Why is constant current region desired in amplifers?

[electricity86]

I read that in CE amplifier, its desirable to bring the amplifier into the constant current region.
Is this region also called forward bias region where iC = beta * iB?
And why is it desirable to insert it into that region?
Thanks.

 

[Willbe]

Because a transistor is a current controlled current source?

 

[electricity86]

I still dont understand why should it be in the constant current region?
In the saturation region, the current is not constant for different values of VCE, why cant it be there?

 

[dknguyen]

I still dont understand why should it be in the constant current region?
In the saturation region, the current is not constant for different values of VCE, why cant it be there?

It can't be in the saturation region for the very reason that the current is not constant for different values of Vce. You have to remember we want to amplify the base signal and we want to amplify it linearily.

You want to be in the constant current region because in this region, the collector current (which produces the output signal via the collector resistor) is mainly dependent on the base current and it's dependence on the the collector-emitter voltage is greatly reduced. In the saturation region, the collector current is dependent on both the base current and collector-emitter voltage. Your gain is not constant for all values of Ib because the gain is now influenced by collector-emitter voltage (which in turn is influenced by collector current running through the resistor producing a voltage).

The voltage output is being produced by the collector current running through the resistor. When the collector current changes due to a change in the input signal current at the base, the voltage across the resistor will change. But so will the collector-emitter voltage since resistor and CE are in series with the same fixed voltage source. In the saturation region, this means that your output voltage is dependent on base current and collector-emitter current. In the constant current region, the collector current is pretty much unaffected by collector-emitter voltage so much of this dependency is removed.

Look at the load line on the attached graph. Imagine what would happen if it was running through the saturation region instead of the constant current region.

 

[kchriste]

I read that in CE amplifier, its desirable to bring the amplifier into the constant current region.

That is because you want the amplifier to be linear with a minimum amount of distortion. If you go into the saturation or cutoff region, you'll have clipping and thus a lot of distortion.

 

[BrownOut]

Is there going to be an exam? Heh!