why a current source is required in a differential amplifier

common mode gain of bjt differential amplifier

In a BJT differential amplifier why a current source is required ? what's it's use......................?

Re: why a current source is required in a differential ampli

desperado1 said:

In a BJT differential amplifier why a current source is required ? what's it's use......................?

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It is not absolutely "required" - in some applications a high resistance in the common emitter path can be also sufficient. The common mode gain goes down when there is much signal feedback in the emitter path.
However, for a good CMRR (that means a very low common mode gain) a good current source is the best choice.

Re: why a current source is required in a differential ampli

LvW said:

desperado1 said:

In a BJT differential amplifier why a current source is required ? what's it's use......................?

Click to expand...

It is not absolutely "required" - in some applications a high resistance in the common emitter path can be also sufficient. The common mode gain goes down when there is much signal feedback in the emitter path.
However, for a good CMRR (that means a very low common mode gain) a good current source is the best choice.

Click to expand...

Thanks Mr. LvW but i have a few queries regarding your replies.
1. What do you mean by the signal feedback in the emitter path ?
2. How does a common mode gain goes down when there is much signal feedback ?
3. what's the advantage of high resistance in the common emitter path ?
4. Does a good current source only ensures good CMRR in common mode case or also in differential mode case ?

i m new to this field so cud you please explain in more basic way..............i know u cud coz u r damn gud in this field.
thanks in advance...

Re: why a current source is required in a differential ampli

desperado1 said:

............but i have a few queries regarding your replies.
1. What do you mean by the signal feedback in the emitter path ?
2. How does a common mode gain goes down when there is much signal feedback ?
3. what's the advantage of high resistance in the common emitter path ?
4. Does a good current source only ensures good CMRR in common mode case or also in differential mode case ?

Click to expand...

1.) For common mode signals the gain is gRc/(1+2gRe) with g being the transistor transconductance. In this mode the fuction of Re is to provide feedback like in each common emitter stage (because the signal voltage across Re counteracts the signal voltage at both base terminals).
2.) The equation shows the relationship
3.) advantage: small common mode gain and high CMRR (this is a wanted property). If you use a good current source its internal resistance approaches infinity. This never can be achieved with resistors.
4.) The common mode gain does not depend on the mode. Dont forget that in each differential signal there is always also a certain portion of common mode signals.

I hope I have expressed myself clear enough.
Regards
LvW

Re: why a current source is required in a differential ampli

1) using current source in emitter to have a stable voltage in emitter and providing a lower Ac in common mode if you use a current source with high output impedance so you will get a better differential amplifier.

2)use a current source also in collector to have a higher Av in differential mode

Re: why a current source is required in a differential ampli

nhashemi said:

.... using current source in emitter to have a stable voltage in emitter ............

Click to expand...

No, the emitter voltage will not be - and shall not be - "stable" ! A current source gives a constant current, but of course the emitter voltage changes according to the "opening/closing" of the transistor in differential mode.

Re: why a current source is required in a differential ampli

well i thought if you design a current source with transistors the condition of Vbe (of mirror transistors) will be better in comparison to using a resistor in emiter of the mirror transistors.