Fundamentals of Amplifier Circuits

What is Amplification

Amplification — Using a small signal to control a large signal; the output is a linearly amplified version of the input.

Vin (small signal) → [Amplifier] → Vout (large signal, proportional to Vin)

Av (Voltage Gain) = Vout / Vin
Ai (Current Gain) = Iout / Iin
Ap (Power Gain) = Pout / Pin

Gain is commonly expressed in decibels (dB):
  Av(dB) = 20 × log₁₀(Vout/Vin)
  Ap(dB) = 10 × log₁₀(Pout/Pin)

Biasing

Why Biasing is Needed

An amplifier requires a DC operating point so that the signal swings within the linear region of the device.

No Bias:            With Bias:
   ↑ Vout             ↑ Vout
   │  ┌──┐            │    ╱╲
   │  │  │            │   ╱  ╲   ← Complete waveform
   │──┘  └──          │──╱    ╲──
   └─ Clipped         └─────────

BJT Voltage Divider Bias (Most Common)

         Vcc
          │
          R1
          │
    ┌─────┼── Base
    │     │     Vb = Vcc × R2/(R1+R2)
    R2    Re     Ve = Vb - 0.7V
    │     │     Ic ≈ Ie = Ve/Re
    └─────┼── GND

Re is key: Provides negative feedback to stabilize the operating point.
Ce (in parallel with Re) is optional: Bypasses AC signals to increase gain.

Three Basic Configurations

Common Emitter / Common Source (CE/CS) — The Workhorse Amplifier

          Vcc
           │
           Rc
           │
        ┌──┴── Vout  (Inverted!)
        │
      C │
Vin ─┤├─B   NPN
  Rb   │
       E │
         │
         Re
         │
        GND

Characteristics:
  Av = -gm × Rc  (High gain, inverted)
  Rin ≈ Rb ∥ rπ  (Medium)
  Rout ≈ Rc       (Medium)
Usage: Primary voltage amplifier

Common Collector / Common Drain (CC/CD) — Emitter/Source Follower

Characteristics:
  Av ≈ 1   (Non-inverted)
  Rin High   (High impedance isolation)
  Rout Low   (Can drive low-impedance loads)

"Follower" = Output voltage follows input voltage
Usage: Buffer, impedance matching, output stage
Application: Voltage followers are used anywhere you need "high input impedance and low output impedance"

Common Base / Common Gate (CB/CG)

Characteristics:
  Av High
  Rin Very Low
  Wide Bandwidth (No Miller Effect)
Usage: High-frequency amplification, current buffer

Comparison of Three Configurations

Frequency Response

Sources of Bandwidth Limitation

Low-frequency attenuation: Coupling capacitors (block DC, pass AC)
High-frequency attenuation: Parasitic capacitances (Miller Effect)

      Av
      ↑
  Av(mid) ┌──────────┐
          │  Flat Region │
  -3dB ───┼──────────┼───
         ╱            ╲
        ╱   BW Bandwidth ╲
  ─────┴──────────────┴─────→ f
       fL            fH

BW = fH - fL  (Typically fH >> fL, so BW ≈ fH)

Miller Effect

In a common-emitter stage, Ccb is connected between input and output:
  Cin(miller) = Ccb × (1 + |Av|)

The capacitance is amplified by a factor of (1+|Av|)!
→ This is the main reason for limited high-frequency response.
→ Common-base stages do not have the Miller Effect → Suitable for high frequencies.

Negative Feedback

Why Add Feedback

Open-loop: High gain but unstable, high distortion, narrow bandwidth
Closed-loop: Sacrifice gain for stability, low distortion, and wide bandwidth

Closed-loop gain: Af = A / (1 + Aβ)
  A: Open-loop gain
  β: Feedback factor

If Aβ >> 1: Af ≈ 1/β  (Gain is determined entirely by external components!)

Four Feedback Topologies

Distortion

Harmonic Distortion (THD): Input is a sine wave, output contains harmonics.
Crossover Distortion: "Dead zone" near the zero-crossing in push-pull output stages.
Clipping: Signal exceeds power rails → Truncated peaks.
Saturation/Cutoff: Transistor operating point deviates from the linear region.

Keywords: Amplifier, Biasing, Common Emitter, Emitter Follower, Gain, Bandwidth, Miller Effect, Negative Feedback