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Introduction
Before the invention of the transistor in 1948, vacuum tubes were used to switch electric signals and power. These tubes were also called valves. In vacuum tubes, the supply of electrons is from the heating of the cathode and the flow of electrons is by varying the voltage between its different electrodes.
There are some drawbacks of vacuum tubes—electrons flow only in one direction, from the cathode to the anode. This kind of vacuum tube consumes high power, has a limited life, is very bulky, and operates at a high voltage.
This solid-state semiconductor device was invented in 1930 to overcome the problem of external heating for the flow of current. Semiconductors are small, consume low power, operate at low voltage, have a long life, and have high reliability.
Transistors were invented in 1947 by J. Bardeen and W. H. Brattain. Their transistor was a point-contact transistor. The first junction transistor, the p-n junction, was invented by William Shockley in 1951. Over time, a new transistor was invented called the Bipolar Junction Transistor or BJT.
A transistor is a semiconductor device having three doped regions—emitter, collector, and base. They form a p-n junction between them. They are of two types.
- n-p-n transistor: There are two segments of n-type semiconductor—emitter and collector—separated by a segment of p-type semiconductor or the base.
- p-n-p transistor: There are two segments of p-type semiconductor—emitter and collector—separated by an n-type of semiconductor known as a base.
Semiconductor
It is a solid device that has conductivity between conductor and non-conductor and also between non-conductor and insulators.
Semiconductors are of pure elements.
Semiconductors are of two types:
- Elemental semiconductors: silicon and germanium
- Compound semiconductors: CdS, GaAs, InP ETC.
Now let’s discuss these three terms: emitter, base, collector.
Emitter: This is present on one side of the transistor. A large number of majority carriers are supplied by the emitter for the current to flow through the transistor. It is of modern size and heavily doped.
Base: This is the central part of a segment. It is thin and lightly doped.
Collector: It is present on either side of the transistor. A major portion of the majority carrier is collected by this segment. It is large and moderately doped.
Functions of a transistor
Basically, the transistor has two different functions. This means it acts in two distinct ways. The charge carrier moves across different regions of the transistor when the proper voltage is applied to its terminals.
- The transistor was invented to function as an amplifier, which produces an enlarged copy of signals from devices.
- It also acts as a switch.
Transistor characteristics and their applications
The transistor can be connected in three ways:
Common emitter (CE), common base (CB), Common collector (CC).
The transistor is most widely used in the CE configuration.
In CE configuration, there are two types of characteristics:
Input characteristics: Input is between the base and the emitter. The variation between base current Ib and the base-emitter voltage Vbe is called input characteristic.
Output characteristics: Output is between the collector and the emitter. The variation between collector current Ic and the collector-emitter voltage Vce is called input characteristic.
Output characteristics are controlled by input characteristics. The collector current changes by the base current.
Plot a graph to understand the input characteristics of the transistor in the CE configuration.
The graph is between the current base and base-emitter voltage.
For the dependence of Ib on Vbe, the common-emitter voltage Vce should be kept fixed. For obtaining the input characteristic, the transistor should be in an inactive state, and the collector-emitter voltage Vce should be kept large enough so that the base-collector junction is in reverse biased. The reverse bias across the base-collector junction is high to operate the transistor as an amplifier. So, an increase in common emitter voltage appears as an increase in common base voltage, and also its effect on base current (Ib) is negligible.
Transistor as an amplifier
When transistors are biased in such a way, they are said to be in an active region. An active region should be V0 and Vi. The rate of change of output with the input is represented by the slope of the linear part of the curve. Output is Vcc– IcRc, not IcRc, and because of that, it is negative, and as the input voltage of the CE amplifier increases, the output voltage decreases. So, in this condition, the output is out of phase with the input.
Small changes in ΔV0 and ΔVi in the output and the input voltage, then ΔV0 / ΔVi is said to be a signal voltage gain Av of the amplifier.
The circuit will behave as a CE amplifier with the voltage gain ΔV0 / ΔVi when Vbb voltage has a fixed value corresponding to the midpoint of the active region. In terms of the resistor in a circuit, we can express voltage gain Av.
We have V0 = VCC
But delta VBE is negligibly small compared to delta IBR B in this circuit. So, the voltage gain in this CE amplifier is given below.
Base current and the collector current would be constant when VBB has a fixed value that corresponds to the middle point of the linear part of the transfer curve. VCE= VCC -I CR C would be constant.
If Vi = 0 then,
Vcc = VCE + IC R L
Transistor as an oscillator
It is an electronic device that produces a periodic, oscillating signal, often a square wave and sine wave. It converts direct current to an alternating current from a power supply. It also produces a continuously alternating wavefront without any input.
The basic parts of the oscillator circuit are the amplifier circuit and the LC network.
The amplifier circuit is used as a transistor, whereas the LC network is of inductor and capacitor, and this LC circuit is used as feedback to feed a part of the output signal back to the input section. It is a very important and necessary condition that feedback should be positive, and the feedback factor must be a unity that is BA= 1. Charging of frequency is done by changing L and C.
It is a device in which DC energy is converted into AC energy at a very high frequency. In this device, only a battery is used for the biasing of the transistor, and no external signal is applied. It produces a continuous and repeated wavefront without any input. They convert a unidirectional flow of current from the DC source into an alternating wavefront.
The objective of an oscillator
An oscillator should be generated at very low sub-audible frequencies.
Conclusion
The amplifier is used in communication at long distances because of its high intensity obtained by the output. It is also used in radio signals. The n-p-n and p-n-p junctions are best for use as an amplifier. There are three types of transistors or amplifiers—common emitter, common base, common collector. It acts as an amplifier only when it is an inactive region of a linear circuit. It is also used for generating electric signals, amplifying purposes, and controlling signals. In simple words, the amplifier is a type of device which amplifies the power or voltage of signals.
