I am trying to build this amplifier on a breadboard, but it starts to oscillate when I put my hand near the push pull transistors. What would cause this and how can I remedy it?
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\$\begingroup\$ The low value of R10 (input) prevents the negative feedback from properly reducing gain and linearizing the first stage (I guess, just eyeballing). So I would raise this, to a few kOhm. and then also add a small cap over R1 and R3 to cut down HF gain. \$\endgroup\$– tobaltCommented Dec 13, 2023 at 15:12
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2 Answers
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First of all, are you sure this circuit is working as intended? I copied it (hopefully not making mistakes)
but the simulation shows clipping of the output from the first transistor stage (measured at TP1):
Why did you choose to use current mirrors as collector loads? Why not simple collector resistors?
Regarding the oscillation, did you measure at what frequency the circuit oscillates? Your amplifier has a very high gain and no way to reduce the gain for frequencies above the audio range (which are not needed and in fact harmful). You need to think about a way to add a compensation mechanism that reduces the gain for high frequencies. A possible starting point for experimentation would be to add ~68-75pF in parallel with 82k resistor, maybe ~33p between bases and collectors of output driver resistors (Q3, Q4 on your schematic) and study AC analysis graphs.
Another advice already given by @glen_geek is careful consideration of feedback from the power stage to the input stages. Block power supply with capacitors (not only large electrolytic, since they have relatively large impedances at higher frequencies but also ceramic ones ~100nF in parallel). Maybe use extra RC filtering like this
or even more comprehensive stabilization of the voltage of the input stage.
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That's a lot of open-loop gain, and breadboard construction doesn't help.
Oscillations could be via Vcc or ground-loops.
Add a monster electrolytic from TIP41C collector to TIP42C collector (Vcc-to-GND).
It is difficult to make Vcc a single-point because so many parts are connected. It is also difficult to make GND a single-point on that breadboard.
The inductive paths may be the cause of oscillations, but even resistive paths can cause problems too, especially where currents are high:
- currents in the 8-ohm load are high.
- GND current path into Class AB final stage is high
- Vcc current path into Class AB final stage is high
Because gain is so high, the ground connection for AudioIn should also be carefully routed to that same single-point GND.
