Can anyone explain how it works? What is the role of the diode in the circuit?

enter image description here


1 Answer 1


It looks like a common emitter amplifier using a PNP transistor so it has a negative power rail with respect to 0V. However there is a positive voltage biasing the base and this will reverse bias the base-emitter juntion turning the transistor off to a certain degree.

When an input signal is of sufficient peak-to-peak amplitude, part of the negative section of the input waveform will cause the base-emitter junction to be forward biased and the output will amplify this part of the input waveform and probably produce something akin to a pulse at the collector.

The diode prevents the base-emitter junction from becoming too reverse biased (either to prevent base-emitter reverse voltage breakdown or just to "shape" the input signal a little bit).

  • \$\begingroup\$ thank you, and one more question, if i want to have the same circuit but with NPN transistor , i should change polarities of sources and also change direction of the diode ? \$\endgroup\$ Commented Nov 1, 2015 at 9:51
  • \$\begingroup\$ @LevonHovsepyan - I have changed my answer considerably because of the bias on the base - you might want to rethink your request. \$\endgroup\$
    – Andy aka
    Commented Nov 1, 2015 at 9:53
  • \$\begingroup\$ @LevonHovsepyan Yes, that might work, but this amplifier has unconstrained gain, so it may nor behave as expected. especially if ti was originally designed for old germanium transistors. \$\endgroup\$ Commented Nov 1, 2015 at 10:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.