Differential amplifier output

[julian403]

Hi, a differential amplifier; What is the different between a simetric output and with the asimetric output?

The next picture is a simetric output

and this is a asimetric output

For example if I want to amplify the different between two signal, I think I must use the simetric output. I do not see a differente between an common emitter and the differential amplifier with asimetric output.

 

[D.A.(Tony)Stewart]

This basic differential amp. has both differential inputs and outputs.
Each has benefits of reducing error from DC offset and nonlinearity.

The Input error is from Vbe offset.
The Output error is from Vce offset.

Eliminating both gets you the basic CE config.
Can you see the difference now?

 

[LvW]

and this is a asimetric output

For example if I want to amplify the different between two signal, I think I must use the simetric output. I do not see a differente between an common emitter and the differential amplifier with asimetric output.

For a "good" diff. amplifier (small common mode gain, large CMRR) one output voltages goes high (+Vout) and the other one goes down (-Vout). Hence, the diff. output is +Vout - (-Vout)=2*|Vout|.
That means: There is no fundamental difference between both modes. The differential output has the advantage that there is no common mode voltage (because of the difference).
Yes - it is correct, that the second circuit acts as a common emitter configuration (with some emitter degeneration) - as far as the inverting output node is concerned. However, using the other (non-inverting) output the two-transistor circuit can be seen as a series combination of a common-collector and a common-base circuit.

 

[julian403]

thank you both. If I want to put as a first stage in an amplifier an differential amplifier. What output I must to do?

And other thing. Why the commun voltage of two signal is the midle of the signal, i means Va/2