Name of the College : Selvam College of Technology
Subject Code/Name : Transistor Bias Stability
Dept : Electronics And Communication Engineering
Degree : B.E
Website : selvamtech.com
Document Type : Question Bank
Download Model Question Paper : https://www.pdfquestion.in/uploads/selvamtech.com/3241-TRANSISTOR%20BIAS%20STABILITY.pdf
Selvam Tech Transistor Bias Stability Question Paper
TWO MARK QUESTIONS & ANSWERS :
UNIT I
1. Why do we choose Q point at the center of the loadline?
The operating point of a transistor is kept fixed usually at the center of the active region in order that the input signal is well amplified. If the point is fixed in the saturation region or the cut off region the positive and negative half cycle gets clipped off respectively.
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2. Name the two techniques used in the stability of the Q point .explain.
Stabilization technique: This refers to the use of resistive biasing circuit which allows IB to vary so as to keep IC relatively constant with variations in Ico and VBE. Compensation techniques: This refers to the use of temperature sensitive devices such as thermistors diodes. They provide compensating voltages & currents to maintain operating point constant.
4. List out the different types of biasing.
Voltage divider bias
Base bias
Emitter feedback bias
Collector feedback bias
5. What do you meant by thermal runway?
Due to the self heating at the collector junction, the collector current rises. This causes damage to the device. This phenomenon is called thermal runway.
6. Why is the transistor called a current controlled device?
The output characteristics of the transistor depend on the input current. So the transistor is called a current controlled device.
7. Define current amplification factor?
It is defined as the ratio of change in output current to the change in input current at constant other side voltage.
8. What are the requirements for biasing circuits?
** The q point must be taken at the Centre of the active region of the output characteristics.
** Stabilize the collector current against the temperature variations.
** Make the q point independent of the transistor parameters.
** When the transistor is replaced, it must be of same type.
9. When does a transistor act as a switch?
The transistor acts as a switch when it is operated at either cutoff region or saturation region.
10. What is biasing?
To use the transistor in any application it is necessary to provide sufficient voltage and current to operate the transistor. This is called biasing.
11. What is operating point?
For the proper operation of the transistor a fixed level of current and voltages are required. This values of currents and voltages defined at a point at which the transistor operate is called operating point.
12. What is stability factor?
Stability factor is defined as the rate of change of collector current with respect to the rate of change of reverse saturation current.
13. What is d.c load line?
The d.c load line is defined as a line on the output characteristics of the transistor which gives the value of Ic & Vce corresponding to zero signal condition.
14. What are the advantages of fixed bias circuit?
This is simple circuit which uses a few components. The operating point can be fixed any where on the Centre of the active region
15. Explain about the various regions in a transistor?
The three regions are active region saturation region cutoff region.
16. Explain about the characteristics of a transistor?
Input characteristics: it is drawn between input voltage & input current while keeping output voltage as constant.
Output characteristics: It is drawn between the output voltage &output current while keeping input current as constant.
UNIT II
MIDBAND ANALYSIS OF SMALL SIGNAL AMPLIFIERS
1. Define small signal equivalent circuit?
The analysis of a non-linear device is complx.signal that takes up a relatively small percentage of an amplifier’s operational range. With small input singles the transistor can be replaced with small signal linear model. This Model is also called small signal equivalent model.
2. Define transconductance?
The change in the drain current due to change in gate to source voltage can be determined using the transconductance factor gm. Id=gm VGS.
3. Define emitter bypass capacitor?
An emitter bypass capacitor CE is connected in parallel with the emitter resistance, RE to provide a low reactance path to the amplified ac signal. If it is not inserted, the amplified ac signal passing through RE will cause a voltage drop across it. This will reduce the output voltage, reducing the gain of the amplifier.
4. Define coupling capacitor?
The coupling capacitor Cs couples the output of the amplifier to the load or to the next stage of the amplifier. It blocks dc and passes only ac part of the amplified signal.
5. Define input resistance
The ratio of V1 to I1 is called input resistance Ri ,of the amplifier. Ri = V1 / I1