Microelectronics Questions and Answers – Bipolar Amplifiers – Q-Point Analysis

This set of Microelectronics Multiple Choice Questions & Answers (MCQs) focuses on “Bipolar Amplifiers – Q-Point Analysis”.

1. What is Q point?
a) Junction point
b) Operational point
c) Operating point
d) Disjunction point

Explanation: The operating point is often regarded as the Q point. The intersection between the load line and the output characteristics of a device is used to find a good operating point for the device for a desired operation. For example, a good operating point for a transistor implies it might be behaving as a good amplifier with reduced distortion.

2. What are the parameters of the load line?
c) Current
d) Voltage

Explanation: The load line represents the output characteristics of a device where the supply voltage and the load are parameters. This load line intersects with the output characteristics of the device which shows how the output current changes with the output voltage. This intersection gives us the Operating point.

3. What would happen if the operating point of a B.J.T. changes suddenly?
a) Distortion
b) Unequal amplification
c) Change in the transconductance
d) Change in the trans-resistance

Explanation: The B.J.T. is a transconductance amplifier since it converts voltage to current. We observe that the parameter gm is a measure of the transconductanceies how much current is generated due to the input voltage. This parameter is directly reflected in the operating point since a higher operating point implies higher transconductance and we get more current for small voltage.

4. Where, in the load line, would we like to set the operating point of a transistor?
a) At the center
b) At the top
c) At the bottom
d) At origin

Explanation: The usual tendency is to keep the operating point at the center of the load line in the active region. In the active region, the transistor is made to behave as a current source and provide amplification. This active region is present at the center of the load line. Observe that the bottom of the load line would give less collector current while the a higher operating point might drive the transistor into saturation.

5. For a transistor, deciding a good q-point is based on solution of the load line and the output characteristics. In the active region, the output characteristic becomes a straight line. What is the slope of the straight line?
a) The conductance due to early effect
b) The resistance due to early effect
c) The transconductance
d) The current gain

Explanation: The slope in the output characteristics, in the active region, is essentially due to the early effect. As the characteristics become nearly a straight line, obtaining the solution becomes easier in the active region.

6. The output characteristic of a transistor in the active mode is given by y=mx+c. The loan line is given by y=nx+d. The operating point is given by ________
a) {nc-md/m-n, (c-d)/(n-m)}
b) {(c-d)/(n-m), nc-md/m-n}
c) {nc-md/m-n, (c-d)/(n-m)}
d) {(c-d)/(n-m), nc-md/n-m}

Explanation: y represents the current while x represents the voltage. Thus, the solution of both the straight lines is the operating point. It is given by the solution (v,i) or (x,y) and hence the correct option is d. Note that option b provides a wrong solution for the current

7. A q-point is located at the center of the load line. If the supply voltage increases, what happens to the operating point?
a) It shifts towards the active region
b) It shifts towards the saturation region
c) It goes into cut-off
d) No change

Explanation: The q-point shifts more into the active region. This can be understood from the fact that an increase in the supply voltage readily increases the output current. For a typical graph showing the Output Characteristics with the load line, we find that the intercept of the load line increases with an increase in the supply voltage and this shows that the center of the load line moves deeper into the active region.

8. A q-point is located at the center of the load line. What would happen to the operating point if the device drives a heavy load?
a) It goes into the inverse-active mode
b) It goes into deep saturation
c) It goes into the active mode
d) Cannot be determined

Explanation: A heavy load draws more current and offers less resistance. This implies that the y-intercept of the load line moves up. The operating point thus shifts deeper into the active region.

9. What kind of an input signal would face a different q-point in every instant of time?
a) Alternating
b) D.C
c) Pulsating D.C.
d) No such input signal

Explanation: Alternating and Pulsating D.C. are slightly different though Pulsating D.C. still alternates between a certain value with time. The q-point thus shifts in every instant of time in case of an alternate signal.

10. The biasing of a transistor is done to reach a good operating point for amplification. If the operating point is selected as the y-intercept of the output characteristics, the transistor is properly biased.
a) True
b) False

Explanation: The y-intercept is present in the cut-off region. This implies that the transistor is off and the above statement is thus false.

11. Ignoring early effect, what is the y-intercept of the load-line for the following circuit?

a) 0, .05mA
b) 0, 1mA
c) 0, 1.5mA
d) 0, 2mA

Explanation: The LOAD resistance in absence of early effect is given by the total resistance connected to the collector node. This is given by R1|| R2|| R3||R4||R5 which results to 200Ω. The y intercept of the load line is (0, VCC/RL) which finally becomes (0, .05mA).

12. The load-line is completely independent of the input voltage.
a) True
b) False

Explanation: The load line is simply dependent on the supply voltage and the load line.

Sanfoundry Global Education & Learning Series – Microelectronics.

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