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3 stages power amplifier project

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Lucky-Luka:
Hi all
I do have a school project to do: a 3 stages power amplifier circuit.

+-16V power supply
+- 175mV input signal swing corresponding to +- 14V output swing. (closed loop voltage gain = 80).
open loop gain of the 3 stages ampli = 2450
resistive load >= 10ohm
amplifier input resistence >= 10kohm
bandwidth >=80 kHz
phase margin 80°
efficiency >=60%
THD <=1%

I have  to use only transistors listed in the ltspice database.
That's why I had to choose 2N3055 (npn) and D45H11 (pnp): the only ones capable of dealing with 10W power dissipation (I've read that from their datasheets).

I've started the project from the output stage.
Class AB output stage using a Vbe multiplier as biasing and a current mirror (current source as first attempt).
I just wanted to see if I could get it right... Something went wrong... Obviously...
I gave it a 14V sinusoidal input with -1.3 DC offset and I hoped to see sinusoidal 14V oscillating around 0V as vo.
I got a distorted sinusoidal waveform oscillating around -280 mV.
How can I fix that? What am I doing wrong?
Any suggestions is appreciated.
Cheers

Zero999:
No this doesn't have an open loop gain of 2450. It's a unity gain power stage.

Connecting the base of Q4 to 0V via V4 like that is messing the biasing up. Look at Q4's base current!

Try AC coupling the input, via a suitable capacitor and a potential divider to bias the output stage, rather than a constant current source.

When you add the other stages, which can be DC coupled, you'll discover that negative feedback will cause Q4's base will sit at two base-emitter drops below 0V.

Lucky-Luka:

--- Quote from: Zero999 on December 18, 2019, 10:07:07 pm ---No this doesn't have an open loop gain of 2450. It's a unity gain power stage.

--- End quote ---
I wasn't clear.
That is the 3 stages circuit open loop gain.


--- Quote from: Zero999 on December 18, 2019, 10:07:07 pm ---Connecting the base of Q4 to 0V via V4 like that is messing the biasing up. Look at Q4's base current!

--- End quote ---
I think I have done as Sedra/Smith shows...


--- Quote from: Zero999 on December 18, 2019, 10:07:07 pm ---Try AC coupling the input, via a suitable capacitor and a potential divider to bias the output stage, rather than a constant current source.

--- End quote ---
How can I do that?

Zero999:
There is nothing wrong with using a constant current source, but it won't work with the input biased around 0V like that. The circuit is designed to be used in the feedback loop of a voltage amplifier which will set the DC bias point to the required voltage using negative feedback.

There are other problems. The VBE multiplier should have 4 base-emitter voltage drops, not two, to bias the output transistors on. The output transistors need emitter ballast resistors to stabilise the operating current, otherwise it won't work in real life, even if you can get it to work in a simulator.

EDIT:
Oh, I missed the -1.3V in the input voltage, but it's not good enough. VBE can't be predicted that accurately.

magic:
This should work as a unity gain buffer with some offset between Vin and Vout, nothing fundamentally wrong with it. The offset should indeed be about two diode drops, but this stage is underbiased so it may do weird things as of now.

Crank up the Vbe multiplier until you get some reasonable idle current through both emitters. Adjust V4 again for zero output, don't blindly assume that 1300mV is the exact number you need because it's not and it depends on transistor type and idle currents.

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