Junction transistors consist of either a thin layer of n-type material sandwiched between two p-type layers (pnp), or a thin layer of p-type material sandwiched between two n-type layers (npn). In each case the three layers are identified as emitter, base and collector.
In common-base’ connection, the current
gain .
The input impedance is low, and the output
impedance is high. It is often used for impedance matching.
In common-emitter connection, the current gain
and is in the region of 50—500. The input and output impedance is similar (medium value).
In common-collector connection (emitter
follower), the gain 
The input impedance is high and the output impedance is of medium value.
The field effect transistor (FET) possesses certain advantages; its temperature stability is better, and it has a high input impedance.
A bipolar transistor connected in the common-emitter mode can be used as a two-state device, i.e. it can be switched ON or OFF, depending upon the input conditions.
Integrated circuits (ICs) can replace discrete component circuits. Typical working area is a few square millimetres, and the absence of interconnecting wires increases the reliability. Digital ICs use two-state logic signals. Linear ICs deal with analogue signals.
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