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I have to design an audio amplifier to drive 500 mW 8 ohm load. I used two SMD transistors (BCP54 NPN and BCP51 PNP) and three BC847Bs, one to drive the BCP transistors, one for voltage gain, and one for current gain.

enter image description here

I calculated that there must be 2.8 V and -2.8 V swings on both transistors to achieve maximum 500 mW power. V=sqroot(PR)=sqroot(500mW8)*sqroot(2) = 2.8 V

I decided that at one peak there will need to be 9V on R3 transistor, on another peak there will need to be 3.4 V on R3 transistor. According to that I calculated values for resistors of Q3, Q4 and Q5 transistors. I want input impedance to be 50 kohm and voltage gain of 100. The problem is that it doesn't work. What have I done wrong?

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    \$\begingroup\$ Maybe you should define what "it does not work" means. What does it do then if it does not do what you expect? Also related, how do you think Q3 can work without proper DC bias on base? \$\endgroup\$
    – Justme
    Commented Mar 14, 2022 at 1:14
  • \$\begingroup\$ There are just weird signals of few nV if I monitor 8 ohm load with oscilosope. Q3 is supposed to get bias by Q4 \$\endgroup\$ Commented Mar 14, 2022 at 1:19
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    \$\begingroup\$ your schematic does not show an 8 ohm load ... in fact, it shows no output connection \$\endgroup\$
    – jsotola
    Commented Mar 14, 2022 at 1:47
  • \$\begingroup\$ How does Q3 get biased from Q4 when you put a capacitor between them? "Biasing" typically requires a DC voltage, which the capacitor blocks... \$\endgroup\$
    – brhans
    Commented Mar 14, 2022 at 1:57
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    \$\begingroup\$ We are not here to do your homework for you. You need to show us that you have made a substantial effort to solve this yourself. \$\endgroup\$ Commented Mar 14, 2022 at 2:09

2 Answers 2

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Two obvious things:-

  1. The load is drawing DC current from the output. You need to connect the speaker through a capacitor.

  2. You have two voltage gain stages with local feedback producing a gain of 'about' 100, but no DC stabilization. The bias point will be hard to adjust correctly and drift horribly.

Another problem is you have no overall negative feedback, so the amplifier will probably have high distortion. To fix this you can apply negative feedback with a resistor from the output to the Emitter of Q4.

If you make Q4 a PNP type then you can apply 100% DC feedback for excellent DC stability, then bypass the Emitter through a resistor and capacitor to ground to set the AC gain.

The circuit looks like this:-

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Any reason you didn't bootstrap R8 to turn it into a constant current source? \$\endgroup\$
    – jonk
    Commented Mar 15, 2022 at 21:56
  • \$\begingroup\$ @jonk I just made the minimum changes necessary to get it working. Didn't want to complicate the circuit - it took long enough just to draw that one! (Internet has been very bad here over the last few days - don't know why.) He only wants 0.5W output into 8 ohms, which shouldn't need bootstrapping on a 12V supply. \$\endgroup\$ Commented Mar 16, 2022 at 2:15
  • \$\begingroup\$ It doesn't add any parts. It just uses the output capacitor in a slightly different topology by repositioning the speaker connections. (I'd do it just because it allows me to put more of the circuit under management.) I was just curious, is all. Thanks. \$\endgroup\$
    – jonk
    Commented Mar 16, 2022 at 2:31
  • \$\begingroup\$ @jonk to do it with no more parts you have to connect the speaker to +ve supply instead of ground. Otherwise you need 2 more parts like this:- electronics.stackexchange.com/questions/539546/… Bootstrapping is a good idea, but if I put that in the circuit I would have to explain how it worked. \$\endgroup\$ Commented Mar 16, 2022 at 21:23
  • \$\begingroup\$ Yeah. I'd tie the speaker high. Just walk around the loop to see a constant voltage (nearly) across R8. It's not that horrible to explain. :) \$\endgroup\$
    – jonk
    Commented Mar 17, 2022 at 2:45
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For a good design, choose a proved one.

Make a dynamic DC analysis to fix the quiet point.
Make a transient analysis to see if Qp is ok, with signal i.e. 1 kHz, some mV, output with no "distortion".
Then, if all is "good", make a AC analysis, to fix bandwidth (capacitors).
In fine, check distortion (harmonic analysis).

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