# audio power amplifier (output stage) class AB design?

i have been assigned to design an AB class audio power (for the final project of a basic course in electronics)amplifier using Opamp : 741 , power transistors BD137,138 , a 12 v. supply , load resistance 8 ohm , voltage gain of 40 dB and cut-off frequency 20 Hz. desperately needing help ! where shall i begin?!

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• Anonymous

This shows a low power amp just using the 741

The ohmage is way off

You'd have to give this some thought because the power transisters will be fed by the op amp and drive the 8 Ohm speaker load.

No quick answer. EE is NOT a self taught discipline (unless you are independently wealthy and/or have all the free time you need).

I would suggest reading ch. 5 in the book, "Analysis and Design of Analog Integrated Circuits" by Gray, Paul, et al

ISBN 0-471-32168-0

My \$0.02

• 4 years ago

40 dB is a gain of 100 They have made it very hard for you ....an 741 opamp needs a plus and minus supply ...if you have to create a pseudo ov (at six volts) it gets tricky pin 7 goes to plus volts pin 4 goes to negative volts pin 2 (-ve) and 3(+ve) should be halfway between plus and minus pin 6 is the output get two 12 volt supplies connect as +12 0V and -12v add a 10k resistor from pin 3 to 0v and a 1uF from the junction of pin 3 and the 10k resistor and use the free end of the 1 uF as the input gain connect a 100k from pin 6 to pin 2 connect a 10k from pin 2 to 0v gain = 1+100k/10k cut off = 1/(2Pi C R) = 1/(6x 1e-6x 10e3) this gives a 3 dB point at 17Hz that's the basic amplifier (with a gain of 100) you have to add the power output stage to pin 6

Try starting out with application notes for 741 from different manufacturers. This site may help with contacts...good luck.

• ZORCH
Lv 6

Your first step is to understand the meaning of class AB. As different than A or B or AB2 or C or D. Much will be clearer after that.