Electronic Devices and Circuits Multiple Choice Questions on “The Common Drain Amplifier”.
1. Output Impedance of Common Drain Amplifier is______________
A. RS+rd
B. RS
C. RS || rd
D. RS||1/gm
Answer: D
Clarification: By using ac equivalent of FET, gm Vgs is made 0. On observing the circuit from output side, ZO=Vo/Io
But Io = Vo((1/rd)+(1/RS)+(gm)),
=> ZO=Vo/Vo((1/rd)+(1/RS)+(gm))= rd |(|RS |)|1/gm
ZO=RS||1/gm.
2. A Common drain amplifier is designed using fixed bias configuration, what is its input impedance?
A. RD + rd
B. RG
C. RD || rd
D. 0
Answer: B
Clarification: From the ac small signal model, the input impedance of FET is open circuit.
Gate resistance RG is in parallel with open circuit resulting in Zin=∞||RG=RG.
3. Which of the following is true about Common Drain amplifier?
A. It has low input impedance
B. It has high output impedance
C. Infinite gain
D. Output will be same as input
Answer: D
Clarification: The voltage gain for a Common Drain Amplifier, Av=1, hence the output at the source terminal follows the input provided to the gate terminal.
4. A sinusoidal signal has 1.5V at t=2S, If this signal is given to Common drain amplifier, what will be its output at t=2S?
A. 7.5V
B. -7.5V
C. 0
D. 1.5V
Answer: D
Clarification: Input at t=2S is 1.5V, |Av|=1
=>output=1×1.5V=1.5V.
5. A sine signal with period 2S has a peak of 2V at 0.5S and at 1.5 it was -2V, if this signal is applied to Common drain configured amplifier, What will be the output at t=1S?
A. 2V
B. -2V
C. 0V
D. 1V
Answer: C
Clarification: For the given sine signal, since the period is 2S, the value of voltage at t=1S will be 0V and this signal is provided as input to Common drain configured amplifier which follows the input signal. Therefore the output will be 0 at t=1S.
6. Which of the following is true about the effects of rd in Drain amplifier?
A. rd Increases output impedance
B. rd Decreases output impedance
C. Output impedance remains the same
D. rd Increases input impedance
Answer: B
Clarification: Typically ZO=c||1/gm
But with rd, ZO=RS||1/gm||rd,
Hence output impedance will decrease.
7. Which of the following is the other name for Common Drain Amplifier?
A. Source Follower
B. Current Booster
C. Voltage booster
D. Voltage limiter
Answer: A
Clarification: The voltage gain for a Common Drain Amplifier, Av=1, hence the output at the source terminal follows the input provided to the gate terminal. Hence we call a Common Drain Amplifier as Source follower as the name itself says it the source terminal follows the input signal.
8. While choosing the operating conditions, the Vgs value was set lesser to Vt. What will the output if the FET is made to work as Source Follower?
A. 0
B. Same as the input
C. Same as input but phase reversed
D. Cannot be determined
Answer: A
Clarification: If FET is needed to work as an amplifier, then the bias point must lie on Saturation, but in the above case the FET is operating in cut-off resulting in off condition.
Hence output will be 0.
9. What will be the gain value of FET if gmo=0?
A. 0
B. 1
C. 2
D. Infinite
Answer: A
Clarification: Av=gm RS/(1+gm RS) but gm is having linear relationship with gmo.
Since gmo=0, voltage gain will be zero resulting in 0 Output.
10. If gm=0.5mS, RS=2KΩ, determine ZO for source follower?
A. 2KΩ
B. 1KΩ
C. 3KΩ
D. 1.5KΩ
Answer: B
Clarification: ZO=RS||1/gm
=>1/gm=1/0.5mS=>2KΩ
2KΩ||2KΩ=1KΩ,
=>ZO=1KΩ.