Amplifier Interview Questions

Q1. What is voltage gain and how is it different from power gain?

Voltage gain Av = Vout/Vin is the ratio of output to input voltage amplitude, while power gain Ap = Pout/Pin accounts for both voltage and current changes and can differ significantly when input and output impedances are different. A common-emitter BC547 amplifier with RC = 4.7 kΩ and re = 26 Ω has Av = -181, but if it drives a 100 Ω load the actual power gain is much less because the output current into 100 Ω is limited by the available Thevenin output resistance. In audio power amplifiers like the TDA2030, specifying both voltage swing and current delivery (Ap in watts) is more meaningful than voltage gain alone.

Follow-up: Why is current gain important in the final stage of an audio amplifier?

Q2. What are the different classes of amplifiers (A, B, AB, C) and what is the efficiency of each?

Class A amplifiers conduct for the full 360° of input cycle, with maximum efficiency 25% (transformer coupled 50%), but produce low distortion, used in precision audio preamps like the MC33078 front end. Class B amplifiers use complementary push-pull stages each conducting 180°, with max efficiency 78.5%, but suffer from crossover distortion at zero-crossing. Class AB (used in the LM386 audio IC) biases both transistors with a small current to overlap slightly, achieving 60–70% efficiency without crossover distortion. Class C amplifiers conduct for less than 180° and are used only in RF power amplifiers (like PA stages in transmitters) where a tuned circuit reconstructs the waveform at up to 90% efficiency.

Follow-up: What is crossover distortion and how does Class AB eliminate it?

Q3. What is the bandwidth of an amplifier and what determines it?

Bandwidth is the frequency range over which the amplifier's gain stays within 3 dB of its midband value, bounded by the lower -3 dB frequency (fL, set by coupling/bypass capacitors) and upper -3 dB frequency (fH, set by transistor capacitances). A BC547 CE amplifier with 10 µF coupling capacitors and 4.7 kΩ collector resistance has fL ≈ 3 Hz and fH limited by fT/β + Miller capacitance, typically a few MHz for small-signal stages. Wider bandwidth requires reducing coupling capacitor values (raises fL), using transistors with higher fT, and using cascode topology to minimize Miller effect.

Follow-up: What is the gain-bandwidth product and why is it constant for a given amplifier topology?

Q4. What is coupling capacitor and bypass capacitor, and what happens if you remove them?

Coupling capacitors (like 10 µF electrolytic) block DC between stages while passing AC signals, allowing each stage to be independently DC biased; removing one couples the DC level of one stage into the next, shifting its operating point and causing distortion or saturation. Bypass capacitors (like 100 µF across RE in a CE stage) short-circuit the emitter resistor at AC frequencies, restoring full AC gain while RE still provides DC bias stability; removing the bypass capacitor reduces gain by the factor (1 + RE/re) from -RC/re to -RC/(RE + re). For a BC547 stage with RE = 470 Ω and re = 26 Ω, removing the 100 µF bypass reduces Av from -181 to -9.6.

Follow-up: How does the value of the bypass capacitor affect the lower -3 dB frequency of the amplifier?

Q5. What is a differential amplifier and why is it used as the input stage of op-amps?

A differential amplifier amplifies the voltage difference between two inputs while rejecting any voltage common to both (common-mode signal), giving high CMRR that makes it immune to power supply noise and ground loops. The LM741 uses a differential pair of NPN transistors with active current mirror load, achieving CMRR = 90 dB with open-loop gain = 200,000. Using a differential pair as input stage means the op-amp is sensitive only to what's different between the two inputs — power supply ripple, temperature drift, and electromagnetic interference on both inputs cancel out automatically.

Follow-up: How does a current mirror improve the CMRR of a differential amplifier?

Q6. What is the output resistance of an amplifier and why does it matter?

Output resistance (Rout) is the Thevenin equivalent resistance seen looking back into the amplifier output; it forms a voltage divider with the load resistance, causing gain to drop when the load is low impedance. A CE amplifier with RC = 4.7 kΩ has Rout ≈ RC || ro ≈ 4.5 kΩ; connecting a 100 Ω speaker directly reduces the gain by 4700/4800 ≈ 97%, making it useless. This is why emitter followers or power buffer stages (like the BD139) with Rout ≈ 5–50 Ω are always added between a voltage amplifier and low-impedance loads like speakers or motors.

Follow-up: How does negative feedback reduce the output resistance of an amplifier?

Q7. What is the input resistance of a CE amplifier and how do you maximize it?

The input resistance of a CE amplifier is Rin = R1 || R2 || (β × re), where R1, R2 are voltage divider bias resistors and β × re is the transistor's base input resistance. For a BC547 with β = 200, IC = 1 mA (re = 26 Ω): β×re = 5.2 kΩ; with R1 = 47 kΩ and R2 = 10 kΩ the input resistance is 47k || 10k || 5.2k ≈ 3.3 kΩ. To maximize Rin, increase β×re by reducing IC, use higher R1/R2 values (at the cost of reduced bias stability), or use an FET-input stage like a 2N4416 where Rin is gigaohms due to the FET gate.

Follow-up: Why does reducing collector current increase input resistance?

Q8. What is frequency response of an amplifier and how is it plotted on a Bode plot?

