Question

Create an real life application where bipolar junction transistor is required and design circuit for the same application

Answer 1

Using a BJT as a Switch

Bipolar junction transistors (Also known as BJTs) can be used as an amplifier, filter, rectifier, oscillator, or even a switch

Using a BJT as a Switch:

Consider a lamp that we wanted to turn on and off with a switch. Such a circuit would be extremely simple, as in the figure below (a).

For the sake of illustration, let’s insert a transistor in place of the switch to show how it can control the flow of electrons through the lamp. Remember that the controlled current through a transistor must go between collector and emitter.

Since it is the current through the lamp that we want to control, we must position the collector and emitter of our transistor where the two contacts of the switch were. We must also make sure that the lamp’s current will move against the direction of the emitter arrow symbol to ensure that the transistor’s junction bias will be correct as in the figure below (b).

(a) mechanical switch, (b) NPN transistor switch, (c) PNP transistor switch.

A PNP transistor could also have been chosen for the job. Its application is shown in the figure above (c). The choice between NPN and PNP is really arbitrary. All that matters is that the proper current directions are maintained for the sake of correct junction biasing (electron flow going against the transistor symbol’s arrow).

In the above figures, the base of either BJT is not connected to a suitable voltage, and no current is flowing through the base. Consequently, the transistor cannot turn on. Perhaps, the simplest thing to do would be to connect a switch between the base and collector wires of the transistor as in figure (a) below.

Transistor: (a) cutoff, lamp off; (b) saturated, lamp on.

BJT as Amplifier

Single Stage RC Coupled CE Amplifier

The figure shows a single stage CE amplifier. C₁ and C₃ are coupling capacitors, they are used for blocking the DC component and passing only ac part they also ensure that the DC basing conditions of the BJT remains unchanged even after input is applied. C₂ is the bypass capacitor which increases the voltage gain and bypasses the R₄ resistor for AC signals.

The BJT is biased in the active region using the necessary biasing components. The Q point is made stable in the active region of the transistor. When input is applied as shown below the base current starts to vary up and down, hence collector current also varies as I_C = β × I_B. Therefore voltage across R₃ varies as the collector current is passing through it. Voltage across R₃ is the amplified one and is 180^o apart from the input signal. Thus voltage across R₃ is coupled to the load and amplification has taken place. If the Q point is maintained to be at the center of the load very less or no waveform distortion will take place. The voltage as well as current gain of the CE amplifier is high (gain is the factor by which the voltage of current increases from input to output). It is commonly used in radios and as low frequency voltage amplifier.

To further increase the gain multistage amplifiers are used. They are connected via capacitor, electrical transformer, R-L or directly coupled depending on the application. The overall gain is the product of gains of individual stages.

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