Bjt differential amplifier

Discussion in 'General Electronics Chat' started by HarrisonG, Sep 3, 2016.

Aug 1, 2016
73
3

I have a few questions about the circuit above.
1. The job of the differential amplifier is to amplify the difference between the two input signals. Can't I just use two CE Amplifiers and connect the emitters together to one emitter resistor? After all, it will do the same job.
2. On most bjt differential amplifier circuits I see that the emitters are connected not to 0v, but to some negative voltage like -12v. Does that mean I need two batteries in series so that the middle point is 0volts, the top is +12v and the bottom -12v?
3. If we do need to commect the emitters to a negative voltage, then why do we do so, what is the reason behind this?
4. How do I calculate the emitter resistor? I mean, in CE Amplifiers I usualy leave 1volt to RE, is it the same case?

2. #12 Expert

Nov 30, 2010
18,093
9,683
That is exactly the functional point that makes it all work.
You must have at least one collector resistor or you can't develop any voltage difference during an input event.
That is a matter of convenience. You can use 1/2 Vcc for the bases if you AC couple the input.
Only for the convenience of a zero DC voltage at the bases. It works really well for an audio amp because you can force the output pair to center at zero DC with that method.
You calculate the emitter resistor to allow the idle current you want. I usually use 1 ma. You might have other needs. There is no reason you can't do a differential pair with 1 amp of idle current if the input signal can force a proper change in current with an emitter resistor so small as to allow an amp of collector current.

HarrisonG likes this.