Frequency response is the variation of gain (and phase) with frequency, typically showing a flat midband region with gain dropping at low frequencies (coupling cap poles) and high frequencies (transistor capacitance poles). On a Bode magnitude plot, each RC pole gives -20 dB/decade rolloff and -45° phase shift at the corner frequency; a 2-pole response rolls off at -40 dB/decade. For a BC547 CE stage, the Bode plot would show flat gain of ~40 dB (×100) from about 10 Hz to 1 MHz, with -3 dB points determined by coupling capacitor values below and transistor fT above.

Follow-up: How do you find the -3 dB frequency experimentally using a function generator and oscilloscope?

Q9. What is harmonic distortion in an amplifier and how is it measured?

Harmonic distortion is the generation of output frequency components at integer multiples of the input frequency due to nonlinearity in the transistor's I-V characteristic, degrading signal fidelity. Total harmonic distortion (THD) for a BC547 CE stage operating at 1 Vpp output is typically 1–5%, while a properly designed push-pull Class AB stage like the LM1875 achieves THD < 0.015% at 1 W output at 1 kHz. THD is measured by applying a pure sine wave, performing an FFT on the output, and summing the power in harmonics relative to the fundamental — the results guide whether a feedback loop or better circuit topology is needed.

Follow-up: How does negative feedback reduce harmonic distortion?

Q10. What is a cascode amplifier and what are its advantages?

A cascode amplifier stacks a common-base (or common-gate in MOSFET) stage on top of a common-emitter stage, so the CB stage absorbs almost all of the VCE voltage swing and shields the CE stage from Miller capacitance multiplication, dramatically extending bandwidth. A 2N2222 + 2N2222 cascode with RC = 2.2 kΩ can achieve -3 dB bandwidth of 50–100 MHz versus less than 5 MHz for a single CE stage at similar gain. The cascode is the fundamental building block of high-frequency LNA designs in receivers and wideband instrumentation amplifiers.

Follow-up: What is the voltage gain of the cascode amplifier compared to a single CE stage?

Q11. What is noise figure (NF) in an amplifier and why is it critical in receiver design?

Noise figure is the ratio (in dB) of the signal-to-noise ratio at the input to the SNR at the output, quantifying how much noise the amplifier itself adds to the signal. The BFU520 low-noise BJT achieves NF = 0.8 dB at 2 GHz, meaning only 0.8 dB of SNR is lost in the LNA stage. In a cellular receiver with -100 dBm sensitivity requirement, the LNA's noise figure directly sets the minimum detectable signal; a 3 dB higher NF means the system loses sensitivity over 1 km of range, which is why Qualcomm's RF designers specify NF < 2 dB for LNAs in basestation receivers.

Follow-up: What is the Friis formula for noise figure in a cascade of amplifier stages?

Q12. What is the difference between single-ended and differential output stages in amplifiers?

A single-ended output references the signal to ground and is susceptible to ground noise and common-mode interference, while a differential output carries the signal on two complementary lines and common-mode noise is rejected by the receiver. The AD8138 differential amplifier driver has 78 dB CMRR and is used to drive long cable runs to ADC inputs in industrial data acquisition systems at Bosch and ABB where electromagnetic interference from motors is severe. Single-ended is sufficient for short PCB connections, but differential signaling (like LVDS used in high-speed serial links at 1.2 Gbps) is essential for boards where high-frequency interference would corrupt single-ended signals.

Follow-up: What is LVDS and where is it used?

Q13. What is the effect of temperature on amplifier gain?

Amplifier gain is affected by temperature through BJT beta variation (increases ~0.5%/°C), transconductance gm = IC/VT (decreases as T increases because VT = kT/q increases), and resistor temperature coefficients. A CE BC547 amplifier designed for Av = -100 at 25°C may show Av = -90 at 75°C because higher temperature raises re = VT/IC, reducing gm. Using emitter degeneration (bypass capacitor removed) makes gain Av ≈ -RC/RE independent of transistor parameters, reducing gain temperature sensitivity to only the resistor TC of ±100 ppm/°C for standard metal film resistors.

Follow-up: How does the transconductance of a MOSFET vary with temperature?

Q14. What is the maximum unambiguous bandwidth of an amplifier determined by the sampling theorem?

If an amplifier's output is sampled by an ADC, the Nyquist theorem requires the amplifier bandwidth to be less than half the sampling rate (fs/2) to avoid aliasing, where high-frequency signal components fold back and corrupt the baseband signal. For a 100 kSPS ADC like the ADS1115 with fs = 100 kHz, an anti-aliasing low-pass filter at the amplifier output must roll off above 50 kHz. In audio recording at 48 kHz sampling rate, amplifier and filter bandwidth must be controlled below 24 kHz, which is why audio codec front-ends include a sharp anti-aliasing filter before the ADC.

Follow-up: What type of filter is typically used as an anti-aliasing filter and why?

Q15. What is an RF power amplifier and how does it differ from a small-signal amplifier?

An RF power amplifier delivers significant power (watts to kilowatts) to an antenna or transmission line at radio frequencies, operating in nonlinear classes (B, C, F) for efficiency and using impedance matching networks to transfer maximum power, unlike a small-signal amplifier which operates linearly to preserve waveform. The RD06HVF1 RF power MOSFET can deliver 6 W at 175 MHz in Class C with 70% efficiency for FM broadcast transmitters. Linearity (EVM, ACLR) matters more in modern modulated systems like LTE, so Doherty amplifier architectures combining Class AB and Class C are used in basestations to achieve both efficiency and linearity simultaneously.

Follow-up: What is 1 dB compression point and why is it used to characterize RF power amplifiers?