Interview questions & answers
Q1. What are the defining characteristics of a common base amplifier?
The common base amplifier has low input impedance, high output impedance, voltage gain greater than 1, current gain less than 1, and is non-inverting. In a practical circuit using a 2N3904 with RC = 10kΩ and RE = 100Ω, voltage gain can reach 100 or more. The near-unity current gain (α ≈ 0.99) means the CB stage is fundamentally a current buffer that converts a low-impedance current input into a high-impedance voltage output.
Follow-up: How does the current gain of the CB configuration compare to the CE configuration?
Q2. Why is the input impedance of a common base amplifier very low?
The input impedance is low because the signal is injected into the emitter, which sees only the small-signal emitter resistance re = VT/IC directly. For IC = 1mA, re = 26Ω, so the input impedance is approximately 26Ω — far lower than the common emitter configuration. This makes the CB amplifier a natural match for low-impedance current sources like photodiodes and current-output sensors.
Follow-up: How do you match a 50Ω antenna or transmission line to a common base input stage?
Q3. What is the current gain of a common base amplifier and what is it called?
The current gain of a CB amplifier is α (alpha), defined as IC/IE, and is always slightly less than 1 (typically 0.95–0.99 for small-signal transistors). For a BC107 with β = 100, α = 100/101 ≈ 0.99. This near-unity current gain means almost all emitter current reaches the collector, making α a measure of transistor efficiency.
Follow-up: What is the relationship between α and β, and how do you derive it?
Q4. Why is the common base amplifier used at high frequencies and in RF circuits?
The CB amplifier is preferred at high frequencies because the base terminal is grounded (AC), which eliminates the Miller effect — the collector-base capacitance CBC does not get multiplied by voltage gain as it does in the common emitter. A cascode amplifier using a 2N5179 in CB configuration can operate usefully above 500MHz where the CE stage alone would have its bandwidth destroyed by Miller multiplication. This makes CB the topology of choice for LNAs and RF front-ends.
Follow-up: What is a cascode amplifier and why does it combine CE and CB stages?
Q5. Does the common base amplifier invert the output signal?
No, the common base amplifier is non-inverting — when emitter current increases (input goes positive), more current flows through the collector load RC, increasing the voltage drop across RC and raising the output voltage at the collector. This is opposite to the common emitter where increased base drive pulls the collector low. The non-inverting property is useful in certain feedback and oscillator topologies.
Follow-up: In which BJT configuration is the output signal inverted with respect to the input?
Q6. How do you calculate the voltage gain of a common base amplifier?
Voltage gain Av = α × RC / re, where re = VT/IC. For IC = 1mA (re = 26Ω), α ≈ 1, and RC = 5.6kΩ, Av = 5600/26 ≈ 215. This gain is the same magnitude as the common emitter stage but without phase inversion, and it can be very large because the output resistance RC is not degenerated.
Follow-up: How would adding an emitter degeneration resistor in series with the input affect CB amplifier gain?
Q7. What is the output impedance of a common base amplifier?
The output impedance of the CB amplifier is very high — ideally it equals the collector output resistance ro of the transistor, which can be hundreds of kilohms for small-signal devices. For a 2N3904 at IC = 1mA, ro ≈ VA/IC where VA (Early voltage) ≈ 100V, giving ro ≈ 100kΩ. This high output impedance makes it behave like a current source at the output, which is exploited in cascode current mirrors.
Follow-up: How does Early effect influence the output impedance of a CB stage?
Q8. What is the practical application of the common base amplifier in a transimpedance amplifier context?
In photodiode receiver front-ends, the CB amplifier is used to amplify the small current from a reverse-biased photodiode (like the BPW34) because its low input impedance quickly absorbs the current without building up a voltage that would slow down the diode response. The CB stage converts this current to voltage across a collector resistor, achieving high bandwidth. This is the basis of high-speed optical receiver circuits used in fiber optic systems.
Follow-up: Why is bandwidth of a photodiode receiver limited by its junction capacitance, and how does the CB input impedance help?
Q9. How does the common base amplifier compare to common emitter in terms of gain-bandwidth product?
Both share the same fT (transition frequency) of the transistor, but the CB amplifier achieves this bandwidth at much higher voltage gains because Miller effect is absent. A 2N5179 with fT = 900MHz in CE configuration with a gain of 20 would show bandwidth of about 45MHz due to Miller effect, whereas the same transistor in CB achieves near-fT bandwidth even at high gain. This is why cascode (CE+CB) is used to get high gain and high bandwidth together.
