Test: Power Transistors

QUESTION: 1

Which of the following devices does not belong to the transistor family?

A.
IGBT
B.
MOSFET
C.
GTO
D.
BJT

Solution:

GTO is gate turn off transistor, it belongs to the Thyristor family. All the other devices belong to the transistor family.

QUESTION: 2

A power transistor is a

A.
three layer, three junction device
B.
three layer, two junction device
C.
two layer, one junction device
D.
four layer, three junction device

Solution:

It has three layers p-n-p or n-p-n forming two p-n junctions.

QUESTION: 3

In a power transistor, ____ is the controlled parameter.

A.
V_BE
B.
V_CE
C.
I_B
D.
I_C

Solution:

The collector current is the controlled parameter.

QUESTION: 4

A power transistor is a _________ device.

A.
two terminal, bipolar, voltage controlled
B.
two terminal, unipolar, current controlled
C.
three terminal, unipolar, voltage controlled
D.
three terminal, bipolar, current controlled

Solution:

Power transistor is simply many BJT’s connected in series parallel on a single silicon chip for power applications. It is a three terminal, bipolar, current controlled device.

QUESTION: 5

In a power transistor, _________ is the controlling parameter.

A.
V_BE
B.
V_CE
C.
I_B
D.
I_C

Solution:

The base current controls the collector current. Hence, the base current Ib is the controlling parameter.

QUESTION: 6

In a power transistor, the I_B vs V_BE curve is

A.
a parabolic curve
B.
an exponentially decaying curve
C.
resembling the diode curve
D.
a straight line Y = I_B

Solution:

The B-E junction of a BJT resembles a p-n junction diode, hence the curve.

QUESTION: 7

For a power transistor, if the base current I_B is increased keeping V_CE constant, then

A.
I_C increases
B.
I_C decreases
C.
I_C remains constant
D.
none of the mentioned

Solution:

Ic is directly proportional to Ic.

QUESTION: 8

The forward current gain α is given by

A.
I_C/I_B
B.
I_C/I_E
C.
I_E/I_C
D.
I_E/I_B

Solution:

Collector current by emitter current is the current gain, its value is close to one but never greater than.

QUESTION: 9

The value of β is given by the expression

A.
I_C/I_B
B.
I_C/I_E
C.
I_E/I_C
D.
I_E/I_B

Solution:

Collector current by the base current is beta, its value is in the range 50 to 300.

QUESTION: 10

A power BJT is used as a power control switch by biasing it in the cut off region (off state) or in the saturation region (on state). In the on state

A.
both the base-emitter & base-collector junctions are forward biased
B.
the base-emitter junction is reverse biased, and the base collector junction is forward biased
C.
the base-emitter junction is forward biased, and the base collector junction is reversed biased
D.
both the base-collector & the base-emitter junctions are reversed biased

Solution:

When base-emitter & base-collector junctions are forward biased only than both the p-n junctions are forward biased and the device is on.

QUESTION: 11

For a power transistor, if the forward current gain α = 0.97, then β = ?

A.
0.03
B.
2.03
C.
49.24
D.
32.33

Solution:

Use the relation α = β/(β+1).

QUESTION: 12

The power electronics devices have a very high efficiency because

A.
cooling is very efficient
B.
the devices traverse active region at high speed & stays at the two states, on and off
C.
the devices never operate in active region
D.
the devices always operate in the active region

Solution:

They are efficient due to their higher transition speeds.

QUESTION: 13

For a power transistor, which of the following relations is true?

A.
Ie>Ic>Ib
B.
Ib>Ic>Ie
C.
Ic>Ie>Ib
D.
Ie=Ib

Solution:

Practically speaking Ie = Ib+Ic. Ie is the highest as it is the sum of the collector and base currents. The base current is the smallest.

QUESTION: 14

High frequency operation of any device is limited by the

A.
forward voltage rating
B.
switching losses
C.
thermal conductivity
D.
heat Sink arrangements

Solution:

Lower the switching losses higher the frequency of operation of the device.

QUESTION: 15

The instantaneous power loss during the delay time of a transistor is given by

A.
Ic Vce
B.
Ib Vbe
C.
Ic Vbe
D.
Ib Vce

Solution:

During the delay time only the collector current flows & base to emitter voltage is zero.

QUESTION: 16

For a power transistor, the average power loss during the delay time can be given by the equation

A.
Ic * Vc
B.
1/T * ∫^Td (Ic Vce) dt
C.
Ic * dVc/dt * T
D.
1/T * ∫^{(Td *Tr)} (Ic Vc) dt

Solution:

During the delay time only, the collector current flows & base to emitter voltage is zero. Hence the average power can be found, simply by integrating it over the total delay time & dividing by the base time period.

QUESTION: 17

A 1mv of i/p gives an output of 1V, the voltage gain as such would be

A.
0.001
B.
0.0001
C.
1000
D.
100

Solution:

1V/1mv = 1000.

QUESTION: 18

Which of the following relations is true for a BJT?

A.
Ic ≈ Ie
B.
Ib ≈ Ic
C.
Ie ≈ Ib
D.
Ib ≈ Ie ≈ Ic

Solution:

The collector & emitter current differ only by the base current, which is very very small.

QUESTION: 19

Choose the correct statement

A.
A transistor will remain on as long the the base current is applied
B.
A transistor remains on after a high to low pulse is applied at the base
C.
A transistor will remain on as long the the collector current is applied
D.
A transistor remains on after a high to low pulse is applied at the collector

Solution:

Unlike the thyristor devices, all the transistor family devices remain in the conducting state as long as the firing pulses are applied. This is a very important property of the transistor devices.

QUESTION: 20

Let’s say that a transistor is operating at the middle of the load line, then a decrease in the current gain would