.. Making the Vbe controlling factor of Ic.. It is all very confusing
transistors
asked 8 hours ago
NullbyteNullbyte
253 bronze badges
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Nullbyte is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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$endgroup$
add a comment |
$begingroup$
Why is it universally stated in so many places that the Base Current Ib controls the collector current Ic in a Bipolar Junction Transistor when I guess it is pretty obvious that its the Emitter Base forward bias voltage that will be responsible for the change in the base current.. Making the Vbe controlling factor of Ic.. It is all very confusing
transistors
asked 8 hours ago
NullbyteNullbyte
253 bronze badges
New contributor
Nullbyte is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
3
$begingroup$
Which came first, the chicken or the egg? Is it the voltage that determines the current flow in a resistor or, is it the current flow that dictates the voltage dropped?
$endgroup$
– Andy aka
8 hours ago
1
$begingroup$
Idk abt that but "voltage is the cause and current is the effect" feels a bit better.. I maybe wrong though
$endgroup$
– Nullbyte
8 hours ago
$begingroup$
According to Shockley transistor equation the collector current is controlled by the voltage on the base.You are talking about the ratio of Ic/Ib. That is a side-effect that base-current flows and that the ratio of base to collector current is somewhat constant in active region.
$endgroup$
– cm64
8 hours ago
$begingroup$
Nullbyte_just one sentence: You are completely right (with your commen above.
$endgroup$
– LvW
5 hours ago
$begingroup$
Far too many engineers seem to actually believe that the base current controls the collector current. But the physics say otherwise. One better paper to read is W M Webster's, "On the Variation of Junction-Transistor Current-Amplification Factor with Emitter Current," from Proceedings of IRE, 1954. If you get that paper and read it, you'll understand more than enough. But to understand why engineers use base current, see Figure 2.17 located here. Required recombination current to sustain collector current holds a ratio over many orders.
$endgroup$
– jonk
5 hours ago
add a comment |
$begingroup$
Why is it universally stated in so many places that the Base Current Ib controls the collector current Ic in a Bipolar Junction Transistor when I guess it is pretty obvious that its the Emitter Base forward bias voltage that will be responsible for the change in the base current.. Making the Vbe controlling factor of Ic.. It is all very confusing
transistors
asked 8 hours ago
NullbyteNullbyte
253 bronze badges
New contributor
Nullbyte is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
Why is it universally stated in so many places that the Base Current Ib controls the collector current Ic in a Bipolar Junction Transistor when I guess it is pretty obvious that its the Emitter Base forward bias voltage that will be responsible for the change in the base current.. Making the Vbe controlling factor of Ic.. It is all very confusing
transistors
transistors
asked 8 hours ago
NullbyteNullbyte
253 bronze badges
New contributor
Nullbyte is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked 8 hours ago
NullbyteNullbyte
253 bronze badges
New contributor
Nullbyte is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked 8 hours ago
NullbyteNullbyte
253 bronze badges
New contributor
Nullbyte is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked 8 hours ago
NullbyteNullbyte
253 bronze badges
asked 8 hours ago
NullbyteNullbyte
253 bronze badges
253 bronze badges
New contributor
Nullbyte is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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New contributor
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3
$begingroup$
Which came first, the chicken or the egg? Is it the voltage that determines the current flow in a resistor or, is it the current flow that dictates the voltage dropped?
$endgroup$
– Andy aka
8 hours ago
1
$begingroup$
Idk abt that but "voltage is the cause and current is the effect" feels a bit better.. I maybe wrong though
$endgroup$
– Nullbyte
8 hours ago
$begingroup$
According to Shockley transistor equation the collector current is controlled by the voltage on the base.You are talking about the ratio of Ic/Ib. That is a side-effect that base-current flows and that the ratio of base to collector current is somewhat constant in active region.
$endgroup$
– cm64
8 hours ago
$begingroup$
Nullbyte_just one sentence: You are completely right (with your commen above.
$endgroup$
– LvW
5 hours ago
$begingroup$
Far too many engineers seem to actually believe that the base current controls the collector current. But the physics say otherwise. One better paper to read is W M Webster's, "On the Variation of Junction-Transistor Current-Amplification Factor with Emitter Current," from Proceedings of IRE, 1954. If you get that paper and read it, you'll understand more than enough. But to understand why engineers use base current, see Figure 2.17 located here. Required recombination current to sustain collector current holds a ratio over many orders.
$endgroup$
– jonk
5 hours ago
add a comment |
3
$begingroup$
Which came first, the chicken or the egg? Is it the voltage that determines the current flow in a resistor or, is it the current flow that dictates the voltage dropped?
