gain frequency response, input and output impedance. Feedback in amplifiers;
negative feedback and stability.
10 questions on each with explained answers
## Gain and Frequency Response
**1. Midband gain region defined by?**
a) Low frequencies only
b) Constant gain where capacitors short, Cstray open
c) High frequencies
d) Cutoff only
**Answer: b) Constant gain where capacitors short, Cstray open** [1]
**Explanation:** Coupling/bypass capacitors act as shorts, transistor/stray capacitances negligible, yielding maximum flat gain. [1]
**2. Low frequency roll-off caused by?**
a) Stray capacitance
b) High XC of coupling/bypass capacitors
c) Miller effect
d) Transconductance drop
**Answer: b) High XC of coupling/bypass capacitors** [1]
**Explanation:** XC = 1/(2Ï€fC) high at low f reduces signal transfer, gain drops -20dB/decade per pole. [1]
**3. High frequency -3dB point determined by?**
a) Coupling capacitors
b) fH where gain drops 3dB from midband
c) DC bias
d) Load resistance
**Answer: b) fH where gain drops 3dB from midband** [1]
**Explanation:** Marks bandwidth limit, typically Miller capacitance dominant. [1]
**4. Bandwidth defined as?**
a) fH only
b) fH - fL
c) Midband gain
d) Gain-bandwidth product
**Answer: b) fH - fL** [1]
**Explanation:** Frequency range where gain within 3dB of maximum. [1]
**5. Miller capacitance effect?**
a) Reduces at high f
b) Multiplies Cgd by (1 + Av)
c) Low frequency only
d) No effect
**Answer: b) Multiplies Cgd by (1 + Av)** [7]
**Explanation:** Feedback capacitance amplified by voltage gain lowers input pole frequency. [7]
**6. Gain drops -20dB/decade means?**
a) Single pole roll-off
b) Two poles
c) Flat response
d) Oscillation
**Answer: a) Single pole roll-off** [7]
**Explanation:** Each RC pole contributes 6dB/octave or 20dB/decade phase/gain slope. [7]
**7. At high frequencies, base current increases due to?**
a) Low XC of Cbe junction
b) Coupling capacitor
c) RE bypass
d) RC load
**Answer: a) Low XC of Cbe junction** [3]
**Explanation:** Shunts signal to ground, reducing β and gain. [3]
**8. Gain-bandwidth product constant for?**
a) All amplifiers
b) Op-amps, fT = Av * BW
c) Power amps only
d) Class A
**Answer: b) Op-amps, fT = Av * BW** [5]
**Explanation:** Unity gain frequency limits closed-loop bandwidth. [5]
**9. Phase shift at cutoff frequency?**
a) 0°
b) 45° for single pole
c) 90°
d) 180°
**Answer: b) 45° for single pole** [7]
**Explanation:** RC network phase φ = tan⁻¹(f/fc) = 45° at fc. [7]
**10. Dominant pole usually from?**
a) Output stage Miller
b) Input coupling
c) Bypass capacitor
d) Load
**Answer: a) Output stage Miller** [7]
**Explanation:** Lowest frequency pole sets overall bandwidth. [7]
## Input and Output Impedance
**1. CE amplifier input impedance?**
a) Very high
b) Low, ≈ β re
c) Infinite
d) RL
**Answer: b) Low, ≈ β re** [11]
**Explanation:** Base-emitter junction forward biased, rπ = β/gm. [11]
**2. Common collector Zin?**
a) Low
b) Very high, β (RE || RL)
c) Medium
d) Zero
**Answer: b) Very high, β (RE || RL)** [12]
**Explanation:** Emitter degeneration multiplies base impedance. [12]
**3. Output impedance CE high due to?**
a) Low re
b) Early effect, rce high
c) RE feedback
d) Coupling C
**Answer: b) Early effect, rce high** [11]
**Explanation:** Collector resistance dominates without emitter degeneration. [11]
**4. Negative feedback effect on Zi?**
a) Decreases
b) Increases for series-shunt
c) No change
d) Infinite
**Answer: b) Increases for series-shunt** [13]
**Explanation:** Voltage sampling, current mixing raises input resistance. [13]
**5. Common base Zin low because?**
a) Emitter forward bias
b) High β
c) Collector grounded
d) RE
**Answer: a) Emitter forward bias** [12]
**Explanation:** ≈ re ≈ 25mV/IE, tens of ohms. [12]
**6. Zo decreases with?**
a) Shunt feedback
b) Series feedback
c) No feedback
d) Open loop
**Answer: a) Shunt feedback** [13]
**Explanation:** Current sampling lowers output resistance. [13]
**7. Emitter degeneration increases?**
a) Zi and Zo
b) Only gain
c) Stability only
d) Distortion
**Answer: a) Zi and Zo** [11]
**Explanation:** RE feedback raises both impedances. [11]
**8. FET CS amplifier Zi?**
a) Low
b) Very high, gate insulated
c) β ro
d) RD
**Answer: b) Very high, gate insulated** [14]
**Explanation:** Ig ≈ 0, megaohms typical. [14]
**9. Buffer amplifier purpose?**
a) High gain
b) High Zi, low Zo matching
c) Phase shift
d) Filtering
**Answer: b) High Zi, low Zo matching** [12]
**Explanation:** CC configuration ideal interface. [12]
**10. Loading effect minimized by?**
a) High Zin amp
b) Low Zo source
c) Both
d) None
