Important short Questions and Answers: Bipolar Junction

BIPOLAR JUNCTION

1.       What is a transistor? What are the types?

Transistor consists of two junctions formed by sandwiching either p type or n type semiconductor between a pair of opposite types.

Two types - *NPN transistor *PNP transistor

2.       Define BJT?

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

3.       Give the hie and heo equations of BJT?

h₁₁ = h_ie - The input impedance of the transistor (corresponding to the emitter resistance re).Unit ohms Ώ.

h₂₂=h_oe  -  The  output  impedance  of  transistor.  This  term  is  usually  specified  as admittance and has to be inverted to convert it to impedance. Units’ siemans S.

4.       Why is the transistor called a current controlled device?

A transistor is called a current controlled device. This is because; collector current is controlled by base current. The changes in collector current are proportional to the corresponding changes in base current.

5.       Explain about the characteristics of a transistor?

In common emitter configuration, input characteristics is the plot obtained by tracing the variation of input current IB with the input voltage VBE. Similarly, the variation of output current IC with the Collector to emitter voltage VCE is known as output characteristics.

6.       Define h parameters?

One of a set of four transistor equivalent-circuit parameters that conveniently specify transistor performance for small voltages and currents in a particular circuit. Also known as hybrid parameter.

7.       Why we use h-parameters to describe a transistor?

Any linear circuit can be analyzed by four parameters (input resistance, reverse voltage gain, forward current gain and output admittance) of mixed dimensions. Since the dimensions of the parameters have mixed units they are referred as h-parameters. The h-parameters are determined by both open circuit and short circuit terminations.

8.       What is operating point?

The Q point or quiescent point or operating point where DC load line intersects proper base current curve. The coordinates of Q point decides the zero values of IC and VCE in a common emitter transistor.

9.       In a bipolar transistor which region is wider and which region is thinner? Why?

The middle region of bipolar junction transistor is called as the base of the transistor. Input signal of small amplitude is applied to the base. This region is thin and lightly doped. The magnified output signal is obtained at the collector. This region is thick and heavily doped.

10.     What do you understand by thermal runaway?

The excess heat produced at the collector base junction may even burn and destroy the transistor. The self destruction of an unbiased transistor is known as thermal runaway. To avoid thermal run away the operating point of the circuit is to be stabilized.

11.     Define the delay time and rise time in the switching characteristics of transistor?

In the transistor switching characteristics the delay time is the time that elapses the application of the input pulse and current to rise to 10 percent of its maximum value. The time required for IC to reach 90% of its maximum level from 10% level is called the rise time.

12. Differentiate FET and BJT (any two)?

13.     When a transistor is used as a switch, in which region of output characteristics it is operated?

When a transistor is used as a switch it is operated alternately in the cut off region and saturation region of the output characteristics.

14. When does a transistor act as a switch?

A transistor should be operated in saturation and cut off regions to use it as a switch. While operating in saturation region, transistor carry heavy current hence considered as ON state. In cut-off, it carries no current and it is equivalent to open switch.

15. Why do the output characteristics of a CB transistor have a slight upward slope?

The emitter and collector are forward biased under the saturation region. Hence a small change in collector voltage causes a significant change in collector current. Therefore the slight upward slope is found in output characteristics.

16. Define transport factor, β?

It is the ratio of injected carrier current reaching at collector base junction to injected carrier current at emitter base junction.

β = I _pC / I _PE

17. Define rise time?

The time required for IC to reach 90% of its maximum level from 10% level is called rise time, tr

18. Define current gain in CE configuration?

The current gain (β) of common emitter configuration is defined as the ratio of change in collector current to change in base current when collector emitter voltage is kept constant.

I_C /  I _B

β is also referred as h_fe.

19. What is meant by biasing a transistor?

Process of maintaining proper flow of zero signal collector current and collector emitter voltage during the passage of signal. Biasing keeps emitter base junction forward biased and collector base junction reverse biased during the passage of signal.

20.     What are the various methods used for transistor biasing? Which one is popular?

Base resistor method

Biasing with feedback resistor

Voltage divider bias

Voltage divider bias is wide popular because it offers excellent stabilization to the circuit.

21.     What are the limitations of h-parameters?

Obtaining the exact value of h-parameters for a particular transistor is quite difficult. Highly suitable only for small ac signals.

22. What is the basic difference between bias compensation and stabilization?

Stabilization is the process of making operating point independent of temperature variations or changes in transistor parameters using dc biasing circuits. In the case of compensation technique, in order to stabilize the Q point, we use temperature sensitive devices like diodes, thermistors, transistors instead of DC biasing circuits.

23.     List the 3 sources of instability of collector current?

•        Individual variations

•        Temperature dependence of collector

•        current Thermal runaway

24.     Define Current Amplification Factor for CE, CC, CB Configuration

25.     Define punch through or reach through.

Extremely large collector voltage, the effective base width may be reduced to zero, resulting in voltage breakdown of a transistor. This phenomenon is known as punch through.

26.     Define pinch off voltage

Pinch off voltage is defined as the drain to source voltage above which drain current becomes almost constant.

27.     A)What happens to transistor when both the junctions are reverse biased?

B) Can transistor circuit be operated as a switch? State how?

A)      The transistor operated in cut-off region and act as a open switch.

B)      A Transistor can be operated as a closed or ON Switch, when both the junctions are forward bias and open or OFF switch when both junction are reverse biased.