a. Transistor Structure and Action
A transistor is made up of three doped regions forging two p-n junctions between them.
The two types of transistors are:
(i) n-p-n transistor: In this type of transistor, a segment of a p-type semiconductor (base) divides two segments of n-type semiconductor (emitter and collector).
(ii) p-n-p transistor: In this type of transistor, a segment of n-type (base) divides two segments of p-type semiconductor (emitter and collector).
The three segments of a transistor have varying levels of thickness and doping.
- Emitter: This segment is of moderate size and heavily doped, and is responsible for the supply of majority carriers required for the flow of current in a transistor.
- Base: This segment is the middle segment, and is very thin and lightly doped.
- Collector: This segment collects the majority carriers supplied by the emitter. The collector segment is moderately doped and bigger than the emitter.
b. Basic Transistor Circuit Configuration and Characteristics
A transistor has only three terminals: Emitter (E), Base (B) and Collector (C). The input and output connections in a transistor circuit are such that at least one of these terminals is common to both the input and the output. The transistor can be connected in any of the three configurations, i.e. Common Emitter (CE), Common Base (CB), Common Collector (CC).
c. Transistor as a Device
The configuration of a transistor, the biasing of the E-B and B-C junction, and the operation region are used to design a transistor in such a way that it can be used as a device. A transistor operating in the cutoff or saturation state behaves as a switch. A transistor operating in the active region is used as an amplifier.
d. Transistor as an Amplifier (CE Configuration)
For a transistor to be used as an amplifier, its operating point should be somewhere in the middle of its active region. If the value of VBB is fixed in relation to a point in the middle of the linear part of the transfer curve, then the dc base current IB and the collector current IC will be constant. The dc voltage VCE = VCC - ICRC also remains constant. The operating values of VCE and IB determine the operating point of the amplifier.
e. Feedback Amplifier and Transistor Oscillator
In an amplifier, a sinusoidal signal is given at the input which is then amplified at the output. An external input is required to deliver the ac signal in the output. Whereas for an oscillator, the ac output is obtained without any external input, meaning that the output for an oscillator is self-sustained. This is done using an amplifier. Part of the output power is sent back as feedback to the input in phase with the initial power. This process is called positive feedback. The feedback is obtained using inductive coupling networks. Different oscillators use different combinations of coupling the output to the input.