Transistor as a Switch

Transistor as a switch:

Transistors are widely used in switching operations. In the Fig, NPN transistor is connected in common emitter configuration and a resistor is connected in series with the base. The load resistance is connected in series with the collector. A pulse type waveform is applied as the input to the transistor through RB. When the input is high, base emitter junction is forward biased and current flows through into the base. The values of and are chosen in such a manner that the base current flowing, is enough to saturate the transistor. When the transistor is saturated, it is said to be ON (maximum current). When the input is low (i.e.) at 0 V, the base emitter junction is not forward biased. So, no base current flows. Hence the transistor is said to be OFF.

Diode as rectifier:

The process in which alternating voltage or alternating current is converted into direct voltage or direct current is known as rectification. The device used for this process is called as rectifier. The junction diode has the property of offering low resistance and allowing current to flow through it, in the forward biased condition. This property is used in the process of rectification.

Half wave rectifier:

A circuit which rectifies half of the a.c wave is called half wave rectifier. Fig shows the circuit for half wave rectification. The a.c. voltage (Vs) to be rectified is obtained across the secondary ends S₁ S₂ of the transformer. The P-end of the diode D is connected to S₁ of the secondary coil of the transformer. The N-end of the diode is connected to the other end S₂ of the secondary coil of the transformer, through a load resistance R_L. The rectified output voltage Vdc appears across the load resistance R_L. During the positive half cycle of the input a.c. voltage Vs, S₁ will be positive and the diode is forward biased and hence it conducts.

Therefore, current flows through the circuit and there is a voltage drop across RL. This gives the output voltage as shown in Fig. During the negative half cycle of the input a.c. voltage (Vs), S₁ will be negative and the diode D is reverse biased. Hence the diode does not conduct. No current flows through the circuit and the voltage drop across R_L will be zero. Hence no output voltage is obtained. Thus corresponding to an alternating input signal, unidirectional pulsating output is obtained. The ratio of d.c. power output to the a.c. power input is known as rectifier efficiency. The efficiency of half wave rectifier is approximately 40.6%.

Bridge Rectifier:

A bridge rectifier is shown in Fig. There are four diodes D₁, D₂, D₃ and D₄ used in the circuit, which are connected to form a network. The input ends A and C of the network are connected to the secondary ends S₁ and S₂ of the transformer. The output ends B and D are connected to the load resistance R_L. During positive input half cycle of the a.c. voltage, the point A is positive with respect to C. The diodes D₁ and D₃ are forward biased and conduct, whereas the diodes D₂ and D₄ are reverse biased and do not conduct. Hence current flows along ABDC through R_L. During negative half cycle, the point C is positive with respect to A. The diodes D₂ and D₄ are forward biased and conduct, whereas the diodes D₁ and D₃ are reverse biased and they do not conduct. Hence current flows along CBDA through . The same process is repeated for subsequent half cycles. It can be seen that, current flows through in the same direction, during both half cycles of the input a.c. signals. The output signal corresponding to the input signal is shown in Fig. The efficiency of the bridge rectifier is approximately 81.2%.

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