Electronic – Amplifier design

amplifierpwm

I am having trouble calculating values for this audio amplifier. Input comes from AVR PWM operating at 44.1 kHz with high being at 3.3V. The speaker is 8 Ohm and speced for 1 Watt.

First two transistors, Q1 and Q2 provide high input impedance for PWM signal, while Q3 and Q4 form push-pull amplifier for the speaker. L1 and C1 form low pass filter, to convert PWM into analog signal.

First off, I am not enterely sure this is the best/correct approach, so if anyone has any better ideas let me know. The second thing is, I am not sure if I correctly calculated values for these resistors. These are the values I calculated earlier (R2, R3 = 47 Ohm, R1 = 330 Ohm, R6 = 470 Ohm, R5 = 1 kOhm, R4 = 470 Ohm).

I would really appreciate if with your anwser you could provide details and explanation of the calculations.

Also note, that AVR pin can source/sink maximum 20mA of current.

schematic

simulate this circuit – Schematic created using CircuitLab

Any help is greately appreciated!

Best Answer

There is a bit of redundancy in your design - both Q1 and Q2 do exactly the same thing - you can get rid of one of them and have R3 and R6 connected to the same point such as the collector of Q1.

The other main problem is that the largest peak-to-peak voltage you'd be able to get on the output is about 1.5 volts because the output transistors are wired as emitter followers in push-pull - to make either transistor conduct, folk tend to use the 0.7 V base-emitter rule - i.e. the base needs to be 0.7 volts greater than the emitter (NPN) and 0.7 volts less than the emitter (PNP).

Given the speaker is 8 ohms and the biggest sinewave from the amp (on a 3V3 supply) is 0.53 volts RMS, the power into the speaker is 35 mW (before things start to clip/distort).

I'd throw it away and get a chip that does this sort of thing - even a H bridge amplifier suitable for motors is going to be better and you'll get a lot more power.