# Electronic Devices and Circuits Questions and Answers – The Transistor as an Amplifier

This set of Electronic Devices and Circuits Questions and Answers for Aptitude test focuses on “The Transistor as an Amplifier”.

1. The emitter current consist of_________
a) electrons passing from collector to emitter
b) holes crossing from base to collector
c) electron current Ine constituted by electrons
d) immobile charge carriers

Explanation: The emitter current consists of two parts. It consists of hole current IpE constituted by holes. The other part is that it consists the electron current InE constituted by electrons.

2. The total emitter current (IE) is given by_________
a) IE = IpE * InE
b) IE = IpE – InE
c) IE = IpE / InE
d) IE = IpE + InE

Explanation: The total emitter current is the sum of InE and IpE. In commercial transistors, the doping of emitter region is made much heavier than base. Hence current by majority charge carriers InE is negligible when compared to current by minority charge carriers IpE.

3. A common base transistor amplifier has an input resistance of 20Ω and output resistance of 100kΩ. If a signal of 400mV is applied between emitter and base, find the voltage amplification. Assume αac to be one.
a) 20
b) 50
c) 30
d) 25

Explanation: IE = V/R=400M/20=20mA
IC=αIE= 1*20mA=20mA. VO=IC*RL=20m*1k=20V
Amplification, A= VO/signal voltage=20V/400m=50.

4. The amplification factor for a transistor is given by_________
a) A=αRL/re
b) A=αRLre
c) A=re/ αRL
d) A=RL/reα

Explanation: One of the most important application of a transistor is an amplifier. A small change in signal voltage produces an appreciable change in emitter current because the input circuit has low resistance (α=∆IC/IE).

5. Why is the silicon mostly chosen when compared to germanium?
a) low power consumption
b) high efficiency
c) greater working temperature
d) large ICBO

Explanation: The normal working temperature of germanium is approximately 70°C .The normal working temperature of silicon is approximately 150°C. The other advantages of using a silicon material are, it has a smaller ICBO and its variations are smaller with temperature.

6. The change in output voltage across the load resistor for a transistor during amplification is_________
a) RL *α*∆IE
b) RL *∆IE
c) RL2*∆IE
d) RL1/2*∆IE

Explanation: A small change of voltage ∆Vi between emitter and base causes a relatively large emitter current change ∆IE. We define by the symbol α that fraction of this current change which is collected and passes through RL.

7. A transistor has an IC of 100mA and IB of 0.5mA. What is the value of αdc?
a) 0.565
b) 0.754
c) 1.24
d) 0.995

Explanation: Emitter current IE=IC+IB=100+0.5=100.5mA.
αdc=IC/IE=100/100.5=0.995.

8. A germanium transistor used as an amplifier has a collector cut off current ICBO=10µA at a temperature 27°C and β=50. What is the collector current when the base current is 0.25mA?
a) 10.76mA
b) 13.01mA
c) 15.67mA
d) 11.88Ma

Explanation: IC=βIB+(1+β)ICBO
IC=50*0.25/1000+51/100000=13.01mA.

9. In a PNP germanium transistor, the cut in voltage is about_________
a) -0.1V
b) -0.01V
c) -0.05V
d) -0.07V

Explanation: The cut in voltage of germanium is lower than that of silicon. If both germanium and silicon are in parallel, Ge starts conducting earlier and stops silicon from conducting.

10. In a PNP transistor operating in active region, the main stream of current is_________
a) drift of holes
b) drift of electrons
c) diffusion of holes
d) diffusion of electrons

Explanation: The emitter-base junction is forward biased while collector-base junction is reversed biased. The transistor now operates in active region. Here, it can be used for amplification purpose.

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

To practice all areas of Electronic Devices and Circuits for Aptitude test, here is complete set of 1000+ Multiple Choice Questions and Answers.

If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]