f an amplifier. One of these is the mode of its operation. Those that operate in a totally linear mode are less efficient than those that run in a less linear mode. As a result amplifiers are classified in terms of their mode of operation. Normal linear amplifiers are said to operate in class A. They are biased to run in the linear region of the amplifying device whether it is a bipolar transistor, FET or even a valve (tube). They are less efficient than other types of amplifier because they conduct current for the whole of the input signal cycle as shown in Figure 6.2. The maximum possible efficiency for a class A amplifier is 50 per cent, with values between 25 and 45 per cent being more normal. Efficiency can be improved by biasing the amplifier to conduct for only part of the cycle. For what is termed class B operation the amplifier conducts over only half the cycle as shown in Figure 6.3. In this case the amplifier is biased at the cutoff point of the device. If an amplifier of this type is operated into a resistive load it effectively rectifies the signal, Output Voltage current at this point Input Current Figure 6.2 Class A operation 184 Newnes Guide to Radio and Communications Technology Output Voltage current at this point Point at which Input transistor turns off 0.7 V Current Conduction for half the waveform Figure 6.3 Class B operation passing only one half of the cycle. Sometimes this type of circuit even may be used as a simple detector. Another application is to use two class B amplifiers operating over opposite halves of the cycle. By summing the outputs both halves of the cycle are present, and a linear representation of the signal is produced. This type of operation is called push-pull, because each half of the amplifier operates on a different half of the cycle. For RF applications a single ended class B amplifier may be used in conjunction with a tuned circuit. In operation the tuned circuit 'rings' when the half wave signal is applied, and this supplies the missing half of the cycle. Amplifie