$endgroup$
– Andy aka
8 hours ago
1
$begingroup$
Idk abt that but "voltage is the cause and current is the effect" feels a bit better.. I maybe wrong though
$endgroup$
– Nullbyte
8 hours ago
$begingroup$
According to Shockley transistor equation the collector current is controlled by the voltage on the base.You are talking about the ratio of Ic/Ib. That is a side-effect that base-current flows and that the ratio of base to collector current is somewhat constant in active region.
$endgroup$
– cm64
8 hours ago
$begingroup$
Nullbyte_just one sentence: You are completely right (with your commen above.
$endgroup$
– LvW
5 hours ago
$begingroup$
Far too many engineers seem to actually believe that the base current controls the collector current. But the physics say otherwise. One better paper to read is W M Webster's, "On the Variation of Junction-Transistor Current-Amplification Factor with Emitter Current," from Proceedings of IRE, 1954. If you get that paper and read it, you'll understand more than enough. But to understand why engineers use base current, see Figure 2.17 located here. Required recombination current to sustain collector current holds a ratio over many orders.
$endgroup$
– jonk
5 hours ago
3
3
$begingroup$
Which came first, the chicken or the egg? Is it the voltage that determines the current flow in a resistor or, is it the current flow that dictates the voltage dropped?
$endgroup$
– Andy aka
8 hours ago
$begingroup$
Which came first, the chicken or the egg? Is it the voltage that determines the current flow in a resistor or, is it the current flow that dictates the voltage dropped?
$endgroup$
– Andy aka
8 hours ago
1
1
$begingroup$
Idk abt that but "voltage is the cause and current is the effect" feels a bit better.. I maybe wrong though
$endgroup$
– Nullbyte
8 hours ago
$begingroup$
Idk abt that but "voltage is the cause and current is the effect" feels a bit better.. I maybe wrong though
$endgroup$
– Nullbyte
8 hours ago
$begingroup$
According to Shockley transistor equation the collector current is controlled by the voltage on the base.You are talking about the ratio of Ic/Ib. That is a side-effect that base-current flows and that the ratio of base to collector current is somewhat constant in active region.
$endgroup$
– cm64
8 hours ago
$begingroup$
According to Shockley transistor equation the collector current is controlled by the voltage on the base.You are talking about the ratio of Ic/Ib. That is a side-effect that base-current flows and that the ratio of base to collector current is somewhat constant in active region.
$endgroup$
– cm64
8 hours ago
$begingroup$
Nullbyte_just one sentence: You are completely right (with your commen above.
$endgroup$
– LvW
5 hours ago
$begingroup$
Nullbyte_just one sentence: You are completely right (with your commen above.
$endgroup$
– LvW
5 hours ago
$begingroup$
Far too many engineers seem to actually believe that the base current controls the collector current. But the physics say otherwise. One better paper to read is W M Webster's, "On the Variation of Junction-Transistor Current-Amplification Factor with Emitter Current," from Proceedings of IRE, 1954. If you get that paper and read it, you'll understand more than enough. But to understand why engineers use base current, see Figure 2.17 located here. Required recombination current to sustain collector current holds a ratio over many orders.
$endgroup$
– jonk
5 hours ago
$begingroup$
Far too many engineers seem to actually believe that the base current controls the collector current. But the physics say otherwise. One better paper to read is W M Webster's, "On the Variation of Junction-Transistor Current-Amplification Factor with Emitter Current," from Proceedings of IRE, 1954. If you get that paper and read it, you'll understand more than enough. But to understand why engineers use base current, see Figure 2.17 located here. Required recombination current to sustain collector current holds a ratio over many orders.
$endgroup$
– jonk
5 hours ago
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
Voltage does not cause current, current does not cause voltage, at least for any meaningful understanding of the word 'cause'. They both co-exist.
When the base-emitter junction of a transistor is biassed, an Ib flows into the base, while a VBE exists across it.
If we now measure the collector-emitter current, we find the ratio to the base current is more or less constant over a very wide range, many orders of magnitude. This is sufficiently useful, that engineers call this ratio beta.
The ratio of collector current to VBE varies with the base current. The ratio of them is still useful, engineers call it the transconductance or gm of the transistor, but it's valid at only one base current setting. So while the BJT is also voltage controlled, as the relationship is non-linear, it's not useful for doing calculations for the initial biassing of the transistor, which usually involves comparing currents over a wide range.
This means that when biassing up a transistor, the beta.Ib expression is most useful for collector current. When using a biassed transistor as an amplifier, the gm.VBE expression is frequently used.
answered 8 hours ago
Neil_UKNeil_UK
84.8k2 gold badges85 silver badges195 bronze badges
$endgroup$
$begingroup$
So we can call a BJT voltage controlled or current controlled.. Or both..?
