I made this circuit and the output signal wasn't really what I was expecting to be.


simulate this circuit – Schematic created using CircuitLab

This is class A amplifier (both Q1 and Q2 conducting both negative and positive half cycles). I was only curious how the signal from V1 is buffered to the output and I am already (at this point) insecure about its operation.

Here is the measurement of a signal I scoped:

enter image description here

As can be seen, there is a "double-amplitude" signal at the point where I probed the circuit.

  • Is this the consequence of Q1 and Q2 constantly conducting the signal?
  • \$\begingroup\$ You should consider replacing R1 and R2 with diodes, also the 47k ohm resistors seem to be quite on the high side, you dont mention what your load will be, if you are planning on using low ohm loads, then I would definitely use smaller resistors for R3 and R4. Also, you are not using any feedback, so distortion of the output waveform is expected. \$\endgroup\$ – S.s. Dec 23 '17 at 23:04
  • \$\begingroup\$ I expect that only one transistor is conducting at any time, as you have shorted their bases together. \$\endgroup\$ – Peter Bennett Dec 23 '17 at 23:11
  • 1
    \$\begingroup\$ The circuit is incorrect, you have Q1 and Q2 bases shorted. @PeterBennett Adding diodes won't help as this makes it a Class AB amplifier \$\endgroup\$ – Jack Creasey Dec 23 '17 at 23:24
  • \$\begingroup\$ @JackCreasey Ah thank you for spoting that mistake. \$\endgroup\$ – Keno Dec 23 '17 at 23:56
  • \$\begingroup\$ Looks like 60Hz mains hum. Increase the scope timebase until the 'double amplitude' signal becomes a single waveform. Do you see the signal waving up and down at a lower frequency? \$\endgroup\$ – Bruce Abbott Dec 24 '17 at 0:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.