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1. BJT Fundamentals & Components

• BJT Basics: A current-controlled device where input base current ($I_B$) controls output collector current ($I_C$); $I_C=\beta \times I_B$.
• Operating Regions:
  • Active: For amplification.
  • Cutoff: Switch OFF state; base-emitter junction reverse-biased, no current flows. Output cannot be amplified as the device is unresponsive.
  • Saturation: Switch ON state; fully conducting at maximum current. Output cannot be amplified as input changes have no effect.
• Resistors & Capacitors:
  • Emitter Resistor ($R_E$): Stabilizes biasing and improves thermal stability.
  • Coupling Capacitor: Blocks DC between stages to maintain independent biasing, passes AC.
  • Bypass Capacitor: Bypasses AC around $R_E$ to prevent negative feedback, maximizing AC gain without affecting DC bias; filters noise.

2. BJT Configurations

• Common Emitter (CE) / Grounded Emitter: Emitter grounded; Input=Base, Output=Collector. E-B is forward-biased, C-B reverse-biased. Most $I_E$ flows through collector.
  • Parameters: High $A_v$ (>1), Moderate $A_i$ ($\beta \approx 50$), High power gain. Moderate $Z_i$ (1k-5k$\Omega$), Low-to-Moderate $Z_o$ (1k-50k$\Omega$, roughly equals $R_C$).
  • Phase Inversion (180°): $V_{in}\uparrow \to I_B\uparrow \to I_C\uparrow \to$ voltage drop across $R_C\uparrow \to V_{out}\downarrow$.
• Common Collector (CC) / Emitter Follower: Collector grounded; Input=Base, Output=Emitter.
  • Parameters: Low $A_v$ (<1, due to $0.7V$ $V_{BE}$ drop: $V_E=V_B-V_{BE}$), High $A_i$ ($\approx 50$), 0° phase shift. High $Z_i$ (20k-1000k$\Omega$, $\approx \beta R_L$), Low $Z_o$ (20-1k$\Omega$).
  • Uses: Buffering high-$Z$ sources to low-$Z$ loads, impedance matching.
  • Bypassing $R_E$ Effect: Massively increases AC voltage gain (removes negative feedback) and high-frequency gain (acts as a high-pass filter), but decreases thermal stability and increases distortion. DC bias remains unaffected.
• Common Base (CB): High $A_v$, Low $A_i$ (<1), Low $Z_i$, High $Z_o$, 0° phase shift. Used for high-frequency applications.

3. Coupling Techniques

• RC Coupling: Connects stages via a load resistor (R) and series capacitor (C).
  • Characteristics: Passes AC, blocks DC. High $Z_i$, Low $Z_o$, High $A_v$. Flat/wide mid-band frequency response. Used in audio/general amps.
  • Frequency Roll-off: Low frequencies drop ($Z_c$ increases, blocking signal); High frequencies drop due to internal capacitance.
  • Pros/Cons: Simple, cheap, good signal quality, DC isolation. Cons: Poor impedance matching, low voltage/power gain, frequency roll-off, temperature sensitive.
• Direct Coupling: Cascaded stages physically wired together; no reactive components.
  • Characteristics: Passes both AC and DC (0 Hz) signals, yielding a completely flat response with no low-frequency cutoff.
  • Pros/Cons: Simple, compact, low distortion, stable phase, no reactive power loss. Cons: Thermal instability (temperature variations altering $V_{BE}$ directly propagate to the next stage), amplifies DC drift/noise, complex biasing networks.

4. Advanced Power Amplifiers

• Darlington Pair: Two cascaded BJTs; Emitter 1 feeds Base 2, collectors tied together, output at Emitter 2. Can be connected in series (higher voltage rating) or parallel (higher current handling).
  • Parameters: Massive current gain (${\beta }_{Total}={\beta }_1\times {\beta }_2\approx 20,000$). Extremely high $Z_i$ (easy to drive) and Low $Z_o$ (excellent power transfer).
  • Pros/Cons: Compact, simplifies design, capable of power amplification. Cons: High voltage drop (1.2-1.4V), slower switching, high power dissipation (heat), sensitive to DC drift, amplifies leakage current.
• Complementary Power Amplifier: Push-pull operation using matched NPN (pushes positive cycle) and PNP (pulls negative cycle) transistors to drive the output without transformers. $A_v<1$ because it is built for current amplification and suffers a 0.7V drop per transistor.
  • Class A: 360° conduction, Q-point centered, 25-50% efficiency, 0 distortion, used in outdoor musical symphonies.
  • Class B: 180° conduction, Q-point on x-axis, 78.5% efficiency, suffers from crossover distortion at the zero-crossing gap.
  • Class AB: Conduction >180° but <360°, Q-point slightly above x-axis. Complex biasing keeps transistors slightly ON to reduce crossover distortion, ~78.5% efficiency.
  • Class C: <180° conduction, Q-point below x-axis, 95% efficiency, maximum distortion, RF/audio power.
  • Pros/Cons: Highly efficient, simple, good power. Cons: Crossover distortion, thermal stability issues, complex biasing (for AB).

5. FET Voltage Amplifiers

• JFET (Junction FET): Unipolar, voltage-controlled device (electric field applied to PN junction). Uses majority carriers only. Applications: Voltage-controlled resistors, amplifiers.
  • Characteristics: Extremely high $Z_i$ (prevents source loading), low noise, highly linear (low distortion). Has lower $A_v$ than a BJT. Leakage gate current can add to $I_D$.
  • Biasing: To prevent distortion and loss of $Z_i$, $V_{GS}$ must never forward bias the diode. Separate bias is expensive/needs tuning; using an $R_S$ resistor stabilizes the Q-point via feedback. For resistive loads, gate bias is chosen so half the supply voltage drops across the load resistor. Bypassing $R_S$ increases gain.
  • Pinch-off Voltage: The $V_{GS}$ where the channel is entirely depleted.
• MOSFET (Metal-Oxide-Semiconductor FET): Voltage-controlled via a gate insulator.
  • Enhancement Mode: Requires a gate voltage ($>V_T$) to create a channel. Used in digital circuits, switches, amplifiers.
  • Depletion Mode (D-MOS): Channel exists at 0V; a threshold voltage turns it OFF. Used in oscillators, V-controlled resistors, amplifiers. D-MOS Amp characteristics: Biased similarly to JFETs. Gate bias increases $Z_i$. For absolute maximum $A_v$: requires high $Z_L$, zero bias, and bypassed $R_S$. In p-channel D-MOS, gate requires negative bias to reduce current.
• FET Operating Regions:
  • Ohmic Region: Channel partially open; operates as a voltage-controlled resistor ($I_D$ linear with $V_{DS}$).
  • Active / Saturation Region: Channel pinched off; operates as a constant-current source ($I_D$ is constant).
  • Cut-off Region: Channel completely closed; open circuit ($I_D=0$).