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Differential Amplifiers MCQs

Practice Differential Amplifiers MCQs with four options, answers and explanations for focused Electronics & Telecommunication Engineering revision.

Question 1
Analog ElectronicsDifferential Amplifiers

CMRR is the ratio of?

  • ACommon-mode gain to differential gain
  • BDifferential gain to common-mode gain
  • COutput resistance to input resistance
  • DBandwidth to slew rate
Report an error
Correct Answer: B. Differential gain to common-mode gain
Explanation: CMRR = Ad/Acm and indicates rejection of signals common to both inputs.
Reference: Standard analog electronics: differential amplifiers
Question 2
Analog ElectronicsDifferential Amplifiers

A differential amplifier ideally amplifies?

  • AThe average of its inputs
  • BThe difference between its input voltages
  • COnly the supply voltage
  • DOnly common-mode signals
Report an error
Correct Answer: B. The difference between its input voltages
Explanation: A differential amplifier responds to the difference between its two input voltages.
Reference: Standard analog electronics: differential amplifiers
Question 3
Analog ElectronicsDifferential Amplifiers

In an ideal BJT differential amplifier with perfectly matched transistors and equal collector resistors, applying the same signal to both inputs ideally produces a differential output of?

  • AZero
  • BEqual to twice the input
  • CInfinite
  • DEqual to the supply voltage
Report an error
Correct Answer: A. Zero
Explanation: An ideal differential amplifier rejects a signal common to both inputs, so the differential output due to a pure common-mode signal is zero.
Reference: Standard analog electronics: differential amplifiers
Question 6
Analog ElectronicsDifferential Amplifiers

The principal purpose of a constant-current source in the tail of a BJT differential pair is to?

  • AImprove bias stability and common-mode rejection
  • BMake both transistors permanently saturated
  • CReduce input resistance to zero
  • DRectify the input signal
Report an error
Correct Answer: A. Improve bias stability and common-mode rejection
Explanation: A high-resistance tail current source keeps the total emitter current nearly constant, improving differential operation and common-mode rejection.
Reference: Standard analog electronics: differential amplifiers
Question 9
Electronic Devices & CircuitsDifferential Amplifiers

In an ideal differential amplifier, common-mode gain should be?

  • AZero
  • BUnity
  • CInfinite
  • DEqual to differential gain
Report an error
Correct Answer: A. Zero
Explanation: An ideal differential amplifier responds only to the difference between its inputs and rejects signals common to both, so ideal common-mode gain is zero.
Reference: Standard analog electronics: differential amplifier
Question 12
Electronic Devices & CircuitsDifferential Amplifiers

In a BJT differential pair with an ideal constant tail-current source, increasing the differential input toward one transistor tends to?

  • ASteer more of the fixed tail current into that transistor
  • BIncrease both collector currents equally without limit
  • CForce both transistors permanently off
  • DEliminate the tail current
Report an error
Correct Answer: A. Steer more of the fixed tail current into that transistor
Explanation: A differential pair steers an approximately fixed tail current between its two devices according to the differential input voltage.
Reference: Standard analog electronics: differential pair current steering
Question 13
Electronic Devices & CircuitsDifferential Amplifiers

In a differential amplifier, using a high-output-resistance tail current source generally improves?

  • ACommon-mode rejection
  • BRectifier efficiency
  • CDiode reverse recovery
  • DLED optical power only
Report an error
Correct Answer: A. Common-mode rejection
Explanation: A high-resistance tail source makes the total tail current less sensitive to common-mode input changes, thereby improving CMRR.
Reference: Standard analog electronics: differential pair tail source
Question 14
Electronic Devices & CircuitsDifferential Amplifiers

For a perfectly balanced BJT differential pair with zero differential input and an ideal tail current of 2 mA, each transistor carries approximately?

  • A0 mA
  • B0.5 mA
  • C1 mA
  • D2 mA
Report an error
Correct Answer: C. 1 mA
Explanation: With matched devices and equal input voltages, the ideal 2 mA tail current divides equally, giving approximately 1 mA in each transistor.
Reference: Standard analog electronics: differential pair bias
Question 15
Electronic Devices & CircuitsDifferential Amplifiers

An active load is often used in an integrated differential amplifier primarily because it can provide?

  • AHigh effective load resistance and increased voltage gain
  • BMechanical isolation
  • CA low-frequency clock
  • DA fixed diode forward drop only
Report an error
Correct Answer: A. High effective load resistance and increased voltage gain
Explanation: Transistor active loads can present a high small-signal resistance while occupying little IC area, which increases amplifier voltage gain.
Reference: Standard analog electronics: active loads
Question 16
Analog ElectronicsDifferential Amplifiers

In a BJT differential pair with an ideal tail current of 1 mA and zero differential input, the quiescent collector current of each matched transistor is approximately?

  • A0.25 mA
  • B0.5 mA
  • C1 mA
  • D2 mA
Report an error
Correct Answer: B. 0.5 mA
Explanation: With zero differential input, an ideal matched pair shares the 1 mA tail current equally, so each transistor carries about 0.5 mA.
Reference: Standard analog electronics: differential pair bias
Question 17
Analog ElectronicsDifferential Amplifiers

For a BJT differential pair at room temperature with total tail current IT, the small-signal differential transconductance from differential input voltage to single-ended collector-current change is approximately?

  • AIT/(4VT)
  • BIT/VT
  • C4IT/VT
  • DVT/IT
Report an error
Correct Answer: A. IT/(4VT)
Explanation: Each transistor carries IT/2 at balance, giving gm=IT/(2VT). For differential excitation, the single-ended collector-current change is gm(vd/2), so the effective differential transconductance is IT/(4VT).
Reference: Standard analog electronics: differential transconductance
Question 19
Analog ElectronicsDifferential Amplifiers

Using a current-mirror active load in a differential amplifier can convert the differential collector currents into?

  • AA useful single-ended output current
  • BA fixed DC supply voltage
  • CA digital clock only
  • DZero output current for every input
Report an error
Correct Answer: A. A useful single-ended output current
Explanation: The mirror redirects one branch-current variation and combines it with the other branch, producing a single-ended output with enhanced gain.
Reference: Standard analog electronics: active-load differential amplifier
Question 20
Analog ElectronicsDifferential Amplifiers

For a long-tailed differential pair, increasing the small-signal resistance of the tail-current source primarily reduces?

  • ACommon-mode gain
  • BDifferential input resistance to zero
  • CDifferential gain to zero
  • DSupply voltage
Report an error
Correct Answer: A. Common-mode gain
Explanation: A high tail resistance makes total current less sensitive to common-mode voltage changes, reducing common-mode gain and improving CMRR.
Reference: Standard analog electronics: common-mode rejection