# Electronic Devices and Circuits Questions and Answers – Frequency Compensation

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Frequency Compensation”.

1. If the ratio RB / RE tends to infinity, what should be the value of stability factor in voltage divider circuit?
a) Unity
b) Zero
c) 1 – βdc
d) 1 + βdc

Explanation: Stability factor is fundamentally dependent on the ratio of base resistance to emitter resistance(RB / RE). Hence, if this ratio is estimated to be small, then the stability factor in such case is equal to unity.
On the contrary, if this ratio RB / RE tends to infinite value, then the value of stability factor in voltage divider circuit becomes nearly equivalent to 1+ βdc.
Therefore, if the ratio is constant then it leads to an increase the stability factor by increasing the value of current gain (βdc). Besides this, smaller values of base resistance also yield improved stabilization.

2. What are the consequences of diode compensation for the change in base-to-emitter voltage (VBE) due to temperature?
a) Temperature compensation takes place by variation in forward voltage (VF)
b) Collector current become sensitive to the change in base-to-emitter voltage (VBE)
c) Depends
d) Nothing happens

Explanation: In some of the negative feedback circuits, the amplification level of AC signals gets ablated suddenly. Since it becomes difficult for the circuits to tolerate the loss of signals, it is essential to reduce the drift in an operating point by means of compensation and stabilization.
Compensation techniques include diode compensation, bias compensation using thermistor, sensistor & so on. They play a major role in providing maximum bias in addition to the thermal stabilization to the circuits.

3. Generally, the resistance of thermistor decreases _______
a) Linearly with an increase in temperature
b) Linearly with the decrease in temperature
c) Exponentially with an increase in temperature
d) Exponentially with the decrease in temperature

Explanation: Thermistor is a device or resistor whose resistance is totally dependent on the temperature. Thermistor is widely applicable for several compensation techniques.
It exhibits variation in its resistance with respect to change in temperature. As the resistance of thermistor decreases with an increase in temperature, this property of thermistor is also regarded as negative temperature coefficient of resistivity.

4. Fidelity is nothing but an ability of amplifier to reproduce ________
a) Input signal without any distortion
b) Output signal without any distortion
c) Phase shift signal
d) Amplitude shift signal

Explanation: Fidelity is the property of amplifier which is usually taken into consideration during the analysis of an amplifier’s frequency response. The ability or a potential of an amplifier in order to reproduce the distortion-less nature of input signal is known as ‘Fidelity’.
Fidelity of an amplifier is dependent on the frequency response and the bandwidth of an amplifier to the greater extent. Fidelity gives an implication of satisfying the condition of distortion less output.
This condition for distortion-less output implies that gain of an amplifier should be independent of frequency (constant) while the phase shift introduced by an amplifier should reveal proportionality with respect to frequency or zero.

5. Which among the following is an output provided by transresistance amplifier?
a) Output current proportional to signal voltage
b) Output voltage proportional to signal current
c) Output voltage proportional to input voltage
d) Output current proportional to signal current

Explanation: An amplifier which produces the output voltage in proportion to the signal current where the proportionality factor is independent of source and load resistances, is known as ‘Transresistance amplifier’. For a practical transresistance amplifier, input resistance must be very less than source resistance and the output resistance must be very less than load resistance.
Generally, it is the ratio of voltage to signal current. Conversely, the transconductance amplifier exhibits output current proportional to signal voltage. Voltage amplifier exhibits the output voltage proportional to input voltage & current amplifier exhibits the output current proportional to the signal current.

6. Which among the below specified conditions is applicable to prevent the occurrence of thermal runaway in voltage divider bias circuit?
a) VCE < VCC / 2
b) VCE = VCC / 2
c) VCE > VCC / 2
d) None of the mentioned

Explanation: Thermal runaway is a growing process which can ultimately harm the transistor due to unnecessary internal heating or increase in ambient temperature. It occurs due to increase in collector current ahead of the maximum specified value. Additionally, it also occurs due to excessive internal power dissipation above the maximum allowed value. Thermal runaway can be disallowed by maintaining the provision of thermal stability in addition to the treatment of heat sink.
According to thermal stability condition, VCE should be less than Vcc/2 results in the Q-point operation at a safe level. On the contrary, if the location of Q-point is at VCE > Vcc/2, then the transistor is more likely to get damaged due to thermal runaway.

