Can anyone explain how it works? What is the role of the diode in the circuit?
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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).
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\$\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\$ Nov 1, 2015 at 9:51
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\$\begingroup\$ @LevonHovsepyan - I have changed my answer considerably because of the bias on the base - you might want to rethink your request. \$\endgroup\$– Andy akaNov 1, 2015 at 9:53
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\$\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\$ Nov 1, 2015 at 10:52