Switching Characteristics of Transistor

Prior to the Lab session:

1. Study the operation and working principle of the Transistor in all regions.
2. Study the procedure for conducting this experiment in the lab.

Objective:

1. To study the Switching characteristics of a transistor.
2. Design Transistor to act as a Switch and verify the operation. Choose V_CC = l0V, I_Cmax = l0 mA, h_fe = 50, V_CESat = 0.2V, Vin = 4Vp-p, V_BESat = 0.6 V.

Apparatus:

Circuit diagram:

Theory:

The Transistor can act as a switch. To operate the transistor as a switch, it has to be operated in saturation region for ON state and to be operated in cut off region for OFF state.

When the Input voltage V_i is negative or zero, transistor is cut-off and no current flows through Rc. Hence V_o is approximately equal to V_CC. When Input Voltage V_i is changed to positive voltage, transistor will be driven into saturation. Then, V_O = V_CC – I_CR_C V_CESat, which is a very small voltage.

Design procedure:

When Transistor is ON, RC = (V_CC - V_CEsat)/I_Cmax

                                         = (10 - 0.2) / 10mA

                                         = 1K

I_B ≥ I_Cmax /h_fe

    ≥ 10mA /50

I_B ≥ 0.2mA

To keep transistor remain in ON, I_B should be greater then I_Bmin = 0.2mA

V_in = I_BR_B + V_BEsat

2V = 0.2mA . R_B + 0.6 V

R_B = 7 K (Choose Practical values as 8.2 K)

Procedure:

1. Connect the circuit as shown in the above figure.
2. Apply the Square wave of 4 Vp-p at frequency of l KHz
3. Observe the waveforms at Collector and Base of the transistor and plot it.

Precautions:

1. Keep the CRO in DC mode while measuring the Output waveform at collector and base,
2. For measuring V_{BE Sat} and V_{CE Sat}  keep volts/div switch at either 0.2 or 0.5 position.
3. When the square wave is being applied, ensure that there is no DC voltage in that. This can be checked by CRO in either in AC or DC mode. There should not be any jumps/distortion in waveform on the screen.

Waveforms:

Inference:

Transistor as a switch has been designed operated and Output waveforms are observed.

Viva Questions:

1. Mention typical values of V_{BE Sat}, V_{CE Sat} for both Si, Ge Transistors?

Ans:

2. Define ON time and OFF time of the transistor?

Ans:

The turn ON time of a transistor, t_ON is the sum of delay time t_d and the rise time t_r, i, e,

t_ON = t_d + t_r

The turn OFF of a transistor, t_OFF is the sum of the storage time and theall time t_ON, i, e,

t_OFF = t_s + t_f

3. In which regions Transistor acts as a switch?

Ans: Transistor acts a switch in saturation region and in cut off region.

4. Define Rise time & fall time of a transistor switch? 

Ans: The rise time t_r is defined as the time required for the collector current waveform to rise from 10% to 90% of its maximum value (I_cs)

The fall time t_f is the time required for I_c to fall from 90% to 10% of I_cs.

5. Define Storage time and delay time?

Ans: 

Storage Time: The storage time t_s is a time interval which elapses between the negative transition of input waveform and the time when I_c has dropped to 90% I_cs.

Delay Time: The delay time of a transistor is the time required for the collector current to rise to 10% of its final value after the application of positive input voltage pulse.

6. What is the phase difference between the input and the output, when the transistor is conducting?

Ans: For CB = 0° or 360°, CE = 180°, CC = 0°

7. What modifications are to be done in the above circuit if we use PNP transistor instead of NPN transistor? 

Ans: We have to apply –Vcc(emitter collector terminal connected w r t ground).

8. How to calculate I_C in the above circuit, when the transistor is ON?

Ans: I_C = V_CC - (V_CE / R_L)

9. What is the output voltage swing for the above circuit?

Ans: Output swing = 0 to 5V (V_SAT)

10. Why square wave is given as input instead of a sinusoidal wave for switching ON and OFF of the transistor?

Ans: For switch function the transistor operated in saturation region and cut off region , so when we applied sinusoidal signal then Possitve portion and negative portion is clipped off. Hence square wave is given for switching ON and OFF.

Design problem:

1. Design transistor switch to get an output of 12Vp-p swing.
2. Can we apply sinusoidal signal to transistor as switch & verify the output for the same.
3. Design a high speed transistor switch.

Outcomes:  After finishing this experiment, the students are able to design a transistor switch circuit and observe the waveforms.