Any study on RF Linear amplifiers or any new data on it always attracts a genuine home brewer. The very first thing to do for a good linear amplifier is feeding it with a pure and stable signal from the oscillator. On frequencies above 7 MHz pre-mixing method is good to increase signal stability. Pre-mixing is generating an oscillator signal by mixing two oscillator signals of higher and lower frequencies and taking the sum from the output after filtering it with a bandpass filter. The present trend of building linear amplifiers with focus on higher gain at lowest possible expense and thus sparing much time and money on getting things straight should necessarily change.
It also is not compulsory that a transmitter needs more than one stage. In C-24/1 we see a 4W CW single transistor transmitter and in C-24/2 there is a 10W CW valve transmitter. Both circuits work with active components and accessories closer to the values shown in the pictures.
If separate stages are used for oscillation and loading this problem can be solved. In C-24/3 such a two stage transmitter circuit is given.
Which is better for RF linear amplifier stages - valve or transistor? On the one side we have heavy expensive high voltage circuits and bulky valves and on the other side there are highly sensitive very intolerant high cost transistors. That discipline which transistors show up at lower frequencies cannot at all be expected at higher frequencies. Because the gain of a transistor increases by 6 db at every octave as the frequency decreases, possibility for low frequency oscillation is very high. Degenerative feed back circuits that checks stage gain proportionally to decreasing operating frequency is required here.
The next serious problem that transistor amplifiers face are issues related to harmonic generation. It demands harmonic filters and decoupling networks after every stage. The purpose of the capacitors found in between the collector and ground are intended to remove VHF/UHF factors in the signal. Capacitors with ten times higher Xc than the collector impedance are recommended here. Parasitic suppressor circuits also are used lavishly. The resistor coil parallel circuits used in series to the valve plates of are called parasitic suppressor chokes. Here the resistor used should be a non-inductive component with around 10 W rating. This could even be a coil wound on a resistor.
Parasitic suppression is possible with low value resistors (10-22 Ohms) connected in series with the base and collector of transistors of 1 Watt or below rating. This is the purpose of ferrite beads used at collector base leads of transistors.
Eliminating positive feed back is handled with neutralisation in which a strength controlled signal of opposite phase is fed back to the input, just like in an AGC. Similar degenerative feed backs are necessary in high beta (β) transistors circuits. Between valve stages Hi-Q circuits are possible because the output-input impedances are high. It gives higher harmonic rejection and selectivity. Just opposite is the case of transistors. There, the degenerative effect created by the low value resistor (un bypassed emitter) connected in series to the emitter protects the transistor from thermal runaway and also helps to increase the signal stability.
It is a general truth that maximum power transfer is possible between stages only when he input and output of the linked stages match perfectly. There are people who create deliberate mismatch in the hope of increasing signal stability. This action seriously affects every transistor used in the stage. If VSWR is higher than 1:2 damage to the final transistor is very possible. Generally a matching circuit and and an SWR protection are used in the output. These all are some of the reasons for suggesting valves at power outputs beyond 100 Watts. Hybrid circuits with transistors upto driver stage and valves in the output stages are common. Commercial equipment FT101 is an example.
It is always better to spare linear amplifier stages for frequency multiplication. If the class of operation is C and if the signal is CW this can be considered; that's it. Another point is the heatsink used for transistors. Coolers itself are necessary for high power transistors. Precisely speaking transistors are better at low power applications. In C-24/4 the circuit for an 8 W 80 meter transmitter is given.
It is always better to spare linear amplifier stages for frequency multiplication. If the class of operation is C and if the signal is CW this can be considered; that's it. Another point is the heatsink used for transistors. Coolers itself are necessary for high power transistors. Precisely speaking transistors are better at low power applications. In C-24/4 the circuit for an 8 W 80 meter transmitter is given.
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