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I'm designing a four transistor audio amplifier circuit.

enter image description here

I simulated it in LTspice. I replaced the BC547 with a TIP142 and the BC557 with a TIP147.

enter image description here

  1. Collector
  2. Base
  3. Emitter

When run mode V out is not amplified. enter image description here,

What is the problem here?

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    \$\begingroup\$ Are you really feeding a 12 V peak signal in that 9 V powered amplifier? \$\endgroup\$ Jun 17, 2022 at 9:11

1 Answer 1

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The BC547 and the BC557 are small signal transistors. The TIP142 and the TIP147 are Darlington power transistors.

The input signal level and the biasing for the small signal transistors won't work with the Darlington power transistors.

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  • \$\begingroup\$ What parameters in the datasheet can distinguish them? \$\endgroup\$
    – MrCR
    Jun 17, 2022 at 8:36
  • \$\begingroup\$ Start with "VBE (on)." 0.7V for the BC parts, about 3V for the TIP parts. \$\endgroup\$
    – JRE
    Jun 17, 2022 at 8:39
  • \$\begingroup\$ LTspice should have the BC parts. Try your circuit using the proper components first. See if that works (make sure you have drawn the circuit correctly.) Once the circuit works as designed, you can swap out the transistors to see what happens. \$\endgroup\$
    – JRE
    Jun 17, 2022 at 8:41
  • \$\begingroup\$ Thank you, i will try it now. \$\endgroup\$
    – MrCR
    Jun 17, 2022 at 8:45
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    \$\begingroup\$ @MrCR For better reading the schematic, just use the already available symbol for transistor, it will make your (and others') life easier. If it's not a .model then change QN or QP to X (use Ctrl+RClick). \$\endgroup\$ Jun 17, 2022 at 8:59

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