I am new to electronics and I am trying to build an audio amplifier with an input signal of 100mV @ 60 hz using 12V as VCC and expect an output of 70-75W power.

I have the following circuit: enter image description here

I copied this one from my notes and simulated it just to find out that it does not amplify. I believe there is something wrong with the biasing. What could be the problem?

  • \$\begingroup\$ You're in luck, you build something (luckily in a simulator so nothing can blow up :-) ) and it doesn't work. Now you can learn something ! Why do you believe the biasing is not OK ? (please motivate) have you checked it ? What should it be ? Are the (DC) currents and voltages what you expect them to be ? And what do you expect ? \$\endgroup\$ Jun 9, 2015 at 15:12
  • \$\begingroup\$ You're definitely not going to get 70-75W with a 1.225k load. \$\endgroup\$
    – Matt Young
    Jun 9, 2015 at 15:15
  • \$\begingroup\$ That is correct, now why would that be ? What would you need ? Questions, questions .... \$\endgroup\$ Jun 9, 2015 at 15:16
  • \$\begingroup\$ I thought because it has very small output power, somehow it might not be working on active mode. Thanks all \$\endgroup\$ Jun 9, 2015 at 15:23
  • \$\begingroup\$ If you're not willing to understand the how and why, then why would you try to build this thing. Why not get some module on eBay that you just hook-up and it works ? Saves you all this trouble. I mean something like this: ebay.nl/itm/… \$\endgroup\$ Jun 9, 2015 at 15:46

1 Answer 1


Its not the biasing - in fact there is no problem.
That circuit is not a voltage amplifier, its a current amplifier - the key feature being that both of your transistors are configured as emitter-followers (gain ~ 1).

What you have there is a common configuration for the output stage of an audio amplifier because it can drive lots of current into your speaker.

You need to add a voltage gain stage in front of it in order to amplify your 100mV signal into something that the output stage can use.
The easiest way is to cheat and use an opamp...


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