can anybody help me to understand how this circuit works?

[fala]

circuitworks 10 help

Hello, I want to understand how this circuit works, it has been taken from National Application note 272- OpAmp Booster design(**broken link removed**). explaination in the document was not enough for me to understand it fully. I actually build this circuit and it works but I need to understand its theory. I understand this far: Q1 & Q2 are voltage dependent current sources and deponding on the output voltage of the opAmp their collector current changes and by drawing current from 1K resistors they impose a Vbe on corresponding Q3 & Q4. Q5 is a constant current source and as article has pointed out it is used to for bias and Q6 I SUPPOSE acts as an emitter follower and Q5 & Q6 will bias push-pull driver Q7&Q8.
now what I don't understand: it is impossible for Q1&Q2 to both be on because if one is on the other reverse biased so only Q3 OR Q4 can be on not both of them. ok when op amp out put is negative and turns on Q1 & Q3 there is no problem because Q3 draws current from 120K resistor but what about opposite when Q2 & Q4 are on then Q3 is supposed to be off so current for Q4 collector comes from where? why the circuit is not symmetrical for positive and negative ?
I will appreciate any idea that helps me to understand it. thank you very much.

 

[IanP]

Re: can anybody help me to understand how this circuit works

Q4 draws current from the base of the PNP transistor - Q6 ..
In NPN transistors current is pumped to the base, in PNP current is drawn from the base ..
Regards,
IanP

 

[artem]

Q1 and Q2 connected as BJT common base amplifiers , while Q3 and Q4 with common emitter . Their common connection (Q3 and Q4 collectors) will have same aplitude as on amplifier output, but to drive output load Q6 Q7 and Q8 are put .

Q5 together with thermocompensation diodes (they also give fixed voltage to Q5 base and define current together with 330 Ohm resistor connected to Q5's emitter) forms current source for Q6's output.
2 diodes connected to Q7 and Q8 are using for biasing them , and define amplifier class (A, AB).


Amplifier has 1 negative feedback defining gain (100 k and 10k resistors) as 100k/10k = 10.
And also correcting capasitor 20 pf and 100 pf provide gain decrease at high freq and stability improvement for opamp and amplifier whole.

100 Oh resistors on Q7 and Q8 emitters , provide curreent protection for output short circuit .

 

[mahdi_p777]

Re: can anybody help me to understand how this circuit works

Q7&Q8 are output transistors in push-pull form . two 100 Ω resistors are for protaction against short circuit at the output. the capacitor is for frequency compensation. all diodes are for biasing. this is AB class of amplifiers