Effect of removing bypass capacitor: In a silicon transistor amplifier, the capacitors Cc and CE are assumed short at signal frequency. If the emitter bypass capacitor CE is disconnected, which statement is TRUE?
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AThe input resistance Ri increases and the magnitude of voltage gain Av decreases
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BThe input resistance Ri decreases and the magnitude of voltage gain Av decreases
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CBoth input resistance Ri and the magnitude of voltage gain Av decrease
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DBoth input resistance Ri and the magnitude of voltage gain Av increase
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EThe voltage gain remains unchanged
Answer
Correct Answer: The input resistance Ri increases and the magnitude of voltage gain Av decreases
Explanation
Introduction / Context:In small-signal transistor amplifiers, bypass capacitors are used to stabilize gain and control input resistance. Removing the emitter bypass capacitor alters feedback conditions, which directly impacts amplifier characteristics.
Given Data / Assumptions:
- Silicon transistor amplifier with emitter resistor RE.
- At signal frequencies, coupling capacitor Cc is short, but CE is disconnected (open).
- We ignore output resistance r0.
Concept / Approach:
With CE present, RE is bypassed, providing high gain and no AC feedback. Without CE, RE introduces negative feedback: this increases input resistance (since base sees β+1 times RE) and reduces AC voltage gain (due to degeneration).
Step-by-Step Solution:
With CE connected: AC ground at emitter → effective emitter resistance ≈ 0.Gain ≈ −Rc/re' (large magnitude).With CE disconnected: emitter resistor RE appears in series for AC signal.Input resistance increases because Rin = (β+1)(re' + RE).Voltage gain magnitude decreases because Av = −Rc/(re' + RE).Verification / Alternative check:
This effect is used deliberately in amplifier design to trade gain for linearity and input resistance stabilization.
Why Other Options Are Wrong:
- (b) Wrong: input resistance increases, not decreases.
- (c) Wrong: input resistance does not decrease.
- (d) Wrong: gain decreases, not increases.
- (e) Wrong: gain is not unchanged; emitter degeneration always reduces gain.
Common Pitfalls:
- Assuming bypass capacitor only affects DC bias; it strongly affects AC small-signal behavior.
- Mixing up input resistance trends when degeneration is added.
Final Answer:
The input resistance Ri increases and the magnitude of voltage gain Av decreases