$endgroup$
– Nullbyte
8 hours ago
3
$begingroup$
Ultimately, every device is controlled by both current and voltage. However, we generally approximate and pick the most useful one for most purposes. When comparing a BJT to a FET, we say that the BJT is current controlled, and the FET is voltage controlled. But as I've said, the gm.VBE expression for collector current is used for a biassed-up transistor, so it's clearly also voltage controlled. What tends to happen is when biassing a transistor, we use beta, and ignore the changes of base voltage, which is a good enough approximation most of the time.
$endgroup$
– Neil_UK
8 hours ago
$begingroup$
So roughly its kinda current controlled for large signals and voltage controlled for small signals right?
$endgroup$
– Nullbyte
8 hours ago
$begingroup$
no. Roughly, it's current controlled.
$endgroup$
– Marcus Müller
8 hours ago
1
$begingroup$
@LvW No, the transistor does what the transistor does. 'Voltage controlled' and 'current controlled' are models, human concepts, views. We pick whatever model is convenient to use at the time. The question is ill-formed, it does not admit only one answer. General rule, no current without a voltage without a current without a voltage without a current ... when do you stop? They co-occur.
$endgroup$
– Neil_UK
5 hours ago
|
show 4 more comments
$begingroup$
It's because current is a defining factor in the relationship between the base-emitter junction and collector current and it makes the most sense to use current here. It's effectively a diode and adding a voltage supply greater than it's saturation voltage without a resistor between them is like shorting it. Sure you could use the diode equation to relate voltage and current here but that only makes it more confusing.
Contrast this to a MOSFET where the gate is an insulator and doesn't allow current through it (ignoring capacitance and leakage current, which aren't relevant to the concept). The MOSFET is literally a voltage controlled device, the voltage is the defining factor in the drain current.
answered 6 hours ago
JaywalkJaywalk
1227 bronze badges
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add a comment |
$begingroup$
Nullbyte, I agree with you, the situation seems to be confusing because some books/articles state that the BJT would be current-controlled and some other say voltage-controlled.
This is a very unsatisfying situation - I really cannot understand, why some (many) people still think (no - they only believe) that the BJT would be current-controlled.
There is not a single proof for this claim.
Can you imagine how two additional charged carriers in the base region should be able to release 1000 additional carriers from the emitter (assuming B=500)?
In contrary - there are many explanations and effects which clearly show that the collector current Ic of a BJT is - of course - controlled by the base-emitter voltage Vgs.
It is really a phenomenon - all designers of classical BJT-based gain stages assume voltage control (low-resistive base voltage divider, voltage-feedback of the emitter resistor Re), but some of them - without realizing what they were doing - still believe in current-control.
Don`t ask me why - I cannot answer.
One explanation may be the fact that for CALCULATION PURPOSES it sems to be simple and convenient to ASSUME current control - and, inded, it works!
However, we should not mix physical principles with design-oriented methods.
As a background, I like to give you a similar example:
We say that in a simple resistive voltage divider the current would produce a voltage across each of the two resistors in proportion to the resistor values (given in Ohms) - right?
No - physically wrong. A current cannot produce this voltage. It is always the voltage that allows/drives a certain current. And the voltages are a only measure of the electric fields within the two resistors.
Hower, for calculation purposes, we are allowed to say: The total current produces a voltage V=IxR. But physically, in the cause-and-effect sense this is wrong.
And the same applies to the base current Ib=Ic/B ...it is a by-product that cannot be avoided. And for calculation/design purposes we may use this equation in the form Ic=Ib*B. But this does not mean that Ic would be controlled by Ib.
answered 6 hours ago
LvWLvW
15.2k2 gold badges13 silver badges31 bronze badges
$endgroup$
$begingroup$
Vbe, not Vgs...
$endgroup$
– Chu
1 hour ago
add a comment |
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3 Answers
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3 Answers
3
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active
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$begingroup$
Voltage does not cause current, current does not cause voltage, at least for any meaningful understanding of the word 'cause'. They both co-exist.
When the base-emitter junction of a transistor is biassed, an Ib flows into the base, while a VBE exists across it.
If we now measure the collector-emitter current, we find the ratio to the base current is more or less constant over a very wide range, many orders of magnitude. This is sufficiently useful, that engineers call this ratio beta.
The ratio of collector current to VBE varies with the base current. The ratio of them is still useful, engineers call it the transconductance or gm of the transistor, but it's valid at only one base current setting. So while the BJT is also voltage controlled, as the relationship is non-linear, it's not useful for doing calculations for the initial biassing of the transistor, which usually involves comparing currents over a wide range.
This means that when biassing up a transistor, the beta.Ib expression is most useful for collector current. When using a biassed transistor as an amplifier, the gm.VBE expression is frequently used.
answered 8 hours ago
Neil_UKNeil_UK
84.8k2 gold badges85 silver badges195 bronze badges
$endgroup$
$begingroup$
So we can call a BJT voltage controlled or current controlled.. Or both..?
