This circuit, from a publication in the 1970s, uses transistors that no longer exist, but can be implemented with more modern equivalent types. Thus, for Q1 and Q2 we can use BC558 and for Q3 a BD136. We can also modify this circuit to operate with NPN transistors, reversing the polarity of the supply. Rx depends on the intensity of the input signal, with values between 0 and 1k. The circuit operates at frequencies up to a few tens of kilohertz.
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