Amplifiers

Amplifiers make small signals bigger. Learn gain in V/V and decibels, common-emitter/source, common-collector (follower) and common-base configurations, input and output impedance, cascading stages, frequency response (coupling capacitors, bandwidth), distortion and amplifier classes A, B, AB, D.

Microphones, antennas and sensors produce microvolts to millivolts. Amplifiers raise these to useful levels without adding too much noise or distortion. Understanding gain, impedance and bandwidth is essential to any analog design.

An analogy

An amplifier is like a lever: a small movement at one end produces a bigger movement at the other — but only within its range before it hits the stops (clipping).

Configurations

Common-emitter/source: high voltage gain, inverting. Common-collector/drain (emitter/source follower): gain ≈ 1, high Z_in, low Z_out — a buffer. Common-base/gate: low Z_in, wide bandwidth — used in RF and cascodes.

Impedance matters

A high source impedance feeding a low Z_in loses signal; a high Z_out driving a low load loses signal. Buffers fix mismatches. Power amplifiers match loads (e.g. 8 Ω speakers) for maximum power transfer.

Frequency response

Coupling and bypass capacitors set the lower corner f_L; transistor and stray capacitances (Miller effect) set f_H. Bandwidth = f_H − f_L. Gain–bandwidth trade-offs appear everywhere.

Power amplifier classes

Class A (always conducting, linear, ≤ 25–50 % efficient), B (push-pull, 78.5 % max, crossover distortion), AB (small bias removes crossover), D (switching PWM, > 90 % efficient — modern audio), and RF classes C, E, F.

Formulas

• Gain in dB: A_dB = 20 log₁₀ |A_v| (power: 10 log₁₀ A_P)
• Cascaded gain: A_total = A₁ × A₂ × … (dB add)
• CE gain (unbypassed R_E): A_v ≈ −R_C / R_E
• Loading: v_load = A_v v_in × R_L/(R_L + Z_out) × Z_in/(Z_in + R_s)
• Coupling capacitor corner: f_L = 1 / (2π R_in C)

Worked example: Gains in dB

Given: Stage 1: ×10, stage 2: ×50

Find: Total gain.

• A = 500
• 20 log 500 = 54 dB (= 20 dB + 34 dB)

Answer: 500 V/V = 54 dB

Worked example: Coupling capacitor

Given: Z_in = 10 kΩ, C = 1 µF

Find: f_L.

• f_L = 1/(2π × 10k × 1µ) = 15.9 Hz

Answer: ≈ 16 Hz — fine for audio

Common mistakes

• Wrong: Ignoring loading between stages. — Right: Account for Z_out and Z_in, or buffer.
• Wrong: Using 20 log for power ratios. — Right: 10 log for power, 20 log for voltage/current.

Applications

• Sensor front-ends: Low-noise preamplifiers.
• Audio: Class AB and D power amplifiers.
• RF receivers: LNAs and IF amplifiers.

Key takeaways

• CE/CS gain, CC/CD buffer, CB/CG wideband.
• dB = 20 log V ratio.
• Mind Z_in and Z_out loading.
• Classes A, B, AB, D trade linearity vs efficiency.

Going further

• Cascode: Removes Miller effect for wide bandwidth.
• Noise figure: Friis formula for cascaded noise.
• Differential pairs: Foundation of op-amps.