Amperra v1.0: The DIY Amplifier

A little class AB amplifier with low distortion

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This little amplifer makes use of an unusual output stage. It has Sziklai output pairs which are configured as a 'Non-Switching' design - meaning that none of the output transistors will completely stop conducting, even when the opposite transistor pair is delivering full output current.
The strangely placed shottky diodes at the output will help with this. Furthermore, they simplify biasing a lot. No need to place milivolts across miliohms in order to get the desired 50mA bias - the bias-setting resistor is quite high at 3.3 Ohms, which makes the circuit very robust to component variations (as evidenced by the current component mismatches ;) )

Furthermore, it has 2-pole compensation, hitting almost 70dB open-loop at 20kHz. LTSpice says that this should result in excellent distortion performance.

Use the KiCAD schematics for reference - the LTSpice schematic doesn't show all resistors, sometimes i included them as ESR into caps. It is just for showing the sim set



5W into 8 Ohms per channel.


Simulated: <-120dB (<0.0001%) just below full scale (5W), 1kHz
Measured: <-90dB (0.003%) just below full scale, 1kHz - but at this point, my measurement equipment becomes the limiting factor


Only uses discrete, cheap and easy-to-source components.

Signal transistors used: 2n3904, 2n3906 (~0.05€ each)

Power Transistors used: BD138, BD139 (~0.30€ each)

Diodes: 1n4148 (~0.02€ each).
I might replace them later, because i've found that my batch of diodes has a quite high forward voltage compared to the vbe of my transistors (0.71V vs 0.65V @I_f=7mA). Ideally, it would be the same or a little lower. The circuit can tolerate this +60mV difference, but it is just at the very edge and leads to lower thermal stability; going below 0mV would be much less problematic and leads to increased stability.

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