$endgroup$
– Nullbyte
8 hours ago
3
$begingroup$
Ultimately, every device is controlled by both current and voltage. However, we generally approximate and pick the most useful one for most purposes. When comparing a BJT to a FET, we say that the BJT is current controlled, and the FET is voltage controlled. But as I've said, the gm.VBE expression for collector current is used for a biassed-up transistor, so it's clearly also voltage controlled. What tends to happen is when biassing a transistor, we use beta, and ignore the changes of base voltage, which is a good enough approximation most of the time.
$endgroup$
– Neil_UK
8 hours ago
$begingroup$
So roughly its kinda current controlled for large signals and voltage controlled for small signals right?
$endgroup$
– Nullbyte
8 hours ago
$begingroup$
no. Roughly, it's current controlled.
$endgroup$
– Marcus Müller
8 hours ago
1
$begingroup$
@LvW No, the transistor does what the transistor does. 'Voltage controlled' and 'current controlled' are models, human concepts, views. We pick whatever model is convenient to use at the time. The question is ill-formed, it does not admit only one answer. General rule, no current without a voltage without a current without a voltage without a current ... when do you stop? They co-occur.
$endgroup$
– Neil_UK
5 hours ago
|
show 4 more comments
$begingroup$
Voltage does not cause current, current does not cause voltage, at least for any meaningful understanding of the word 'cause'. They both co-exist.
When the base-emitter junction of a transistor is biassed, an Ib flows into the base, while a VBE exists across it.
If we now measure the collector-emitter current, we find the ratio to the base current is more or less constant over a very wide range, many orders of magnitude. This is sufficiently useful, that engineers call this ratio beta.
The ratio of collector current to VBE varies with the base current. The ratio of them is still useful, engineers call it the transconductance or gm of the transistor, but it's valid at only one base current setting. So while the BJT is also voltage controlled, as the relationship is non-linear, it's not useful for doing calculations for the initial biassing of the transistor, which usually involves comparing currents over a wide range.
This means that when biassing up a transistor, the beta.Ib expression is most useful for collector current. When using a biassed transistor as an amplifier, the gm.VBE expression is frequently used.
answered 8 hours ago
Neil_UKNeil_UK
84.8k2 gold badges85 silver badges195 bronze badges
$endgroup$
$begingroup$
So we can call a BJT voltage controlled or current controlled.. Or both..?
$endgroup$
– Nullbyte
8 hours ago
3
$begingroup$
Ultimately, every device is controlled by both current and voltage. However, we generally approximate and pick the most useful one for most purposes. When comparing a BJT to a FET, we say that the BJT is current controlled, and the FET is voltage controlled. But as I've said, the gm.VBE expression for collector current is used for a biassed-up transistor, so it's clearly also voltage controlled. What tends to happen is when biassing a transistor, we use beta, and ignore the changes of base voltage, which is a good enough approximation most of the time.
$endgroup$
– Neil_UK
8 hours ago
$begingroup$
So roughly its kinda current controlled for large signals and voltage controlled for small signals right?
$endgroup$
– Nullbyte
8 hours ago
$begingroup$
no. Roughly, it's current controlled.
$endgroup$
– Marcus Müller
8 hours ago
1
$begingroup$
@LvW No, the transistor does what the transistor does. 'Voltage controlled' and 'current controlled' are models, human concepts, views. We pick whatever model is convenient to use at the time. The question is ill-formed, it does not admit only one answer. General rule, no current without a voltage without a current without a voltage without a current ... when do you stop? They co-occur.
$endgroup$
– Neil_UK
5 hours ago
|
show 4 more comments
$begingroup$
Voltage does not cause current, current does not cause voltage, at least for any meaningful understanding of the word 'cause'. They both co-exist.
When the base-emitter junction of a transistor is biassed, an Ib flows into the base, while a VBE exists across it.
If we now measure the collector-emitter current, we find the ratio to the base current is more or less constant over a very wide range, many orders of magnitude. This is sufficiently useful, that engineers call this ratio beta.
The ratio of collector current to VBE varies with the base current. The ratio of them is still useful, engineers call it the transconductance or gm of the transistor, but it's valid at only one base current setting. So while the BJT is also voltage controlled, as the relationship is non-linear, it's not useful for doing calculations for the initial biassing of the transistor, which usually involves comparing currents over a wide range.
This means that when biassing up a transistor, the beta.Ib expression is most useful for collector current. When using a biassed transistor as an amplifier, the gm.VBE expression is frequently used.
answered 8 hours ago
Neil_UKNeil_UK
84.8k2 gold badges85 silver badges195 bronze badges
$endgroup$
Voltage does not cause current, current does not cause voltage, at least for any meaningful understanding of the word 'cause'. They both co-exist.
When the base-emitter junction of a transistor is biassed, an Ib flows into the base, while a VBE exists across it.
