Question about differential amplifier configurations

[crutschow]

But explain to me this then. Why do they use these on the input of allot of chips? What do they do to get the extra power gain? They don't feed both base's do they?

Probably the main reason to use a differential input is to cancel the dc offset drift of the base-emitter voltage with temperature. The two base emitter voltages drift in the same direction and thus cancel each other (quite well if they are close together on the same chip), therefore the dc output bias level remains constant. This is important for DC coupled circuits, and to keep the internal bias levels constant in AC input coupled circuits. Most ICs, even if they are AC coupled on the input, have several interal dc coupled stages.

An ideal differential stage's output differential voltage gain is essentially the same as a single transistor stage. Thus there is no gain advantage to a differential stage.

 

[Roff]

An ideal differential stage's output differential voltage gain is essentially the same as a single transistor stage. Thus there is no gain advantage to a differential stage.

To clarify this, it is true when each transistor has the same collector load resistor and is running at the same emitter current. For example, if you had a single transistor CE stage, running at Ie=1mA, and Rc=1k, then a diff amp with a tail current of 2mA (1mA per side), and 1k collector loads, would have the same gain when measured between the collectors.
Another advantage to diff pairs is the elimination of even harmonic distortion on the differential output. Single CE stages (without feedback) have very high even harmonic distortion.

 

[MrAl]

No, I realize that but I had some strange idea that the single ended configuration I was using would help kill noise. I think I read it wrong...lol. Maybe I can lie my way out of this one.

Hi again,


I am not sure if i made it clear or not before, but what i was suggesting
was that the circuit you posted be changed a little so that it is made
into a true two input diff amplifier. This would give you some CMR
if in fact that is what you really need for your application. I dont
know everything about your application though so i cant make
that call. It would help if everyone knew a little more about your
intended app.
If you do decide that you really need CMR then you can
ask someone here to redraw it for you to make it work that way.

 

[Space Varmint]

Thank you Ron and Carl. Wow! Two heavy hitters right there. I like the idea of limited harmonic distortion. I like that allot in radio circuits. I think I have a market for this thing when I'm done with it. That could be one of it's features...lol