# Microwave Engineering Questions and Answers – Heterojunction BJT – 2

This set of Microwave Engineering Multiple Choice Questions & Answers focuses on “Heterojunction BJT – 2”.

1. The hybrid-π model of a BJT is useful for analysis at all frequency ranges and variation of other transistor parameters.
a) true
b) false

Explanation: The element values of the hybrid-π model are fairly constant over a wide range of operating points, bias conditions, load conditions and frequency. Otherwise, the element values become frequency, bias or load dependent in which case the hybrid –π model becomes less useful.

2. If the S11 and S22 parameters of a common emitter operated BJT is high:
a) then the output and input ports are matched well
b) there is mismatch in the ports
c) the gain of the amplifier is high
d) none of the mentioned

Explanation: S11 and S22 parameters of a two port network signify the amount of signal to the same port that is excited by the source, a high value of these values imply that these ports are not impedance matched properly.

3. If a common emitter configuration BJT is treated as a two port network, the gain of the amplifier is roughly given by the S parameter:
a) S11
b) S12
c) S21
d) S22

Explanation: When a BJT is represented as a two port network, where the base is port 1 and collector is port 2. Gain of the amplifier is given by the parameter S21. This parameter drops quickly with an increase in the operating frequency.

4. Short circuit current gain of BJT is given by the expression:
a) gm/ωC
b) ωC/ gm
c) gm/C
d) none of the mentioned

Explanation: Short circuit current gain of BJT is defined as the ratio of output collector current to the input base current assuming the base resistance to be zero. The frequency at which the short circuit current gain of the amplifier is unity is called upper frequency limit.

5. The output collector to emitter current of a BJT amplifier is independent of the input base current of the amplifier.
a) true
b) false

Explanation: BJT is a current controlled device. Output collector current is controlled by the input base current. If the input base current is increased, the collector current also increases.
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6. The current gain of a BJT ________ with frequency.
a) increases
b) decreases
c) remains constant
d) none of the mentioned

Explanation: The short circuit current gain of a BJT amplifier is given by the expression gm/ωC. From the equation, it is seen that gain is inversely proportional to frequency. As the frequency of operation of BJT increases, current gain of the transistor reduces.

7. If a transistor has a short circuit current gain of 25 and the capacitance measured in the hybrid-π model of the transistor was 60 pF. Then the threshold frequency of operation of the transistor is:
a) 60 MHz
b) 45.6 GHz
c) 66.3 GHz
d) 34.8 GHz

Explanation: The threshold frequency for a BJT is given by gm/2πC. substituting the given values; the threshold operating frequency of the transistor is 66.3 GHz.

8. Hetero junction bipolar transistors have the same working principle and operation as that of a BJT.
a) true
b) false

Explanation: The operation of hetero junction BJT is same as that of BJT, but an HBT has a base emitter junction made from a compound semiconductor material such as GaAs in junction with thin layers of other materials.

9. Advantage of HJT over BJT is that it has:
a) higher gain
b) high frequency of operation
c) sophisticated construction
d) none of the mentioned

Explanation: The gain of BJT reduces with the increase in the operating frequency. This problem is overcome in HJT which gives sustained gain even at frequencies of about 100 GHz. They are also suitable for application in low power circuits.

10. The S21 parameter of a HJT increases with increase in the operating frequency of the transistor.
a) true
b) false

Explanation: S21 parameter of a transistor signifies the gain of the transistor. With the increase in the operating frequency of the transistor, the gain of the transistor reduces, but it is to be noted that the fall in gain of a HJT is not as rapid as fall in gain in a BJT.

Sanfoundry Global Education & Learning Series – Microwave Engineering.
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