Friday, November 23, 2012

Multiple Choice Questions on Transistors including EAMCET Engineering 2004 and 2005 Questions

“The pursuit of truth and beauty is a sphere of activity in which we are permitted to remain children all our lives.”
– Albert Einstein

A very important semiconductor device you need to study is the transistor, the invention of which brought about a revolution in electronics. Questions involving transistors have been discussed earlier on this site. You can access them by trying a search for ‘transistor’ using the search box provided on this page or by clicking on the label ‘transistor’ below this post. Today we shall discuss a few more questions on transistors
(1) In an amplifier circuit using a transistor the collector current changes by1.224 mA when the emitter current changes by 1.228 mA. What is the common emitter small signal current gain of the transistor used in the circuit?
(a) 0.997 (very nearly)
(b) 1.003 (very nearly)
(c) 122 (very nearly)
(d) 307
(e) 306
The small signal common emitter current gain is the ratio of the change in collector current to the change in base current. Since the emitter current is the sum of the collector current and the base current, the change in base current in the present case is 1.228 mA 1.224 mA which is equal to 0.004 mA.
Therefore common emitter small signal current gain (βac) = 1.224/0.004 = 306.
(2) In an n-p-n transistor in CE configuration
(i) the emitter is more heavily doped than the collector
(ii) emitter and collector can be interchanged
(iii) the base region is very thin but is heavily doped
(iv) the conventional current flows from base to emitter
(a) (i) and (ii) are correct
(b) (i) and (iii) are correct
(c) (i) and (iv) are correct
(d) (ii) and (iii) are correct
The above question appeared in EAMCET Engineering 2004 question paper.
Option (c) is the answer. Note that the configuration deoes not matter.
(3) An n-p-n transistor power amplifier in CE configuration gives
(a) voltage amplification only
(b) current amplification only
(c) both current and voltage amplifications
(d) only power gain of unity
This question appeared in EAMCET engineering 2005 question paper. In common emitter configuration a transistor (n-p-n as well as p-n-p) gives both current and voltage amplifications so that there will be appreciable power gain. Therefore the correct option is (c).

(4) The adjoining figure shows an n-p-n transistor operated in the common emitter configuration in which the potential divider resistors R1 and R2 provide the base voltage required for forward biasing the base emitter junction. A student wrongly selected too small a value for R1 so that the transistor got saturated. (When a transistor is saturated, its emitter to collector voltage is nearly zero). The collector load resistor RC is 5 KΩ and the emitter resistor RE (used for bias stabilization) is 1 KΩ. If βdc of the transistor is 200 what is the minimum base current required for saturating the transistor?
Even though this question may appear to be difficult at the first glance, it is really simple. You have to first calculate the current I which will produce a voltage drop of 12 volts across 6 KΩ.
[Since the emitter to collector voltage is nearly zero when the transistor is saturated, the entire supply voltage of 12 volts must be dropped across the resistors RC and RE].
Therefore we have
            I × 6 KΩ = 12 V so that I = 2mA
The emitter current and collector current are nearly the same (because of large current gain βdc) so that we can take the collector current to be 2 mA. The base current required for producing a collector current (IC) of 2 mA is the saturating base current in this circuit. Therefore the minimum base current IB required for saturating the transistor is given by
            IB = IC/βdc = 2 mA/200 = 0.01 mA = 10 μA.

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