Transistors are current amplifiers. We sometimes need a voltage amplifier – for example, to boost the small signal from a microphone to an acceptable level. Although an op.amp provides the ideal solution, a simple one or two-transistor amplifier can be very useful, particularly if the quality of the signal is not important, e.g. to allow a microphone to activate another circuit such as a sound-switch or an audio driven lightchaser unit. A single transistor voltage amplifier is shown in Fig The design of a quality transistor amplifier is extremely complex, so we will look at the principle of using this simple arrangement. A transistor will start to turn on when the voltage at its base, in relation to that at its emitter, is at around 0·5V (never an absolutely predictable figure). If the base voltage is increased, the transistor will turn-on more (in other words the resistance from collector to base will fall), until it is fully turned on, or saturated, when its base voltage is around 0·8V (again never an absolutely predictable figure).
Transistor as a voltage amplifier Circuit
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