Understanding a Push-pull amplifier schematic.

[nylonman]

Hi, I'm trying to understand the operation of the schematic I attach.
http://imageshack.us/photo/my-images/853/abampschematic.png/
What's the point of having the input at the Q1 collector? I can understand the more common structure shown in wikipedia ( http://en.wikipedia.org/wiki/Push–pull_output ) but I have trouble with the first one. Can anyone explain me how it works? Thanks.

 

[Averagesupernova]

The wiki link didn't work for me. The collector of Q1 is not the input. It is the input to the last stage, but the output of the previous stage. The image looks incomplete towards the bottom of Q1.

 

[nylonman]

I suppose that Q1 causes a variation of its collector current according to the input somewhere in the previous stage. I see that if Q1 isn't conducting at all the last stage 'pushes' current to the load (not shown here). But what about 'pulling' it?

Try this link: http://en.wikipedia.org/wiki/Push-pull_output

 

[Averagesupernova]

Without showing any more of the schematic in your first link you cannot really say if Q1 ever goes completely out of conduction. Based on my experience I would say that it should not. There just isn't enough there to tell any more than this. What you can be sure of is that the output stage has an AC voltage gain of one and the emitters of the output transistors will follow pretty closely what is on the collector of Q1.

 

[Studiot]

Your image is a bog standard class A driver for a complementary pair of emitter followers operating in class AB.

The input of the stage is to the base of Q1. It's output is developed, largely across R1 and feeds both bases of the output pair. R2 and the two diodes form the small bias to (nearly) remove crossover distortion.

Since Q2 and Q3 are of opposite polarity each conducts on half a cycle and is cut off on the other half, making the 'push-pull' arrangement.

It is called push-pull because the output is alternately pushed up towards the +ve supply as Q2 conducts and Q3 is cut off and then pulled down towards the negative supply as Q3 conducts and Q2 is cut off.

go well

 

[jsgruszynski]

Q1 is simply the prior stage. Push-pulls are usually output stages so there is generally prior stages more optimal for lower level signal amplification. Q1 is that. It happens its output comes out of the collector (and input the base, hence CE configuration) but it could have been different, e.g. CB cascode output.

 

[nylonman]

Ok, thanks! I understand it now but there is another detail: The book says that R4 and R3 improve thermal stability since there is a voltage drop of few tenths across them that can be controlled with R2. But wouldn't it be better that there wasn't any quiescent current at all?

 

[Studiot]

Ok, thanks! I understand it now but there is another detail: The book says that R4 and R3 improve thermal stability since there is a voltage drop of few tenths across them that can be controlled with R2. But wouldn't it be better that there wasn't any quiescent current at all?

No sure what you have understood here.

R3 and R4 are emitter resistors that provide a small amount of local emitter feedback to each output transistor. This has nothing to do with with the push-pull arangement and is basic to the emitter follower (common collector) configuration that Q2 and Q3 are in.

If you do not understand this I suggest you go back and revise that part of your notes it is really basic.

In order to be just at the edge of conduction each of Q2 and Q3 need to be biased the equivalent of one forward pn junction - the base emitter junction. Hence the two diodes.
Using a diode instead of a resistor thermally stabilises the transistor since the diode junction reacts the same as the transistor junction to temperature change, unlike a resistor.

The purpose of R2 is twofold.
Firstly the voltage across it provides the extra bias to compensate for R3 and R4.
Secondly adding a small base bias on top of this puts Q3 and Q4 just slightly beyond the edge of conduction into actual conduction. This is done to eliminate the unwanted highly non-linear transfer characteristic at the origin called crossover distortion.

You may be confused with the use of the term bias.

R2 and the diodes provide voltage bias to the bases of the two output transistors.

This results in a small collector bias current in both transistors. It is this bias current that is meant when referring to setting the amplifier bias.

go well

 

[nylonman]

Ok, I was confused with the purpose of adding some extra bias voltage with R2. Tanks for your help.