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Topic: Design an amplifier which can provide 1w to a speaker (Read 866 times) previous topic - next topic

Design an amplifier which can provide 1w to a speaker

Hello!

I need help designing an amplifier that is capable of providing 1 W to an 8 Ohm speaker. I have 3.3 V, 5 V or 18 V supply available to power it.

I also have only one power supply available that has no negative output.

The signal is coming from an AVR as a 3.3 V PWM signal. I have following components available:

NPN, PNP transistors,
N-channel MOSFETs,
TL494 Pulse-Width-Modulation Control Circuit,
JRC4558, LM358 op amps,
NJM13700 TRANSCONDUCTANCE AMPLIFIER

and other standard equipment
The last three ICs I got from an old 500W amplifier (that's why I posted them here since they are useful).

I already experimented with push-pull, common emitter but just couldn't get it to work (not enough power and distortions).
Thanks to Olin Lathrop I designed a bridged class D amplifier, but its not working as it should. First off, here is the schematic:


simulate this circuit – Schematic created using CircuitLab

NOT, AND, NAND gates are all made from NAND logic gates.

NOT1 inverts the PWM signal to drive the other half of H-bridge, combined with AND3, and D6 (which is output pin on MCU), they drive the other half of the H-bridge only when D6 is high, preventing it to be on, when PWM output is low.

NAND1 prevents both halves of the H-bridge to be on at the same time (at least thats what I though it would do).

NAND1, AND1 drives one half of the H-bridge, when the PWM signal is high.

NAND1, AND2 drives second half of the H-bridge, when the PWM signal is low.

The purpose of these logic gates is also, to convert 3.3 V PWM signal to 5 V.

PROBLEM:

If I connect only one half of the H-bridge I get expected distorted sound output. Now, when I connect the second half, it makes a really bad noise and PWM signal stops.

What I think happens:

When I did some measurements with osciloscope I expected that when I would add(+) both waveforms, from AND1 and AND2, I would get a flat +5 V line, but in reality there are some spikes reaching to 0 V and 10 V on both rising and falling edges of the waveforms. So I suspect that, due to this, both of the halves of the H-bridge are on for a small amount of time, to short circuit whole thing to ground.

I am not an expert on this matter, so I would really appreciate any help from you.