1W Push-Pull Power Amp Design

audioimpedancetransformer

I need a simple single supply amplifier that draws a lot of current when signal is applied. Without going into why, let's just say this is supposed to be part of a compressor. Distortion performance is not important. What is important is that it has a high ratio of current drawn in relation to the signal. So if the signal is low, the current drawn is low. If the signal is high, the current drawn is 100X as much.

So I have modeled and breadboarded the following circuit:

not so good push-pull

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This circuit works fine (with a 100pF cap across the PI outputs) but it doesn't draw nearly enough current. The transformer is a standard Radio Shack style output transformer (actually an Eagle LT700) with a 1K2 primary and 3R4 secondary.

Ultimately the problem is that I cannot pull more than 2mA through the power transistors. Can someone explain why?

My first guess is that the impedance relation between the phase inverter and power section stinks. But it's not crystal clear how to resolve that.

Ultimately I would like a current gain of 100:1. If I could make a 1W power amp out of this circuit, that would be ideal.

Any ideas?

Best Answer

Here's my thoughts. You want a power amplifier to drive a speaker right? And you also want to take progressively more current from its power supply as the signal gets bigger. Why don't you use a regular 1W amplifer IC and design, on the side, a small circuit that rectifies the peak signal amplitude and uses this to drive a current source that takes the required current from the PSU.

What does this solve? Firstly you can get 1W IC amplifiers from lots of places and they are not complex to build or get working. Secondly, a peak signal capture circuit (given that you can drive it from the amplifier output) is just a diode, a capacitor and a resistor. Thirdly, a circuit that takes current from the supply based on a demand signal (the output from the diode, capacitor and resistor) is easy too; it's an op-amp, a power transistor on a heatsink and a 1W, 1 ohm resistor.

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