3.7 Receiver with the HF Amplifier
In HF amplifier the signal coming from the radio station is being amplified in its original form. In our case, this means that AM signal is led at input of the HF amplifier, and on its output the same shaped signal is obtained, only with bigger amplitude. This device got its name because it is used to amplify HF signals, although more precise term for it is the Selective Voltage Amplifier (that's how it is called in professional books).
Electrical diagram of a direct receiver consisting of HF amplifier, diode detector and LF (audio) amplifier is presented on Pic.3.24. This receiver does not have a selective input circuit. It would be better that way, the selectivity of the receiver would be better, but "technical reasons" made us not to include it: the double variable capacitor (like the one in the first issue of PE) i.e. the one consisting of two equal variable capacitors connected onto the common shaft, is very hard to find these days. The active element of the HF amplifier from Pic.3.24 is the transistor BC557 that operates in the common base junction. The station signal that is being amplified is led onto the emitter (i.e. between the emitter and ground), and the amplified signal is taken from the collector (i.e. between it and ground). Working principle is similar to the one of the input circuit explained with the Pic. 3.1-b. To refresh your memory: we have been considering an example where four signals of equal amplitudes but different frequencies, were present in the antenna: fs1, fs2, fs3 and fs4. They were causing four different currents to flow through the LC oscillatory circuit: Is1, Is2, Is3, and Is4. All these currents were creating some voltage on the ends of the LC circuit, but the one caused by the current Is2 was significantly (about 20 times) bigger, due to the oscillatory circuit being set to its frequency. The parallel oscillatory circuit that is on Pic.3.24 as the collector .....is exactly the same as the the one on pic.3.1.
It also has the same role, therefore HF amplifier from pic.3.24 having the same selectivity as the input circuit in all the receivers described so far, with addition of extra amplification. This is being accomplished in the following manner: under the simultaneous load of 4 voltages that are coming to emitter from the antenna, their frequencies being fS1, fS2, fS3 and fS4, four currents flow simultaneously through the transistor. They share the same circuit: from positive battery pole, through P1, then transistor (in direction emitter-collector), over the LC circuit to the minus battery pole. All of them therefore flow simultaneously through the LC circuit as well. The resonance frequency of this circuit is set (by C) to be equal to the frequency of one of the currents and it acts upon it as a huge resistor (200 kOhms, as on pic.3.2-b). According to Ohm’s Law, this current creates voltage on the oscillatory circuit. For other 3 currents the circuit acts as a resistor with much smaller resistance (less than 20 kOhms, as on pic.3.2-b) and they create much smaller voltage in the circuitry (10x smaller, as in our example). The important difference in operation of circuits from pics. 3.1 and 3.24 is that all currents are much smaller in the latter case (because of the amplifying effect of transistor), therefore the voltages on the LC circuit being much bigger.
* With P1 potentiometer the signal amplitude from antenna to the input of HF amplifier is regulated. If, on your device, you find the slider for all stations to be in rightmost position, put a resistor instead of potentiometer, and connect the antenna with emitter.
* As with all input circuits, when connecting the capacitor C care should be taken to connect the rotor to the ground (G-point on pic.3.7-a).
* The R3 resistor comprises with C2 and C3 capacitors the LF filter which prevents the feedback (that would lead to unstable operation) between the LF circuitry and HF amplifier. If the feedback still occurs, the R3 resistance should be increased.
* In LF part of the receiver the audio amplifier with LM386 IC is used. That is by no means necessary, any audio receiver will do.
* There is also a better variation of HF amplifier, with increased selectivity. Its electronic diagram is shown on pic.3.29-b.