Diode Detector Circuits

Author: J.B. Hoag

A two-electrode tube or diode can be used as a detector with much more satisfactory results than the crystal just described.

Fig. 17 C. A simple diode detector circuit

In the circuit of Fig. 17 C, the r.f. current is rectified by the diode D and flows through the load resistance R. Since (in the conventional sense) current can only flow from the plate to the filament of the tube, the voltage drop across R is + at the top and — at the bottom, as shown. This voltage varies in strength and frequency in the same way as the modulations of the modulated carrier wave or r.f. input. A typical value of R is 250,000 ohms. Condenser C must have a reactance for the given r.f. which is small compared with the resistance of R. If C is too large some of the a.f. will be lost. A typical value is 250 pF.

Fig. 17 D. A full-wave diode detector

In Fig. 17 D, full-wave rectification is used, the principle of operation being the same as described elsewhere. The audio-frequency voltages are taken out of the circuit through the condenser C₁ (= 0.1 μfd.) and the voltage divider or "volume control" R₁ (from 0.5 to 1 megohm).