# Audio power amplifier design help

For a school project, I need to design an 8 W audio amplifier with BJTs. The power supply is from 9 V to -9 V and the input is 0.8 Vp 1 kHz. A speaker with 8-ohm impedance and a voltage gain of 10 is wanted. I tried using Class AB design but couldn't get over 700 mA of output current, the power supply makes things really hard. Do you have any tips on how can I design a working amplifier circuit?

I added the design I'm currently working on and the max output I can get. I try to increase the VOUT by increasing R6 but it gets clipped in the negative cycle.

• Well, we won't hand you the design. Show us the class AB design that you made. I think the best advice for "tips on how I can design" is just to study existing successful audio amplifier designs. There are many, many of them available online. Commented Jan 10, 2022 at 16:27
• We don't know what tips you already used. Post your design so we can tell what is wrong with it. Unless the problem is not the amp, but the power supply. Did you happen to use 9V batteries? Commented Jan 10, 2022 at 16:29
• the power supply makes things really hard Why? Commented Jan 10, 2022 at 16:30
• TIP#1 - An 18 volt supply rail could just about sustain a 16 volt p-p output waveform. Assuming that waveform is sinusoidal means an RMS value that is 16/2.82 volts = 5.66 V RMS. Into an 8 ohm speaker, that's a max power of 4 watts. This means you can't use a conventional push-pull class AB (or otherwise) output stage. Commented Jan 10, 2022 at 16:31
• Bah, what a spoiler Neil. Commented Jan 10, 2022 at 17:52

Your first problem is your choice of a Darlington follower for your output stage. The voltage across a Darlington cannot be less than two diode drops, which would amount to about 1.5 volts. So the output can never be greater than about 7.5 volts for a 9-volt supply - and this is an absolute limit. In your case your getting about 7 volts swing, which is a very reasonable number.

But this is not your big problem. That derives from

I need to design an 8W audio amplifier with BJTs. The power supply is from 9V to -9V and the input is 0.8Vp 1 kHz. A speaker with 8-ohm impedance and a voltage gain of 10 is wanted.

I'm afraid you haven't thought this through. Generally, an 8W audio amplifier means 8W RMS with a sinewave input such as 1 KHz. What will produce this? Well, obviously 8V RMS into an 8 ohm load. However, the peak voltage of a sine wave is approximately 1.414 times the RMS, which means you need a 12-volt supply, plus another 2 volts to account for your output stage limitations. Since this requires both a + and - excursion around ground, you need about a +/- 14 volt supply. That is, as long as you are trying to do it the way you are.

Pay close attention to Neil_UKs comment.

On the other hand, this implies that your required voltage gain is not 10, it's 14 and a bit. Let's say that the 8W output is 8W peak, which only requires a gain fo 10. This you can do with 9 volt supplies. The trick is not to use a Darlington configuration. Something like

simulate this circuit – Schematic created using CircuitLab

this will do. The final output transistors can be driven to produce a Vce voltage of about 0.1 to 0.2 volts, which gives you the 8 volts you need.

Of course, you can't do this open loop, or at least not reliably. You probably can tweak the base drive currents to give you what you want, but in real life it wouldn't work due to temperature instability.