Why this class AB complementary buffer isn't working?

Hi,
Everyone, My English is not good,I have a doubt with this circuit.i think it can not work,but I am wrong,can everyone tell me,why?
Thanks!

I have a doubt

In order to work, without the signal generator, you must check the operation point, that is Q1 and Q2 must be correctly biased. And the point between D1 and D2 must be cero.
I supouse that the point between R3 and R5 is the output.

I hope this can help you.

Re: I have a doubt

It's a usual class AB complementary buffer with some non-ideal properties, but operational so far. Using current sources instead of the 1 k resistors would make bias curent more independant of operation point. Also the actual bias current depends on diode to transistor area ratio and can't be prededicted from the schematic.

Re: I have a doubt

Hi,
About 4.4mA of base current is a lot for a signal generator output to overcome and have a significant efffect (OK, I know this is a virtual sig generator, so it will probably have a very low AC & DC impedance), I think you should make the biasing resistor values larger so that the standing current is lower, preferably add a second resitor between each devices base and ground to stabilise the bias and prevent possible thermal runaway. If this is a power amp stage then you could add a low power driver stage in front of it. The biasing of transistors is very important as stated by previous respondants. As drawn both transisotrs are likely to be almost fully conducting (0.5A) under quiescent conditions, this is not normal for a push-pull amp or even for a class A amp.

Bob

Re: I have a doubt

DrBob13 said:

As drawn both transisotrs are likely to be almost fully conducting (0.5A) under quiescent conditions, this is not normal for a push-pull amp or even for a class A amp.

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I disagree.
The diodes are a fairly good match to the base-emitter junctions of the transistor and the emitter resistors aid in turning off the transistors. There might be a current in the transistors that is too low resulting in crossover distortion.

Re: I have a doubt

Thank you for your reply,my mean one transistors is on,the other is off, i=0?the transistors Working in what state?

I have a doubt

The transistors are supposed to have a small idle current in them so they operate in class-AB with very low crossover distortion.
The NPN works hard to pull up the output and the PNP works hard to pull down the output.

Re: I have a doubt

Remove R1 and R3,R5 switch to 1 ohm

Re: I have a doubt

zlatkoMM said:

Remove R1 and R3,R5 switch to 1 ohm

Click to expand...

R1 can be removed only if the signal generator is moved to drive the base of Q1 and D2.

Re: I have a doubt

You have a very valid point Audioguru, Whoops! Humble Pie, I did get it wrong. Too late at night when I answered the post I think and you are right about not removing R1 without moving the sig gen.
Bob

Re: I have a doubt

One may have different specifications regarding input and output impedance, but the dimensioning is basically acceptable. If the diodes are similar to transistor BE, you get about 10 mA bias current. Due to the large emitter resistor value, it's almost insensitive to exemplar variation and thermal drift.