Transistor RF power amplifiers differ significantly in circuit design from valve amplifiers. This arises mainly because of the much lower input and output impedances of the transistor compared with the thermionic valve. These impedances decrease as the power level increases.
It must be remembered that transistors have very little thermal overload capacity and so the standard precautions for their use are particularly important in high-power amplifiers, ie close attention must be paid to the dc-rating factor and heat sinking. Considerable dc-rating may be advisable if amplitude modulation is to be used, and therefore SSB or NBFM is to be preferred.
Layout and bypassing of the collector supply must be carefully considered to avoid the creation of instability and hence probably interference (see Chapter 8). The antenna must be accurately matched to the transmitter output as transistor output stages are very sensitive in this respect.
Adjustment and operation are generally more critical.