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I am currently examining the "El Cheapo" amplifier and despite its apparent obviousness I'm still not sure about NPN/PNP interchangeability.

Is there any particular reason why there is PNP in the emitter follower and not NPN? I mean, can I just recalculate biasing and use NPN if I find it more suitable without any drawbacks?

Also, there are examples of single transistor input stage in some books, e.g. Douglas Self's that use PNP and I feel a bit unsecure because of it :P

I realise that AB amp requires NPN/PNP combination but other than that, I find NPN simplier to use, perhaps more intuitive - hence my doubts.

El Cheapo

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  • \$\begingroup\$ Given that both the input and output of Q1 are capacitatively coupled, I can't think of a good reason why you couldn't implement this amplifier stage with reversed polarity. I will watch this question to see if anyone else can think of a reason why it should be implemented in pnp rather than npn. \$\endgroup\$ Commented Nov 30, 2020 at 18:57
  • \$\begingroup\$ Just had a thought. What is \$V_{cc}\$? Could they get away with lower voltage capacitors by implementing the way they did? \$\endgroup\$ Commented Nov 30, 2020 at 18:59
  • \$\begingroup\$ PNP transistors tend to be slightly lower noise. This may be part of Doug Self's thinking; he was interested in audio quality. \$\endgroup\$ Commented Nov 30, 2020 at 20:02
  • \$\begingroup\$ @MathKeepsMeBusy - Exactly, it was - most likely - about cheaper caps. \$\endgroup\$ Commented Dec 1, 2020 at 9:22

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If the output from Q1 is biased close to ground then a PNP will drive effectively close to ground better than an NPN would. As drawn, the PNP has an effective impedance of 150k(R2) divided by the Hfe of Q1 which is likely to be 100 or so. If we turn the input circuit upside down and use R3 as a pull-down then the output impedance is 33k. Keeping the output of Q1 biased to a low voltage improves supply rejection and minimises the voltage on C2, which could reduce cost and possibly microphonic effects. So a PNP is preferable but an NPN could be used.

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