Important Short Questions and Answers: Transistors

TRANSISTORS

1. Draw the input and output characteristics of a transistor in CE configuration and mark the cutoff saturation and active regions.

2. State the advantage of optocouples?

·                                Control circuits are well protected due to electrical isolation.

·                                Wideband signal transmission is possible.

·                                Due to unidirected signal transfer , noise from the output side does not get coupled to the input side.

·                                Interfacing with logic circuit is easily possible.

3. Why is collector region wider than emitter region in BJT?

In BJT collector region is wider and base region is thinner. The collector is made wider so as make heat dissipation easier whereas thinner base will increase the value of β of the transistor.

4. In a BJT,the emitter current is 12 mA and the emitter current is 1.02 times the collector current. Find the base current.

I_E= I_C + I_B = 1.02 I_C (Given)

I_B = 0.02 I_C

But                     I_C =  I_E / 1.02

=                                                 12/1.02

=                                                 11.76 Ma

I_B = 0.02 * 11.76 *10^-3

=                                                 235.2 µA.

5. Differentiate FET and BJT ( two points)?

FET

Unipolar device (that is current conduction by only one type of either electron or hole).

High input impedance due to reverse bias.

Gain is characterized by trans conductance

Low noise level

BJT

Bipolar device (current conduction by both electron and hole).

Low input impedance due to forward bias.

Gain is characterized by voltage gain

High noise level

6. Define pinch off voltage in FET.

The pinch off voltage V_P is defined as the value of V_DS beyond which the drain current becomes constant.

7. Why are power transistor provided with heat sinks?

To avoid thermal runway , which will damage the transistor due to internal heating the power transistor provided with heat sinks.

8. What are the special features of FET

·                                               It is a voltage controlled device.

·                                               It is equivalent to a controlled current source.

·                                               The gate source junction is always reverse biased.

·                                               Very small gate current.

·                                               High input resistance and input capacitance.

·                                               Can be used as a switch or as an amplifier.

·                                               It can be used as voltage variable resistance VVR.

9. Will a transistor result if two diodes are connected back to back?

No, because

·                                               The diode equivalent circuit cannot give the integrated effect of the transistor and the base terminal has no control over the current flowing through the diode.

·                                               The reverse biased diode representing the collector junction will not allow the current reverse to flow.

10.            Why FET is called unipolar device?

FET is a unipolar device, that means the current flowing through it is only due to one type of charge particles, holes or electrons . Transistor on the other hand is a bipolar device as holes and electrons both contribute to the flow of current.

11. What is a bipolar junction transistor?

A bipolar junction transistor is a three terminal semiconductor deice in which the operation depends on the interaction of both majority and minority carriers.

12.            Define the different operating regions of transistor.

The different operating regions of transistor are

Active Region: It is defined in which transistor function is biased in reverse direction and emitter function in forward direction.

Cutoff Region: The region in which the collector and emitter functions are both reverse biased.

Saturation Region: The region in which both the collector and emitter functions are forward biased.

13.            Explaip npn and pnp transistor.

npn Transistor: In npn transistor, P-type semiconductor is sandwiched between two n-type semiconductors. The emitter region is made up of n-type semiconductor base region is made of p-type semiconductor, collector region is made of n-type semiconductor.

pnp Transistor: In pnp transistor, n-type semiconductor is sandwiched between two P-type semiconductor. Emitter region is made of P-type, collector region is made of P-type and the base region is made of n-type, semiconductor.

14. Define Transistor current.

The emitter current (1E) is the sum of the collector current (Ic) and the base current (IB), is called transistor current'. IE = Ic + IB;. IB is very small compared to IE or Ic.

15. What are the three types of configuration in transistors?

Depending on the input, output and .common terminal a transistor are connected in 3 configurations;

i) Common base configuration

ii)  Common emitter configuration

iii) Common collector-Configuration.

16. What is early effect or base and the modulation?

As the collector by voltage Vcc is made to increase the reverse bias, the space charge width between collector and base tends to increase with the result that the effective width of the base decreases. This dependency of base width on collector to emitter -voltage is known as early effect.