Power Amplifier Lecture Notes

Series Fed Class A Amplifier

• Power Transistor:
  • Generally, less than 100 gain.
  • Used for handling power/current with little voltage gain.
• DC Bias Operation:
  • Equation: ( V_{CC} = I_B R_B + V_{BE} )
  • Collector current ( I_C = I_B )
  • Voltage equation: ( V_{CC} = V_{CE} + I_C R_C )
• Efficiency:
  • Optimal Q point at the midpoint of the DC load line.
  • General efficiency formula: ( % = \frac{P_o (ac)}{P_{in} (dc)} \times 100% )
  • Ideal efficiency max is 25%, practical is 10-20%._

Transformer Coupled Class A Amplifier

• Transformers:
  • Turn ratio ( (\frac{1}{2})^2 R_L = 2 R_L )
  • Maximum permissible decrease in ( V_{CE} ) is ( V_{CEQ} = V_{CC} ).
  • Intercept on ( V_{CE} ) axis should be ( 2V_{CC} ).
• Efficiency:
  • Maximum efficiency is about 50%, practically less than 40% due to various losses._

Class B Amplifier

• General Efficiency:
  • Equation: ( % = \frac{P_o (ac)}{P_{in} (dc)} \times 100% )
  • Maximum efficiency can be 78.5%.
• Power Dissipation:
  • Equation for power loss: ( P_D(2Q) = P_i(dc) - P_o(ac) )
  • Maximum power dissipation when ( V_{LP} = 2V_{CC} )._

Phase Splitter Circuit

• **Push-Pull Class B Amplifier: **
  • Uses transformers to couple output and input.
  • Cross-over distortion occurs at zero-voltage condition.
  • Class AB biasing reduces distortion by keeping transistors ON longer.

Complementary Symmetry Class B Push Pull Amplifier

• Transistors:
  • Requires matched transistors and separate voltage supplies.
  • Using Kirchhoff's law to determine max output voltage.

Darlington and Quasi Complementary Push Pull Amplifiers

• Darlington Connected Transistors:
  • Higher output current, lower resistance.
  • Provides efficient push-pull operation.

Tuned Power Amplifiers

• Purpose:
  • Amplifies selective frequency using LC networks.
  • Acts like a band-pass filter.
• Single Tuned Amplifier:
  • Uses parallel LCR circuits.
  • Gain and bandwidth determined by component values.

Stagger-tuned Voltage Amplifier

• **Design: **
  • Uses multiple stages with slightly different frequencies for increased bandwidth.
  • For optimized tuning, achieves Butterworth response.

Synchronous Tuning

• Provides sharp curves with minimal bandwidth.
• 3 dB bandwidth related to tuned circuit characteristics.