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help with Class AB amplifier
rex1232:
Hey guys :)
I am in need of help with understanding the part of the circuit which is marked in the red circel thing (R9 all the way to D10)
The first part by that I mean the differential amp stage (Q2,Q3) and the constant current source (Q1, D8 D9) I understand.
But Q16 and Q17 looks like another diff amp to me, but is it?
I have been told the part which looks like a constant current source (D10,R11,R13,Q6) is actually a current mirror.
Can anyone explain what function this part has?
ps. (someone told me that the top part (d10 etc) was used as a temperature regulated current source, which would work by D10 changing its voltage drop over it if the temperature changed.) Is there any truth to that?
Thank u in advance
magic:
Swap D10 and R11 (which makes no difference to the functioning of the circuit) and you will see it's a current mirror, with 180Ω emitter degeneration. R9,Q16,Q17 are indeed a second differential stage, C3 is likely some sort of feedback from later stages.
It seems Q7,R12,R14 are Vbe multiplier and the whole circled part is the VAS. Q17 pulls the output down, Q16 pulls it up through the mirror. C3 is the VAS compensation capacitor.
Jay_Diddy_B:
Hi,
Audio amplifier normally have three stages:
1) Input stage Q1, Q2 and Q3
This is a long-tailed pair with a current source, Q1, D8, D9 in the emitter.
2) Voltage amplifier stage VAS
This is Q6, Q16 and Q17
3) Current amplifier stage
Q10, Q12, Q14 and Q15
There are two Darlington emitter followers, voltage gain=1
Q8 and Q9 are for overcurrent protection
Q7 is vbe multiplier to compensate for vbe in the output transistor stage.
In the VAS, D10, R11, R13 Q6 is a current mirror. it would look more like a current mirror if D10 was replaced with a diode connected transistor.
Q16 and Q17 is another differential amplifier
The current mirror load gives higher gain than if resistors were used.
Regards,
Jay_Diddy_B
xavier60:
The negative feedback path is also important to be aware of. It sets the overall gain to some practical value within the audio pass band plus some extra.
As well, it stabilizes the output DC to near zero volts.
David Hess:
--- Quote from: rex1232 on November 22, 2019, 07:10:55 pm ---But Q16 and Q17 looks like another diff amp to me, but is it?
--- End quote ---
Yes, Q16 and Q17 form another differential amplifier. Note that the operating point (tail current) is set by the common mode output voltage of the previous differential amplifier.
--- Quote ---I have been told the part which looks like a constant current source (D10,R11,R13,Q6) is actually a current mirror.
--- End quote ---
Yes, it is a current mirror. As magic points out, swapping D10 and R11 makes this more apparent.
--- Quote ---Can anyone explain what function this part has?
--- End quote ---
That part is the VAS stage. The current mirror converts the differential output current from the differential pair into a single ended output voltage. Douglas Self refer to this configuration as the Hitachi push-pull VAS and apparently it became popular in the 1980s for driving the large gate capacitance of power MOSFET output amplifiers.
--- Quote ---ps. (someone told me that the top part (d10 etc) was used as a temperature regulated current source, which would work by D10 changing its voltage drop over it if the temperature changed.) Is there any truth to that?
--- End quote ---
D10 roughly compensates for the Vbe and temperature coefficient of Vbe of Q6. The temperature compensation is incidental and not required for proper operation in this application. Often this configuration gets away with leaving out D10. A more precision design would use the base-emitter junction of another BD140 in place of D10 but that would be more expensive for little if any gain.
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