If we now measure the collector-emitter current, we find the ratio to the base current is more or less constant over a very wide range, many orders of magnitude. This is sufficiently useful, that engineers call this ratio beta.
The ratio of collector current to VBE varies with the base current. The ratio of them is still useful, engineers call it the transconductance or gm of the transistor, but it's valid at only one base current setting. So while the BJT is also voltage controlled, as the relationship is non-linear, it's not useful for doing calculations for the initial biassing of the transistor, which usually involves comparing currents over a wide range.
This means that when biassing up a transistor, the beta.Ib expression is most useful for collector current. When using a biassed transistor as an amplifier, the gm.VBE expression is frequently used.
answered 8 hours ago
Neil_UKNeil_UK
84.8k2 gold badges85 silver badges195 bronze badges
edited 8 hours ago
answered 8 hours ago
Neil_UKNeil_UK
84.8k2 gold badges85 silver badges195 bronze badges
answered 8 hours ago
Neil_UKNeil_UK
84.8k2 gold badges85 silver badges195 bronze badges
answered 8 hours ago
Neil_UKNeil_UK
84.8k2 gold badges85 silver badges195 bronze badges
84.8k2 gold badges85 silver badges195 bronze badges
$begingroup$
So we can call a BJT voltage controlled or current controlled.. Or both..?
$endgroup$
– Nullbyte
8 hours ago
3
$begingroup$
Ultimately, every device is controlled by both current and voltage. However, we generally approximate and pick the most useful one for most purposes. When comparing a BJT to a FET, we say that the BJT is current controlled, and the FET is voltage controlled. But as I've said, the gm.VBE expression for collector current is used for a biassed-up transistor, so it's clearly also voltage controlled. What tends to happen is when biassing a transistor, we use beta, and ignore the changes of base voltage, which is a good enough approximation most of the time.
$endgroup$
– Neil_UK
8 hours ago
$begingroup$
So roughly its kinda current controlled for large signals and voltage controlled for small signals right?
$endgroup$
– Nullbyte
8 hours ago
$begingroup$
no. Roughly, it's current controlled.
$endgroup$
– Marcus Müller
8 hours ago
1
$begingroup$
@LvW No, the transistor does what the transistor does. 'Voltage controlled' and 'current controlled' are models, human concepts, views. We pick whatever model is convenient to use at the time. The question is ill-formed, it does not admit only one answer. General rule, no current without a voltage without a current without a voltage without a current ... when do you stop? They co-occur.
$endgroup$
– Neil_UK
5 hours ago
|
show 4 more comments
$begingroup$
So we can call a BJT voltage controlled or current controlled.. Or both..?
$endgroup$
– Nullbyte
8 hours ago
3
$begingroup$
Ultimately, every device is controlled by both current and voltage. However, we generally approximate and pick the most useful one for most purposes. When comparing a BJT to a FET, we say that the BJT is current controlled, and the FET is voltage controlled. But as I've said, the gm.VBE expression for collector current is used for a biassed-up transistor, so it's clearly also voltage controlled. What tends to happen is when biassing a transistor, we use beta, and ignore the changes of base voltage, which is a good enough approximation most of the time.
$endgroup$
– Neil_UK
8 hours ago
$begingroup$
So roughly its kinda current controlled for large signals and voltage controlled for small signals right?
$endgroup$
– Nullbyte
8 hours ago
$begingroup$
no. Roughly, it's current controlled.
$endgroup$
– Marcus Müller
8 hours ago
1
$begingroup$
@LvW No, the transistor does what the transistor does. 'Voltage controlled' and 'current controlled' are models, human concepts, views. We pick whatever model is convenient to use at the time. The question is ill-formed, it does not admit only one answer. General rule, no current without a voltage without a current without a voltage without a current ... when do you stop? They co-occur.
$endgroup$
– Neil_UK
5 hours ago
$begingroup$
So we can call a BJT voltage controlled or current controlled.. Or both..?
$endgroup$
– Nullbyte
8 hours ago
$begingroup$
So we can call a BJT voltage controlled or current controlled.. Or both..?
$endgroup$
– Nullbyte
8 hours ago
3
3
$begingroup$
Ultimately, every device is controlled by both current and voltage. However, we generally approximate and pick the most useful one for most purposes. When comparing a BJT to a FET, we say that the BJT is current controlled, and the FET is voltage controlled. But as I've said, the gm.VBE expression for collector current is used for a biassed-up transistor, so it's clearly also voltage controlled. What tends to happen is when biassing a transistor, we use beta, and ignore the changes of base voltage, which is a good enough approximation most of the time.
