be as little as 10 ohms falling to an ohm or less as powers rise above a watt or SO. Accordingly not all types of LC matching networks are suitable for these circumstances. Also in some circumstances a mismatch is introduced into the design to control the power distribution in the amplifier chain and to aid stability. The Q of the tuned circuits is another important factor. This is the 'quality' factor of the circuit and is determined by taking the bandwidth and dividing it by the resonant frequency of the circuit. While a high Q is desirable to reduce the level of spurious signals, many transistor designs use low Q circuits to reduce the possibility of instability. Most solid stage amplifiers use matching networks with loaded Q levels of five or less. Those amplifiers that use valve or tube technology, and a few are still in use, can use higher levels of Q, typically between 10 and 15 because of the higher grid circuit impedances. To indicate the type of circuits that may be used, the circuits of typical amplifiers using discrete components are shown in Figure 6.6. In this, first a transformer is used to give the required impedance transformation whereas in the second a capacitive divider is used. It can be seen that there is a high level of decoupling provided on the supply to prevent high levels of RF energy appearing on the common supply used by all circuits in the transmitter. Many of the spurious signals emanating from an RF amplifier are harmonics of the wanted signal, especially when the amplifier is run in class C. To remove them the final amplifier often uses a low-pass filter after any matching circuitry. The general format for these filters is shown in Figure 6.7. It can be seen that the filter consists of series inductors with Transmitters 189 (a) Transformer coupling (b) Capacitor coupling Figure 6.6 Typical methods used to couple RF amplifiers in a transmitter 190 Newnes Guide to Radio and Communications Technology -3 dB Cutoff frequency Figure 6.7 An example of a low-pass filter with a typical fre