CET Staging & Prognostic Factors 3 — Questions and Answers
Question 1: Which factor most significantly determines the input impedance of a common-emitter transistor amplifier stage?
- Collector resistance
- Supply voltage
- Base bias resistors and transistor beta (Correct answer)
- Emitter bypass capacitor value
Correct answer: Base bias resistors and transistor beta
The input impedance of a CE stage is primarily set by the parallel combination of bias resistors and the transistor's beta times the emitter resistance.
Question 2: An amplifier stage has an output impedance of 10 kΩ driving a load of 1 kΩ. What happens to the voltage across the load relative to the open-circuit output voltage?
- It increases by 10x
- It remains the same
- It drops significantly due to loading (Correct answer)
- It doubles
Correct answer: It drops significantly due to loading
The load forms a voltage divider with the output impedance; with a 10:1 ratio, most voltage drops across the internal impedance.
Question 3: What is the function of a buffer stage inserted between two amplifier stages?
- Increase voltage gain
- Provide high input impedance and low output impedance to prevent loading (Correct answer)
- Filter high-frequency noise
- Shift the DC operating point
Correct answer: Provide high input impedance and low output impedance to prevent loading
A buffer stage (often an emitter or source follower) presents high input impedance and low output impedance to isolate stages and prevent loading effects.
Question 4: In a cascaded amplifier system, thermal noise contributed by each stage:
- Cancels out between stages
- Accumulates and is amplified by subsequent stages (Correct answer)
- Only affects the last stage
- Remains constant throughout
Correct answer: Accumulates and is amplified by subsequent stages
Noise introduced in early stages is amplified along with the signal by all subsequent stages, making the first stage's noise contribution most critical.
Question 5: What is the significance of the 'noise figure' (NF) in a multistage receiver front-end?
- It indicates total harmonic distortion
- It measures how much the stage degrades signal-to-noise ratio (Correct answer)
- It specifies the maximum input signal level
- It defines the operating bandwidth
Correct answer: It measures how much the stage degrades signal-to-noise ratio
Noise figure quantifies the degradation of SNR as a signal passes through the amplifier stage; lower NF means less added noise.
Question 6: Friis's formula for cascaded noise figures shows that the overall noise figure is dominated primarily by:
- The last stage
- The stage with the highest gain
- The first stage (Correct answer)
- The stage with the lowest impedance
Correct answer: The first stage
Friis's formula shows that the first stage's noise figure contributes most to overall system noise, especially when it has low gain.
Question 7: Which topology is commonly used as the first stage in a low-noise amplifier (LNA) design?
- Common-base
- Common-emitter with inductive degeneration (Correct answer)
- Common-collector
- Class C tuned amplifier
Correct answer: Common-emitter with inductive degeneration
Common-emitter with inductive degeneration provides good noise performance and simultaneous noise and input impedance matching in LNA designs.
CET Staging & Prognostic Factors 3
Which factor most significantly determines the input impedance of a common-emitter transistor amplifier stage?