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# Why is a transistor a current controlled device

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#### Faiyaz

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Why is a transistor a current controlled device when actually it is the potential across the BE junction that controls the operation of a transistor?
Or am I wrong in making such a statement? :?:

The Transistor: A current controlled device

I actually think it's because the gain of the transistor (amplification) is mostly determined by the current gain. Please look into it a little more, I will do so myself and post another contribution.

Because the current that is allowed trough flow trough the collector is dependant on the base current.

Faiyaz said:
Why is a transistor a current controlled device when actually it is the potential across the BE junction that controls the operation of a transistor?
Or am I wrong in making such a statement? :?:
You can look at it either way, since Vbe and base current are related by an equation. The transconductance, gm=Ic/Vbe, is very nonlinear (it's exponential). The current gain (Hfe=Ic/Ib) is relatively constant. Also, the input impedance of a common emitter transistor is relatively low, meaning the base voltage changes very little with change in input current. Unless you need a logarithmic transfer function, and some applications do, it is generally much easier to think of the BJT as a current-controlled device.

Then I am not wrong in stating that a transister is a voltage controlled device?
Although the input impedance may be low, but it is a fact that ultimately it is the VBE that controls the flow of current through the emitter junction which in turn controls collecter current.
Variations in VBE brings about a change in the emitter current. isn't it?

Faiyaz said:
Then I am not wrong in stating that a transister is a voltage controlled device?
Although the input impedance may be low, but it is a fact that ultimately it is the VBE that controls the flow of current through the emitter junction which in turn controls collecter current.
Variations in VBE brings about a change in the emitter current. isn't it?
Read my previous post again. You are correct in saying that a bipolar transistor is a voltage-controlled device. You are also correct if you say that it is a current-controlled device.

Thank you ROn H. For once there is some one who agrees with me. I actually lost a job because of making such a statement.

I agree V and I controlled! Why would anyone argue?!
These devices have I-V curves that they operate on and if you can force the V part, you will get the I part and vice versa.

Just like a diode! You can either forward bias it with a voltage and its current will be whatever it is on the I-V curve. Or, you can force a current through it and its voltage drop will be whatever it is on the I-V curve.

I think the notion of I controlled versus V controlled comes primarily from what the typical (or easy) methods are to control the devices. For example with a MOSFET, the gate current is very very low. While you can inject a current to bias the device, this is typically not done because it is difficult to accurately achieve such low currents. So instead, the VGS is controlled and so it is to be called a "voltage" controlled device. But it is really either one strictly speaking.

And I think since everyone is so interested in the Ib to Ic control of bipolar devices, this is why this one is called current controlled.

Out of curiosity, what kind of a person would fire you over such a subjective topic? Why would it be relevant to your job what they are "called" ??

Faiyaz said:
Thank you ROn H. For once there is some one who agrees with me. I actually lost a job because of making such a statement.

You may already know all this.

The base-emitter diode current (Ib) is

Ib = Is*( e^(q*Vbe/(n*k*T)) - 1 ),

where
Is is the diode saturation current (reverse bias leakage),
Vbe=base-emitter voltage,
e=2.71828...., the Naperian logarithm base,
q=1.602*10^-19 coulombs, the charge on an electron,
k=1.3807*10^-23 joules per kelvin, Boltzmann's constant,
1<n<2, the diode ideality factor (n=1 nominally, but varies),
T=temperature, degrees kelvin.

nkT/q is approximately 26mv at room temperature, so

Ib=Is*(e^(Vbe/.026)-1), and if Vbe>>.026,

Ib=Is*e^(Vbe/.026)

Since collector current Ic =beta*Ib,

Ic=beta*Is*e^(Vbe/.026) at room temperature, proving that the BJT is a voltage-controlled device.

Having said that, I still think it is easier to think of it as a current-controlled device, i.e., Ic=beta*Ib.

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