RFAmplifier Classes: A, B, AB, and C - A Comparison
amplifier
class a
class b
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class c
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This article compares different amplifier classes: Class A, Class B, Class AB, and Class C, highlighting their key characteristics and performance.
Amplifier Class-A Operation
- In Class-A operation, the amplifier operates in the active region for the entire input cycle (2π).
- Stage efficiency = PL(max) / PS x 100% where PS is the DC input power.
- Load Power = PL = (VL)2 / RL = (VPP)2 / (8 * RL)
Amplifier Class-B Operation
- In Class-B operation, current flows for only half of the input cycle (π).
- The Q-point is located at the cutoff point on the DC and AC load lines. VCEQ = VCC / 2
- AC load power of a Class-B push-pull amplifier: PL = (VPP)2 / (8 * RL)
Amplifier Class-AB Operation
- In Class-AB operation, the amplifier operates between half and the full input cycle (between π and 2π).
Amplifier Class-C Operation
- In Class-C operation, the amplifier operates for less than half of the input cycle (less than π).
- It’s primarily a tuned amplifier.
- AC load power for a Class-C amplifier: PL = (VPP)2 / (8 * RL)
- Where VPP is the peak-to-peak load voltage and RL is the load resistance.
Comparison Table: Class A, B, AB, and C
| Amplifier Class Type | Q point | Iq | Conduction Angle | Maximum Efficiency |
|---|---|---|---|---|
| Class A | 0.5 | 0.5 | 2π | 50% |
| Class B | 0 | 0 | π | 78% |
| Class AB | 0-0.5 | 0-0.5 | π-2π | 50-78% |
| Class C | <0 | 0 | 0-π | Approaches 100% |
To operate a transistor (FET) in a particular class, the gate and drain DC voltages must be carefully biased to a specific operating point, called the Quiescent point or Q-point. Class A offers the least distortion, while Class C offers the maximum distortion of the output signal.