$endgroup$
– Neil_UK
8 hours ago
$begingroup$
Ultimately, every device is controlled by both current and voltage. However, we generally approximate and pick the most useful one for most purposes. When comparing a BJT to a FET, we say that the BJT is current controlled, and the FET is voltage controlled. But as I've said, the gm.VBE expression for collector current is used for a biassed-up transistor, so it's clearly also voltage controlled. What tends to happen is when biassing a transistor, we use beta, and ignore the changes of base voltage, which is a good enough approximation most of the time.
$endgroup$
– Neil_UK
8 hours ago
$begingroup$
So roughly its kinda current controlled for large signals and voltage controlled for small signals right?
$endgroup$
– Nullbyte
8 hours ago
$begingroup$
So roughly its kinda current controlled for large signals and voltage controlled for small signals right?
$endgroup$
– Nullbyte
8 hours ago
$begingroup$
no. Roughly, it's current controlled.
$endgroup$
– Marcus Müller
8 hours ago
$begingroup$
no. Roughly, it's current controlled.
$endgroup$
– Marcus Müller
8 hours ago
1
1
$begingroup$
@LvW No, the transistor does what the transistor does. 'Voltage controlled' and 'current controlled' are models, human concepts, views. We pick whatever model is convenient to use at the time. The question is ill-formed, it does not admit only one answer. General rule, no current without a voltage without a current without a voltage without a current ... when do you stop? They co-occur.
$endgroup$
– Neil_UK
5 hours ago
$begingroup$
@LvW No, the transistor does what the transistor does. 'Voltage controlled' and 'current controlled' are models, human concepts, views. We pick whatever model is convenient to use at the time. The question is ill-formed, it does not admit only one answer. General rule, no current without a voltage without a current without a voltage without a current ... when do you stop? They co-occur.
$endgroup$
– Neil_UK
5 hours ago
|
show 4 more comments
$begingroup$
It's because current is a defining factor in the relationship between the base-emitter junction and collector current and it makes the most sense to use current here. It's effectively a diode and adding a voltage supply greater than it's saturation voltage without a resistor between them is like shorting it. Sure you could use the diode equation to relate voltage and current here but that only makes it more confusing.
Contrast this to a MOSFET where the gate is an insulator and doesn't allow current through it (ignoring capacitance and leakage current, which aren't relevant to the concept). The MOSFET is literally a voltage controlled device, the voltage is the defining factor in the drain current.
answered 6 hours ago
JaywalkJaywalk
1227 bronze badges
$endgroup$
add a comment |
$begingroup$
It's because current is a defining factor in the relationship between the base-emitter junction and collector current and it makes the most sense to use current here. It's effectively a diode and adding a voltage supply greater than it's saturation voltage without a resistor between them is like shorting it. Sure you could use the diode equation to relate voltage and current here but that only makes it more confusing.
Contrast this to a MOSFET where the gate is an insulator and doesn't allow current through it (ignoring capacitance and leakage current, which aren't relevant to the concept). The MOSFET is literally a voltage controlled device, the voltage is the defining factor in the drain current.
answered 6 hours ago
JaywalkJaywalk
1227 bronze badges
$endgroup$
add a comment |
$begingroup$
It's because current is a defining factor in the relationship between the base-emitter junction and collector current and it makes the most sense to use current here. It's effectively a diode and adding a voltage supply greater than it's saturation voltage without a resistor between them is like shorting it. Sure you could use the diode equation to relate voltage and current here but that only makes it more confusing.
Contrast this to a MOSFET where the gate is an insulator and doesn't allow current through it (ignoring capacitance and leakage current, which aren't relevant to the concept). The MOSFET is literally a voltage controlled device, the voltage is the defining factor in the drain current.
answered 6 hours ago
JaywalkJaywalk
1227 bronze badges
$endgroup$
It's because current is a defining factor in the relationship between the base-emitter junction and collector current and it makes the most sense to use current here. It's effectively a diode and adding a voltage supply greater than it's saturation voltage without a resistor between them is like shorting it. Sure you could use the diode equation to relate voltage and current here but that only makes it more confusing.
Contrast this to a MOSFET where the gate is an insulator and doesn't allow current through it (ignoring capacitance and leakage current, which aren't relevant to the concept). The MOSFET is literally a voltage controlled device, the voltage is the defining factor in the drain current.
answered 6 hours ago
JaywalkJaywalk
1227 bronze badges
answered 6 hours ago
JaywalkJaywalk
1227 bronze badges
answered 6 hours ago
JaywalkJaywalk
1227 bronze badges
answered 6 hours ago
JaywalkJaywalk
1227 bronze badges
1227 bronze badges
add a comment |
add a comment |
$begingroup$
Nullbyte, I agree with you, the situation seems to be confusing because some books/articles state that the BJT would be current-controlled and some other say voltage-controlled.
This is a very unsatisfying situation - I really cannot understand, why some (many) people still think (no - they only believe) that the BJT would be current-controlled.
There is not a single proof for this claim.