7. Which of the following process plays a crucial role in devising the independency of operating point over the variations in temperature or transistor parameters?
a) Bias stabilization
b) Bias compensation
c) Bias stabilization & compensation
d) No process

Explanation: Bias stabilization is a process which makes the operating (Q) point independent of change in temperature or any change in transistor parameters.
As Q-point exhibits its independent nature over the several variations, it plays a major role in providing the stability to greater extent especially for self-bias, fixed-bias and collector to base bias circuits. Since the stability factor of circuit gets unnatural due to several variations, bias stabilization has a provision of maintaining the Q-point condition irrespective of the changes in temperature or transistor parameters and in that way ensuring the confined level of bias stability to circuits.

8. What should be the level of input resistance to allow the occurrence of source loading in common base amplifier configuration?
a) Low
b) High
c) Moderate
d) Stable

Explanation: As per the configuration of CB amplifier, it is obvious that its input resistance is very low whereas its output resistance is extremely high. However, the lower value of input resistance allows the provision of source loading in common base amplifier circuit. Thus, there is no current amplification due to unity current gain. These all reasons ultimately contribute to high level of voltage gain.

9. The value of dBm in power measurement is predictable by assuming the reference, which is equal to ________
a) 1mW
b) 10mW
c) 1/10 mW
d) 1/100 mW

Explanation: Basically, the Decibel (dB) is a logarithmic unit which is used to state the gain of audio frequency amplifiers. This is so for the reason that the response of a human ear to the sound intensity exhibits logarithmic nature.
Apart from this, dBm is a unit which is often used in power measurement. dBm refers to the dB value which is estimated by assumption of a reference whose value is equal to 1mW.

10. The cut-off frequency (fβ) is basically the frequency at which the short circuit __________
a) CB gain of transistor drops by 3 dB from its value at low frequency
b) CE gain of transistor drops by 3 dB from its value at low frequency
c) CC gain of transistor drops by 3 dB from its value at low frequency
d) CC gain of transistor drops by 3 dB from its value at high frequency

Explanation: Cut-off frequency (fβ) is basically the frequency at which short circuit CE gain of transistor drops by 3dB from its value at low frequency.
Thus, the frequency range upto the cut-off frequency(fβ) is regarded as the bandwidth of circuit. Also, the current gain reduces to 70.7% of the low frequency current gain.
Likewise, the frequency at which short circuit CB gain of transistor drops by 3dB from its value next to low frequency is known as cut-off frequency (fα).
The relationship between these cut-off frequencies indicates that the 3dB frequency of CB configuration is (1 + hfe) times greater or higher as compared to that of CE configuration.

11. Which among the following will possess a higher bandwidth, if two transistors are provided with unity gain frequency?
a) Transistor with lower hfe
b) Transistor with higher hfe
c) Transistor with lower hre
d) Transistor with higher hre

Explanation: In general, the short circuit CE gain of a transistor gets reduced to unity at f = fT. This implies that the transistor loses its capability of current amplification at f = fT.
As the gain-bandwidth product always remains constant, the transistor with lower hfe will definitely possesses higher bandwidth if the two transistors are made available with equal unity gain frequency.

12. What is an angle of phase shift for each designed RC network in the Phase Shift Oscillator circuit?
a) 60o
b) 90o
c) 180o
d) 45o

Explanation: R-C phase shift oscillator consists of three identical basic R-C phase shifting kind of networks approved in the cascade configuration.
Thus, the total phase shift introduced by three stage R-C network is equal to 180o, which implies that the output of network leads its input by 180o. As a result, it infers that each phase-shift stage acquires the phase shift of about 60o.
RC network is also renowned as ‘Ladder network’. The phase-shift of about 180o is generated particularly at one particular frequency, which is measured to be the frequency of operation of the oscillator.

Sanfoundry Global Education & Learning Series – Electronic Devices and Circuits.

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