IF amplifier for 455kHz with FET input
This IF amplifier uses a junction field effect transistor (JFET) in the input, otherwise bipolar junction transistors (BJT = bipolar junction transistor) are used. Due to the high-impedance input of the FET, the input-side resonant circuit is hardly damped. The coupling winding may be connected via a capacitor (e.g., 150pF) to the coupling winding of the preceding converter. In this way, e.g., this circuit can be connected just behind the previously described IF converter for 10.7 MHz / 455 kHz, as to obtain a IF unit for a dual conversion superhet. The value of the capacitor affects the coupling and thus the bandwidth. The output-side bandpass filter also consists of two single resonant circuits, which however are operated with capacitive high-point coupling. Overall, similar selection properties result in this way, as in a tube super with single-stage IF amplifier. If these are not sufficient, a ceramic filter can be inserted at the input.
At the output labeled "ZF", e.g. an FM or SSB demodulator can be connected. The actually the AGC voltage generation serving diode can also be used for AM demodulation. The demodulated signal is applied to the port labeled "NF". In addition, the voltage for an S meter can be tapped at its cathode. The DC amp for the AGC works with the left transistor BC237B. With increasing signal voltage its conductivity increases between emitter and collector. The associated current flow causes an increasing voltage drop across the drain resistor and thus on the one hand a reduction in the drain voltage of the JFET, whereby its gain decreases. On the other hand, the output signal of the JFET stage is more and more short-circuited from the collector to the ground, so that the control operates in two ways. In this way a control efficiency can be achieved, as it would otherwise be achievable by simple bias changes only over several stages. In contrast, with this circuit, the regulation of one IF stage for a complete superhet receiver is already completely sufficient. The second IF stage, formed with the right BC237B and a BC307B, provides high voltage gain and low enough output impedance to feed the signal to the left resonant circuit via its coupling winding. Despite the comparatively low effort, the circuit shows excellent overall properties. Selectivity and gain control are significantly better, as in the typical two-stage arrangement found in many transistorized broadcast and CB radios. In that case were two transistor stages in grounded emitter circuit usual, coupled at the input side, between the stages, as well as between the output and the demodulator via each one of the IF single resonant circuits used here (yellow, white, black).