Can you imagine how two additional charged carriers in the base region should be able to release 1000 additional carriers from the emitter (assuming B=500)?
In contrary - there are many explanations and effects which clearly show that the collector current Ic of a BJT is - of course - controlled by the base-emitter voltage Vgs.
It is really a phenomenon - all designers of classical BJT-based gain stages assume voltage control (low-resistive base voltage divider, voltage-feedback of the emitter resistor Re), but some of them - without realizing what they were doing - still believe in current-control.
Don`t ask me why - I cannot answer.
One explanation may be the fact that for CALCULATION PURPOSES it sems to be simple and convenient to ASSUME current control - and, inded, it works!
However, we should not mix physical principles with design-oriented methods.
As a background, I like to give you a similar example:
We say that in a simple resistive voltage divider the current would produce a voltage across each of the two resistors in proportion to the resistor values (given in Ohms) - right?
No - physically wrong. A current cannot produce this voltage. It is always the voltage that allows/drives a certain current. And the voltages are a only measure of the electric fields within the two resistors.
Hower, for calculation purposes, we are allowed to say: The total current produces a voltage V=IxR. But physically, in the cause-and-effect sense this is wrong.
And the same applies to the base current Ib=Ic/B ...it is a by-product that cannot be avoided. And for calculation/design purposes we may use this equation in the form Ic=Ib*B. But this does not mean that Ic would be controlled by Ib.
answered 6 hours ago
LvWLvW
15.2k2 gold badges13 silver badges31 bronze badges
$endgroup$
$begingroup$
Vbe, not Vgs...
$endgroup$
– Chu
1 hour ago
add a comment |
$begingroup$
Nullbyte, I agree with you, the situation seems to be confusing because some books/articles state that the BJT would be current-controlled and some other say voltage-controlled.
This is a very unsatisfying situation - I really cannot understand, why some (many) people still think (no - they only believe) that the BJT would be current-controlled.
There is not a single proof for this claim.
Can you imagine how two additional charged carriers in the base region should be able to release 1000 additional carriers from the emitter (assuming B=500)?
In contrary - there are many explanations and effects which clearly show that the collector current Ic of a BJT is - of course - controlled by the base-emitter voltage Vgs.
It is really a phenomenon - all designers of classical BJT-based gain stages assume voltage control (low-resistive base voltage divider, voltage-feedback of the emitter resistor Re), but some of them - without realizing what they were doing - still believe in current-control.
Don`t ask me why - I cannot answer.
One explanation may be the fact that for CALCULATION PURPOSES it sems to be simple and convenient to ASSUME current control - and, inded, it works!
However, we should not mix physical principles with design-oriented methods.
As a background, I like to give you a similar example:
We say that in a simple resistive voltage divider the current would produce a voltage across each of the two resistors in proportion to the resistor values (given in Ohms) - right?
No - physically wrong. A current cannot produce this voltage. It is always the voltage that allows/drives a certain current. And the voltages are a only measure of the electric fields within the two resistors.
Hower, for calculation purposes, we are allowed to say: The total current produces a voltage V=IxR. But physically, in the cause-and-effect sense this is wrong.
And the same applies to the base current Ib=Ic/B ...it is a by-product that cannot be avoided. And for calculation/design purposes we may use this equation in the form Ic=Ib*B. But this does not mean that Ic would be controlled by Ib.
answered 6 hours ago
LvWLvW
15.2k2 gold badges13 silver badges31 bronze badges
$endgroup$
$begingroup$
Vbe, not Vgs...
$endgroup$
– Chu
1 hour ago
add a comment |
$begingroup$
Nullbyte, I agree with you, the situation seems to be confusing because some books/articles state that the BJT would be current-controlled and some other say voltage-controlled.
This is a very unsatisfying situation - I really cannot understand, why some (many) people still think (no - they only believe) that the BJT would be current-controlled.
There is not a single proof for this claim.
Can you imagine how two additional charged carriers in the base region should be able to release 1000 additional carriers from the emitter (assuming B=500)?
In contrary - there are many explanations and effects which clearly show that the collector current Ic of a BJT is - of course - controlled by the base-emitter voltage Vgs.
It is really a phenomenon - all designers of classical BJT-based gain stages assume voltage control (low-resistive base voltage divider, voltage-feedback of the emitter resistor Re), but some of them - without realizing what they were doing - still believe in current-control.
Don`t ask me why - I cannot answer.
One explanation may be the fact that for CALCULATION PURPOSES it sems to be simple and convenient to ASSUME current control - and, inded, it works!
However, we should not mix physical principles with design-oriented methods.
As a background, I like to give you a similar example:
We say that in a simple resistive voltage divider the current would produce a voltage across each of the two resistors in proportion to the resistor values (given in Ohms) - right?
