Help me with the calculations of coupling capacitors for my power amplifier

Devezon08

Joined Nov 14, 2023
3
I am trying to design a power amplifier using a differential amplifier with constant current source, low pass filter (fc = 3100 hz), buffer amplifier and push pull amplifier. I used a value of 50 for beta. I am struggling to design the right amount of coupling capacitor values between stages. Right now, I just assumed it to be 47 microfarads, the circuitry and results are below. How do I design it properly?

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Ian0

Joined Aug 7, 2020
9,503
You need more help than just coupling capacitors. That circuit has a lot of problems:
1) No base bias resistor for Q1.
2) D1 and D2 are shorted out
3) The output at Q5 emitter is limited to a swing of half the positive supply, so the output will never reach maximum
when you have sorted out all those, then the cutoff frequency of the coupling capacitors is
f=1/(2πRC) just the same as any other filter.
C is your coupling capacitor, and R is the input impedance of the next stage, which is not so easy to work out.
for instance, the input impedance of the second stage.
Its maximum value will be the parallel combination of R13 and R14, but it will be reduced by the input impedance of the transistor, which, as an emitter follower, will be somewhere in the region of Q5 beta x the input impedance of the next stage.

Devezon08

Joined Nov 14, 2023
3
You need more help than just coupling capacitors. That circuit has a lot of problems:
1) No base bias resistor for Q1.
2) D1 and D2 are shorted out
3) The output at Q5 emitter is limited to a swing of half the positive supply, so the output will never reach maximum
when you have sorted out all those, then the cutoff frequency of the coupling capacitors is
f=1/(2πRC) just the same as any other filter.
C is your coupling capacitor, and R is the input impedance of the next stage, which is not so easy to work out.
for instance, the input impedance of the second stage.
Its maximum value will be the parallel combination of R13 and R14, but it will be reduced by the input impedance of the transistor, which, as an emitter follower, will be somewhere in the region of Q5 beta x the input impedance of the next stage.
Thank you. For no. 1, I removed the collector resistor in Q1, I mistakenly added it. Supposedly I'm just going to use a single ended output diff amp, that's why I removed it. As for no. 2, I added that because it fixes the crossover distortion of the push pull amplifier. As for no. 3, can you please give a hint or tip to how I can fix that? I'm also quite struggling with the input and output impedance of push pull amplifier, I will have to figure it out.

Ian0

Joined Aug 7, 2020
9,503
The input impedance of an emitter follower is approximately Rload/β., but β varies with collector current and is not a well-defined parameter.

is there a reason why you want to make an amplifier out of three individual stages each with its own local feedback? Conventionally, the three stages would be DC coupled, and have overall global feedback.

Devezon08

Joined Nov 14, 2023
3
I just want to know if these three individual stages would work as a power amplifier. Can you please give suggestions?

Ian0

Joined Aug 7, 2020
9,503
I just want to know if these three individual stages would work as a power amplifier. Can you please give suggestions?
Well, sort of.
The input and output stages are OK, but the middle stage will need some voltage amplification, so a PNP common-emitter stage with local feedback would be better.
But it‘s never going to be as good as a DC coupled amplifier with global feedback.

Audioguru again

Joined Oct 21, 2019
6,610
You do not have a power amplifier, instead it is a low power opamp with a fairly high output impedance.
The errors are said previously and since it has low voltage gain and no overall negative feedback it will have distortion and sound bad.
Why does it have an extremely low impedance lowpass filter at the input that will short the signal source? The 3400hz cutoff will sound very muffled like an old telephone or old AM radio. We can hear to 20kHz.
Your schematic had its parts very far apart so I cropped then enlarged it.

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LowQCab

Joined Nov 6, 2012
3,938
If You just want to see if You can do it, it can be a very satisfying project,
but You never out-perform a simple Single-Chip-Amp that is simple and dirt-cheap.

What are your goals/specifications for this Amp ?
.
.
.

Audioguru again

Joined Oct 21, 2019
6,610
Why is it called a "Power Amplifier"? With its 24V supply and 7V peak clipping into 10k ohms, its output power is only 0.003W with high distortion..
With a 24V supply and an 8 ohms speaker, the output power should be 6.3W with low distortion.