EE321 - Analog Electronics Notes on Power Amplifiers and Classifications

Recap of Common-Emitter Amplifier

• Quiescent Conditions:
  • Given values:
  • $V_{C} = 15V$
  • $I_{C} = 15mA$
• For small-signal analysis, the base-emitter voltage $V_{BE}$ varies as base current changes.

Biasing in Amplifiers

• Characteristic curve outlines relationship between base-emitter voltage and collector current.
• The graph demonstrates the exponential relationship,
  • $I<em>{C} = I</em>{S} (e^{(V<em>{BE}/V</em>{T})} - 1)$
    where $V<em>{T}$ is thermal voltage and $I</em>{S}$ is reverse saturation current.

Small Signal Operation

• As input voltage $V_{in}$ changes:
  • Base-emitter voltage $V_{BE}$ mirrors this change.
  • Collector current is given by $I<em>{C} = g</em>{m} imes V<em>{BE}$ (where $g</em>{m}$ is mutual conductance).
• Voltage across collector resistor $R_{C}$ impacts overall output voltage.

Mutual Conductance, $g_{m}$

• Defined as: g{m} = rac{dI{C}}{dV_{BE}}
• It’s not a physical conductance but a measure of the slope of the $I<em>{C}$ - $V</em>{BE}$ curve; varies with bias current.

Voltage Gain of Amplifier

• Voltage gain formula derived from output voltage and input current:
  • A{v} = -rac{R{C}}{r_{in}}
• Typical configuration can yield a gain of:
  • $A_{v} = -100$
• Output characteristics significantly depend on the load.

Loaded Common-Emitter Amplifier

• For a common-emitter configuration,
• Gain decreases with lower load impedance, which also influences $g<em>{m}$ and resulting $r</em>{in}$.

Common-Emitter Amplifier Limitations

• Often difficult to achieve both high voltage and current gain.
• Multi-stage amplifiers are solutions to bridge efficiency gaps:
  • Use differential amplifiers for initial stage,
  • Common-emitter stages for voltage gain.

Classifications of Power Amplifiers

• Different classes include Class A, B, AB, C, and D which define operational characteristics based on conduction angle:
  • Class A: High linearity, low efficiency, conduction angle of $360^{ ext{o}}$.
  • Class B: More efficient (up to $78.5 ext{ m}$) with a conduction angle of $180^{ ext{o}}$.
  • Class AB: Combination of A and B; operational mostly at >180 and <360 degrees.
  • Class C: Ideal for tuned circuits, conducts less than half the cycle, high efficiency.
  • Class D: Utilizes pulse width modulation; efficient (theoretically approximates $100 ext{ m}$).

Efficiency and Power Dissipation

• Defined as:
  • ext{Efficiency, } ext{η} = rac{P{out}}{P{in}}
• Calculating efficiency involves further considerations of output power from load and total power supply.
• High dissipation leads to heat management challenges.

Thermal Considerations and Heat Management

• Transistor temperature rises due to power dissipation, which, if not managed, changes device characteristics and may lead to failure.
• Use heatsinks to improve thermal dissipation efficiency, expressed as:
  • $θ<em>{JA} = θ</em>{JC} + θ_{CA}$
• Thermal derating curve helps specify maximum thermal conditions under operational constraints.

Design Considerations for Power Amplifiers

• Must include:
  • Selection of appropriate output stage based on application requirements,
  • Linearity vs. efficiency trade-offs,
  • Safe operational parameters to avoid thermal runaway, especially in Class AB designs.

Examples of Circuit Topologies

• Class A: emitter followers maintaining a constant bias current.
• Class B: unbiased configuration for lower power dissipation and minimized heat.
• Class AB: arrangement to minimize cross-over distortion while ensuring high efficiency.
• Class C & D: required for applications needing high efficiency with special requirements on load characteristics.