Chapter 23 - RF Power Amplifiers, low level AF amplifier

 

Already it was told that in SSB equipments it is at the source stages that output power control arrangements are set. Level control is done by doing signal attenuation and gain reduction of active stages in in linear amplifiers.  In outstanding power amplifier circuits an AGC also is seen. In every RF stages especially in solid state packs, degenerative feed back arrangement is necessary in every stage. Because transistors have considerable inter electrode capacitance inside, and corresponding regenerative feed back may end up in transistor failure, an external circuit that cancels this inner feed back is necessary. 

In some transistors a protection circuit is built inside in it. In valves an external degenerative feed back circuit can perfectly cancel the inter electrode feedback inside. 

Unwanted signal interference is possible in all linear amplifier stages. Wave Traps, similar to that we used in SSB Tx mixer output stages. Parallel tuned trap, series tuned traps and a combination of both - many types of traps depending upon the requirement are in use.  In fig; C-23/1 the details of both types of traps are given.  Whatever be the frequency at the input no unwanted signals cross the output.  In RF Amplifier stages, do not think that single ended stages are the only choice. Cascade amplifiers require no neutralisation circuits; they are also noted for low noise level and high stage gain. 

In fig. C-23/2 Valve/transistor cascade arrangement is shown. The cascade arrangement we discuss now and the stage arrangement in which more than one amplifier stages are set in series addd with tuned circuits intended for narrow band cascading are different. (cascade and cascade). 

If the tuned circuits at RF Amplifier outputs are tuned to the harmonics of the input signal, frequency multiplying is possible.  The power gain for the second harmonic will be around 60% when compared with the output power with a tuned circuit exactly done for the input signal.  At 3rd harmonic, output power will be reduced further proportionally. But a push-pull doubler circuit can cut this sort of losses too. An example is the ripple frequency in full wave rectifiers. 

Here, since the audio power need to be very high, low, medium level modulations are found to be common. In low power transmitters low level modulation works fine.  In C-23/3the audio amplifier circuit for a low level modulation circuit is given. In QRP circuits it is the power supply line that is modulated. In one sense it is plate modulation itself. 

The PCB layout for the circuit in C-23/4A is given in C-23/4B

In fig. C-23/5, we have given the block diagram of a simple AM QRP transmitter. We had already explained every stages of such a QRP through various chapters, except the final power amplifier stage. 

An effective transmitter not only requires a decently built QRP transmitter but also a well tuned Antenna that matches perfectly with the Power amplifier output. After our study on linear amplifiers are over we will turn to Antennas too. 

Chapter 22                                                                             Chapter 24