**Answer: c) Both** [11]
**Explanation:** Zin >> Zsource, Zload >> Zo prevents gain loss. [11]
## Feedback in Amplifiers
**1. Feedback topology classified by?**
a) Gain type only
b) Mixing (series/shunt) and sampling (series/shunt)
c) Frequency
d) Class
**Answer: b) Mixing (series/shunt) and sampling (series/shunt)** [13]
**Explanation:** Determines Zi, Zo, gain type changes. [13]
**2. Negative feedback samples?**
a) Inverted output
b) Output subtracted from input
c) Added to input
d) No sampling
**Answer: b) Output subtracted from input** [13]
**Explanation:** Error = Vin - β Vout drives correction. [13]
**3. Advantages of negative feedback?**
a) Increases distortion
b) Reduces distortion, stabilizes gain
c) Low bandwidth
d) High noise
**Answer: b) Reduces distortion, stabilizes gain** [13]
**Explanation:** Loop gain >>1 linearizes response. [13]
**4. Series-shunt feedback is?**
a) Transresistance
b) Voltage amplifier
c) Current
d) Power
**Answer: b) Voltage amplifier** [13]
**Explanation:** Voltage mixing/sampling, high Zi low Zo. [13]
**5. Loop gain Aβ represents?**
a) Closed loop gain
b) Open loop gain times feedback fraction
c) Distortion
d) Phase
**Answer: b) Open loop gain times feedback fraction** [13]
**Explanation:** Determines desensitization factor 1/(1+Aβ). [13]
**6. Feedback reduces?**
a) Bandwidth
b) Increases BW by 1+Aβ
c) Gain infinite
d) Noise only
**Answer: b) Increases BW by 1+Aβ** [13]
**Explanation:** Trades gain for extended frequency range. [13]
**7. Shunt-shunt feedback topology?**
a) Voltage
b) Transresistance amp
c) Current
d) Buffer
**Answer: b) Transresistance amp** [13]
**Explanation:** Current mixing, voltage sampling. [13]
**8. Positive feedback causes?**
a) Stability
b) Oscillation if Aβ=1, 0°/360° phase
c) Gain reduction
d) Distortion reduction
**Answer: b) Oscillation if Aβ=1, 0°/360° phase** [13]
**Explanation:** Barkhausen criteria for sustained oscillation. [13]
**9. Desensitivity factor?**
a) 1 + Aβ
b) Gain increase
c) Distortion multiplier
d) BW reduction
**Answer: a) 1 + Aβ** [13]
**Explanation:** Closed-loop gain = Aol/(1+Aβ) ≈ 1/β. [13]
**10. Emitter degeneration example of?**
a) Shunt-series
b) Series-series current feedback
c) Voltage
d) No feedback
**Answer: b) Series-series current feedback** [11]
**Explanation:** Senses Ie, mixes with input series. [11]
## Negative Feedback and Stability
**1. Stability determined by?**
a) Gain only
b) Phase margin >45°, gain margin >6dB
c) Distortion
d) BW
**Answer: b) Phase margin >45°, gain margin >6dB** [4]
**Explanation:** Prevents oscillation in closed loop. [4]
**2. Nyquist stability criterion?**
a) Gain >1 at 180°
b) Encircles -1 point indicates instability
c) Phase 0°
d) BW infinite
**Answer: b) Encircles -1 point indicates instability** [4]
**Explanation:** Plots Aβ vs frequency encircling critical point unstable. [4]
**3. Phase margin definition?**
a) Gain at 180°
b) Phase from -180° when |Aβ|=1
c) BW
d) Distortion
**Answer: b) Phase from -180° when |Aβ|=1** [4]
**Explanation:** 45-60° typical for good damping. [4]
**4. Gain margin?**
a) Phase when gain=1
b) dB from 0dB when phase=-180°
c) Opposite phase margin
d) No relation
**Answer: b) dB from 0dB when phase=-180°** [4]
**Explanation:** >20dB safe, 40dB conservative. [4]
**5. Compensation improves stability by?**
a) Reducing phase lag
b) Adding dominant pole, reducing gain early
c) Increasing gain
d) No phase change
**Answer: b) Adding dominant pole, reducing gain early** [5]
**Explanation:** Ensures |Aβ|<1 before 180° phase shift. [5]
**6. Negative feedback stable if?**
a) Aβ >1 at 180°
b) |Aβ| <1 when ∠Aβ=-180°
c) Positive loop
d) Zero phase
**Answer: b) |Aβ| <1 when ∠Aβ=-180°** [4]
**Explanation:** Prevents growing oscillations. [4]
**7. Lead compensation?**
a) Reduces BW
b) Adds phase lead, increases phase margin
c) Lag only
d) Gain reduction
**Answer: b) Adds phase lead, increases phase margin** [5]
**Explanation:** RC network boosts high frequency phase. [5]
**8. Unconditional stability means?**
a) Oscillates some loads
b) Stable all gains ≤1
c) Only unity gain
d) Unstable
**Answer: b) Stable all gains ≤1** [4]
**Explanation:** No right-half plane poles. [4]
**9. Bode plot used for?**
a) Time response
b) Gain/phase vs log f stability analysis
c) DC only
d) Nonlinear
**Answer: b) Gain/phase vs log f stability analysis** [1]
**Explanation:** Predicts margins from asymptotic approximations. [1]
**10. Ringing indicates?**
a) Perfect stability
b) Low phase margin, underdamped
c) Overdamped
d) No feedback
**Answer: b) Low phase margin, underdamped** [4]
**Explanation:** Step response overshoot/oscillation from poor margins. [4]