Follow-up: Define fT of a transistor and how is it measured?
Q10. What biasing challenges does the common base amplifier present compared to common emitter?
In the CB configuration, the signal is fed into the emitter, so a DC bias current must flow through the emitter even as the AC signal source is connected, requiring careful design to avoid loading or disturbing the bias point. Typically, the base is held at a fixed DC voltage via a voltage divider, and a large bypass capacitor (10µF or more) is placed at the base to make it an AC ground. Forgetting the base bypass capacitor is the most common practical mistake — without it, the circuit behaves unpredictably.
Follow-up: What is the effect on gain if the base bypass capacitor in a CB amplifier is removed?
Q11. Explain the role of the common base amplifier in a cascode configuration.
In a cascode, the CE stage amplifies the input signal and passes its output current to the emitter of a CB stage, which presents a low-impedance current sink to the CE collector — reducing the CE collector voltage swing and therefore killing the Miller effect. An LNA cascode using BFR92 transistors achieves gains of 15–20dB at 2GHz with noise figures under 2dB. The CB stage in the cascode does the impedance transformation and isolates input from output, improving both gain and stability.
Follow-up: What is the noise advantage of a cascode LNA over a single-stage CE LNA?
Q12. What is the relationship between α and β in a BJT?
α = β/(β+1), and conversely β = α/(1−α), derived from the current relationships IE = IC + IB, IC = αIE, and IC = βIB. For β = 150, α = 150/151 ≈ 0.993. At low β values (say β = 10), α = 0.909, which means 9% of emitter current is lost to base recombination — a significant inefficiency that shows why high-β transistors are preferred for CB stages.
Follow-up: How does β variation over temperature affect the CB amplifier's performance?
Q13. How is the common base amplifier used in oscillator circuits?
The CB amplifier is used in Colpitts and Clapp oscillators where its non-inverting gain and high output impedance are leveraged to sustain oscillation with a capacitive feedback network at the emitter. In a Colpitts VCO for a 10MHz crystal oscillator, a common-base stage maintains the oscillation by compensating for tank circuit losses. The CB stage's non-inverting nature means the feedback network needs to provide 0° phase shift, which the capacitive divider naturally does.
Follow-up: How does changing the capacitor ratio in a Colpitts oscillator affect the feedback and oscillation condition?
Q14. Why is the common base amplifier rarely used as a standalone general-purpose amplifier?
The CB amplifier's low input impedance (typically 20–50Ω) makes it difficult to interface with most signal sources without impedance mismatch and signal loss. An audio sensor with a 10kΩ source impedance driving a CB input of 26Ω would lose virtually all signal voltage due to this mismatch. It is therefore used only in specific applications — high-frequency RF, current-input circuits, and cascode configurations — where its properties are a direct advantage.
Follow-up: How would you design an impedance-matching network to interface a 600Ω audio source with a CB amplifier input?
Q15. What is the noise performance of the common base amplifier at high frequencies?
The common base amplifier can achieve excellent noise performance at RF frequencies because the base is grounded, reducing the contribution of base resistance (rbb) to the noise figure. In an LNA using the BFG540 transistor in CB configuration at 900MHz, noise figures of 1.5–2dB are achievable. The noise performance degrades if the base is not properly RF-grounded, as any residual base impedance introduces noise feedback.
Follow-up: What is noise figure and how do you measure it for an RF amplifier stage?
Common misconceptions
Misconception: The common base amplifier has current gain greater than 1 because it is called an amplifier.
Correct: The CB amplifier's current gain α is always less than 1 (typically 0.95–0.99); its amplification comes from voltage gain, not current gain.
Misconception: The output of a common base amplifier is taken from the base terminal.
Correct: In the common base configuration, the base is the common (grounded) terminal; input is at the emitter and output is taken from the collector.
Misconception: The CB amplifier inverts the output like the common emitter amplifier.
Correct: The common base amplifier is non-inverting — rising emitter current increases collector current and raises output voltage across the collector load.
Misconception: Miller effect is a major bandwidth limiter in the common base amplifier just as in the common emitter.
Correct: Miller effect is negligible in the CB configuration because the base is at AC ground, so CBC sees nearly zero voltage gain across it and is not multiplied.