No - physically wrong. A current cannot produce this voltage. It is always the voltage that allows/drives a certain current. And the voltages are a only measure of the electric fields within the two resistors.
Hower, for calculation purposes, we are allowed to say: The total current produces a voltage V=IxR. But physically, in the cause-and-effect sense this is wrong.
And the same applies to the base current Ib=Ic/B ...it is a by-product that cannot be avoided. And for calculation/design purposes we may use this equation in the form Ic=Ib*B. But this does not mean that Ic would be controlled by Ib.
answered 6 hours ago
LvWLvW
15.2k2 gold badges13 silver badges31 bronze badges
$endgroup$
Nullbyte, I agree with you, the situation seems to be confusing because some books/articles state that the BJT would be current-controlled and some other say voltage-controlled.
This is a very unsatisfying situation - I really cannot understand, why some (many) people still think (no - they only believe) that the BJT would be current-controlled.
There is not a single proof for this claim.
Can you imagine how two additional charged carriers in the base region should be able to release 1000 additional carriers from the emitter (assuming B=500)?
In contrary - there are many explanations and effects which clearly show that the collector current Ic of a BJT is - of course - controlled by the base-emitter voltage Vgs.
It is really a phenomenon - all designers of classical BJT-based gain stages assume voltage control (low-resistive base voltage divider, voltage-feedback of the emitter resistor Re), but some of them - without realizing what they were doing - still believe in current-control.
Don`t ask me why - I cannot answer.
One explanation may be the fact that for CALCULATION PURPOSES it sems to be simple and convenient to ASSUME current control - and, inded, it works!
However, we should not mix physical principles with design-oriented methods.
As a background, I like to give you a similar example:
We say that in a simple resistive voltage divider the current would produce a voltage across each of the two resistors in proportion to the resistor values (given in Ohms) - right?
No - physically wrong. A current cannot produce this voltage. It is always the voltage that allows/drives a certain current. And the voltages are a only measure of the electric fields within the two resistors.
Hower, for calculation purposes, we are allowed to say: The total current produces a voltage V=IxR. But physically, in the cause-and-effect sense this is wrong.
And the same applies to the base current Ib=Ic/B ...it is a by-product that cannot be avoided. And for calculation/design purposes we may use this equation in the form Ic=Ib*B. But this does not mean that Ic would be controlled by Ib.
answered 6 hours ago
LvWLvW
15.2k2 gold badges13 silver badges31 bronze badges
edited 5 hours ago
answered 6 hours ago
LvWLvW
15.2k2 gold badges13 silver badges31 bronze badges
answered 6 hours ago
LvWLvW
15.2k2 gold badges13 silver badges31 bronze badges
answered 6 hours ago
LvWLvW
15.2k2 gold badges13 silver badges31 bronze badges
15.2k2 gold badges13 silver badges31 bronze badges
$begingroup$
Vbe, not Vgs...
$endgroup$
– Chu
1 hour ago
add a comment |
$begingroup$
Vbe, not Vgs...
$endgroup$
– Chu
1 hour ago
$begingroup$
Vbe, not Vgs...
$endgroup$
– Chu
1 hour ago
$begingroup$
Vbe, not Vgs...
$endgroup$
– Chu
1 hour ago
add a comment |
Nullbyte is a new contributor. Be nice, and check out our Code of Conduct.
Nullbyte is a new contributor. Be nice, and check out our Code of Conduct.
Nullbyte is a new contributor. Be nice, and check out our Code of Conduct.
Nullbyte is a new contributor. Be nice, and check out our Code of Conduct.
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3
$begingroup$
Which came first, the chicken or the egg? Is it the voltage that determines the current flow in a resistor or, is it the current flow that dictates the voltage dropped?
$endgroup$
– Andy aka
8 hours ago
1
$begingroup$
Idk abt that but "voltage is the cause and current is the effect" feels a bit better.. I maybe wrong though
$endgroup$
– Nullbyte
8 hours ago
$begingroup$
According to Shockley transistor equation the collector current is controlled by the voltage on the base.You are talking about the ratio of Ic/Ib. That is a side-effect that base-current flows and that the ratio of base to collector current is somewhat constant in active region.
$endgroup$
– cm64
8 hours ago
$begingroup$
Nullbyte_just one sentence: You are completely right (with your commen above.
$endgroup$
– LvW
5 hours ago
$begingroup$
Far too many engineers seem to actually believe that the base current controls the collector current. But the physics say otherwise. One better paper to read is W M Webster's, "On the Variation of Junction-Transistor Current-Amplification Factor with Emitter Current," from Proceedings of IRE, 1954. If you get that paper and read it, you'll understand more than enough. But to understand why engineers use base current, see Figure 2.17 located here. Required recombination current to sustain collector current holds a ratio over many orders.
$endgroup$
– jonk
5 hours ago