ELECTRONICS

Q.1: What is a bipolar junction transistor? Why it is so named?
Ans: BJT: A BJT consist of two back to back P – N junctions manufactured in a single piece of a semiconductor crystal. These two junctions give rise to three regions called emitter, base and collector. Junction transistor is simply a sandwich of one type of semiconductor material between two layers of the other type. The two junctions are emitter- base (E / B) and collector – base (C / B) junction.
It is so named because –
i. It is smaller and light weight.
ii. It has no heater requirement or heater loss.
iii. It has rugged construction.
iv. It is more efficient since less power was absorbed by the device itself.
v. It is instantly available for use, requiring no warm – up period.
vi. Here lower operating voltages were possible.
Q.2: Describe the construction of a transistor.
Ans: Transistor construction: A transistor is a three layer semiconductor device consisting of either two n - type and one p - type layers of material. The former is called an n-p-n transistor while the later is called a p-n-p transistor. The emitter layer is heavily doped, the base layer is lightly doped and the collector layer is only lightly doped. The output layers have widths much greater than the inner p or n type material. The first layer is called emitter and is indicated by capital E, the mid layer is called base and is indicated by B and the third layer is called collector and indicated by capital C.
Q.4: Mention the rules for biasing a transistor.
Ans: Rules for biasing a transistor:
1. Base – Emitter junction is always forward biased.
2. Collector – Base junction is always reverse biased.
Q.5: What do you mean by transistor configuration?
Ans: In a circuit a transistor can be connected in three different ways. This is called transistor configuration. There are three types of transistor configuration –
1. Common base configuration: In a common base configuration mid layer of the transistor is common to outer two layers that is the base is common to both the input and output sides of the configuration. In addition, the base is usually the terminal closest to or at ground potential.
The input set for the common – base amplifier relates an input current to an input voltage for various levels of output voltage. The output set relates an output current to an output voltage for various levels of input current. The output or collector set of characteristics has three basic regions of interest the active, cutoff, and saturation regions.
In the active region the base emitter junction is forward biased, whereas the collector base junction is reverse biased. In the cutoff region the base emitter and collector base junctions of a transistor are both reversed biased. In the saturation region the base emitter and collector base junctions are forward biased.











2. Common emitter configuration: The most frequently encountered transistor configuration for the p-n-p and n-p-n transistors is called the common emitter configuration. In a common emitter configuration the emitter is common to both the input and output terminals. Two sets of characteristic are the input or base emitter circuit and the output or collector emitter circuit.
In the active region of a common emitter amplifier the base emitter junction is forward biased, whereas the collector base amplifier is reversed biased.









3. Common collector configuration: The third and final transistor configuration is the common collector configuration. In a common collector configuration the collector is common to both the base and emitter terminals. The common collector configuration has high input impedance, opposite to that of the common base and common emitter configurations.
A common collector circuit configuration is provided with the load resistor connected from emitter to ground. It can be designed using the common emitter characteristics. The output characteristics of the common collector configuration are the same for the common emitter configuration.





Q.6: Explain the operation of a transistor in common base (CB) configuration and draw input and output characteristics.
Or,
Explain the operation of a transistor in common base (CB) configuration and draw input and output characteristics. Identify cutoff region, saturation region and active region on the output characteristics.
Ans: The operation of a transistor in common base (CB) configuration: For the operation of a transistor, the transistor should be properly biased. The general rule for the biasing of a transistor is that emitter base junction should be forward biased and collector base junction should be reverse biased. The circuit diagram for a biased p-n-p transistor is given bellow.









Here battery VEE makes the emitter base junction forward biased and battery VCC makes the collector base junctions reverse biased. Since emitter base junction is forward biased, the depletion region is reduced, due to applied voltage, resulting a heavy flow of majority carriers from p to the n type material. On the other hand, since collector base junction is reverse biased, depletion region increased which stops the flow of majority carriers. The huge number of injected charge carriers will be minority carriers in the n type material and will see an easy path to flow through collector base reverse biased junction to the collector. Only a few will go through the high resistive path to the base terminal. The direction of current flow through the transistor is shown by arrow sign.
Applying Kirchhoff’s law of current flow to the transistor we obtain,
IE = IC + IB
That is the emitter current is the sum of collector and base currents. The collector current is composed of two components the majority and the minority components. This can be written as
IC = IC majority + ICO minority.
The minority component ICO is very small.
Input and output characteristics of a transistor in common base configuration: For a common base configuration IE = IC + IB. To fully describe the behavior of a three terminal device requires two sets of characteristics – the input characteristics and the output characteristics.
The input characteristic for common base transistor is shown bellow which relates the input current IE to input voltage VBE for various levels of output voltage, VCB. This curve is identical to a forward biased p-n junction.