Question

What is the maximum collector efficiency of a transformer coupled Class A power amplifier?

• A. 30%
• B. 80%
• C. 45%
• D. 50%

Hint:

Amplifier Efficiencies (Max Theoretical). Class A (Series-fed): 25%. Class A (Transformer-coupled): 50%. Class B: 78.5%. Class C:>80% (often near 90%).

Answer 1

The Correct Option is D

The collector efficiency (\(\eta\)) of a power amplifier is the ratio of the AC power delivered to the load to the DC power drawn from the supply
For a Class A amplifier, the active device (transistor) conducts for the entire 360 degrees of the input cycle
- For a simple series-fed (resistive load in collector) Class A amplifier, the maximum theoretical efficiency is limited to 25%
- When a transformer is used to couple the load to the collector, it allows for better impedance matching and permits the collector voltage to swing ideally from near zero up to \(2V_{CC}\) (where \(V_{CC}\) is the DC supply voltage)
This larger AC voltage swing across the load allows for greater AC output power from the same DC input power compared to the series-fed case
The maximum theoretical collector efficiency for a transformer-coupled Class A amplifier is 50%