Measurements and Fault-finding on Valve and Transistor Amplifiers

The capability of an amplifier is usually expressed in terms of its voltage gain — that is, the ratio of the output to the input signal amplitude — or its power gain — that is, the ratio of the output to the input signal power. For such tests all gain controls will be set at maximum and a complete specification will show the gain against a range of frequencies (see section 4.1). For tests to establish the correct working of the amplifier such detail is rarely required, a test at a mid-band frequency usually being sufficient to provide such information. Such a test is known as a dynamic test and is useful in the case of an amplifier containing several stages as a quick method of locating the faulty stage should the amplifier not be functioning correctly. Dynamic tests to locate a faulty stage are best illustrated by considering the block diagram of figure 11.1, which shows an amplifier consisting of four stages of amplification, each of which may be either a valve or a transistor amplifier. If the application of a signal at the required level to the input terminals does not produce the expected output then the system contains a fault. The simplest method of detecting the faulty stage is to apply a signal from a suitable source to the input terminals of block C. A satisfactory output signal indicates that stages C and D are satisfactory and that the fault must lie in either A or B. Using this type of test the faulty stage may be quickly located, and current, voltage and resistance measurements may then be used to locate the faulty component, such measurements being known as static tests.