IAN LANG ELECTRONICS
No2. Class C Amplifiers/ Harmonic Generators
In this stage we amplify or make stronger our generated carrier wave. We may also change frequency.
The base idea behind a class C amplifier is that the transistor or, as is more likely in this case valve, is biased beyond the cut-off point and a rough pulse turns it on and off. They do not give a linear response if left untuned, and we cannot pass a signal through them without it becoming distorted. They do however possess two interesting qualities which make them useful for RF communications technologies.
Firstly they are extremely efficient at turning DC into AC, some 90% efficiency is easily achievable.
For our second advantage we need to remember that we are not yet modulating the carrier and will not be until it has passed through the class C chain. If we now put a tuned load on the circuit, we can restore the distorted carrier to a sinusoidal form. If we make the tuned load resonate at some determined frequency, we can change the rate of oscillation. The circuit below uses an NPN bipolar junction transistor, but it could just as easily be a valve if you added a supply for the heater :
The pulses at the gate cause pulse currents at the collector and the resulting collector voltage becomes a sine wave due to the action of the tank circuit. When the tank is tuned to the required harmonic of the input it acts as a filter, rejecting all other frequencies. Chaining several of these together can create very high frequencies. Buffer amplifiers (where gain is unity, impedance in is high and out is low) are usually put between the class c amplifiers to prevent loading. The tank circuit is built to high specs to prevent instability. An LC circuit will not have the tolerance of a crystal oscillator but the overall stability comes from the original crystal circuit.
On this frequency doubler the tank (LC circuit at the collector) Should be tuned to a harmonic of the input frequency. A harmonic is an integral multiple of the base frequency, such as 2, 4, 6 or 6, 12, 18 etc. In the former case, 4 would be the second harmonic of 2, 6 the third and so on. It is recommended to start with the second, as the third would be weaker, and once amplified by the stage, use the third in the next. In this way for a 10 MHz oscillator we can go 20, 60, 180 MHz and that takes us into the lower end of Band III. Of course for a 1 MHz signal, we could tune the tank to 1 MHz and get the same frequency, but more powerful carrier.