Class C Amplifier Theory: RF Efficiency, Tank Circuits, and Design Math

A Class C amplifier is a highly efficient radio frequency (RF) power amplifier where the active transistor conducts for less than 180 degrees of the input signal cycle, relying on a tuned LC tank circuit to reconstruct the full output sine wave. In a real circuit, shifting from a linear Class AB topology to Class C changes the transistor's DC bias point from slightly above cutoff to deeply below cutoff. This eliminates quiescent current draw and pushes DC-to-RF efficiency up to 90%, but it completely destroys amplitude linearity. Because it clips the bottom half (and part of the top half) of the input waveform, it is strictly used for constant-envelope RF signals like FM, CW (Morse code), and unmodulated carrier waves.

Makers and students frequently confuse Class C with Class D amplifiers. While both are highly efficient, Class D uses high-frequency PWM switching and low-pass filtering for audio reproduction, whereas Class C relies on resonant LC filtering for continuous-wave RF transmission. You will never find a Class C stage in a hi-fi audio system.

Amplifier Classes at a Glance: The Efficiency vs. Linearity Trade-off

To understand why Class C exists, you have to look at the fundamental trade-off in amplifier design: linearity versus efficiency. The table below maps out the standard amplifier classes. Notice how the conduction angle shrinks as theoretical efficiency climbs.

The LC Tank Circuit: Reconstructing the Sine Wave

If the transistor only conducts for, say, 120 degrees of the cycle, the output current looks like a series of sharp, narrow pulses. How does this become a clean sine wave at the antenna? The answer is the parallel LC (inductor-capacitor) tank circuit, also known as a flywheel circuit.

Think of pushing a child on a playground swing. You don't push continuously through the entire arc of the swing (Class A). Instead, you give one sharp, brief push at the bottom of the arc (the Class C conduction pulse). The momentum of the swing and the pull of gravity (the LC tank's stored energy) carry the child through the rest of the upward and downward trajectory. As long as you push at the exact resonant frequency of the swing, the motion remains a smooth, continuous arc.

In the circuit, when the transistor fires, it dumps a pulse of DC current into the inductor, building a magnetic field. When the transistor turns off, the inductor's collapsing magnetic field charges the capacitor. The capacitor then discharges back through the inductor. This energy sloshes back and forth at the resonant frequency ($f_r$), filling in the 'missing' 240+ degrees of the waveform where the transistor is completely off. The Q factor (Quality factor) of the tank circuit determines how well it filters out the harsh harmonics generated by the sharp current pulses.

Worked Numeric Example: 27 MHz CB Radio Final Stage

Let's design the output tank circuit for a 27.0 MHz (11-meter CB band) Class C final amplifier using a classic 2SC1971 RF power transistor. We will calculate the required inductance and evaluate the thermal performance compared to a linear alternative.

1. Calculating the LC Tank Components

The resonant frequency formula is:

$f_r = \frac{1}{2\pi\sqrt{LC}}$

Rearranging to solve for Inductance (L):

$L = \frac{1}{4\pi^2 f_r^2 C}$

In high-power RF, we typically select a standard capacitor value that can handle the high circulating currents without arcing. Let's choose a 150 pF high-voltage silver mica capacitor for our tank.

• $f_r = 27 \times 10^6$ Hz
• $C = 150 \times 10^{-12}$ F
• $4\pi^2 \approx 39.478$

$L = \frac{1}{39.478 \times (27 \times 10^6)^2 \times (150 \times 10^{-12})}$

$L = \frac{1}{39.478 \times 729 \times 10^{12} \times 150 \times 10^{-12}}$

$L = \frac{1}{4,316,919} \approx 231.6 \text{ nH}$

Practical Build Note: You won't find a 231.6 nH off-the-shelf inductor. In practice, you would wind an air-core coil using 14 AWG magnet wire (approx. 4 to 5 turns on a 10mm diameter form) and use a variable trimmer capacitor in parallel to tune the circuit to exact resonance on the bench using a grid dip meter or spectrum analyzer.

2. Efficiency and Thermal Math

Assume our 13.8V mobile power supply delivers a peak DC current of 3.0A to the transistor.

• DC Input Power: $13.8\text{V} \times 3.0\text{A} = 41.4\text{W}$
• Class C RF Output (at 80% efficiency): $41.4\text{W} \times 0.80 = 33.1\text{W}$
• Heat Dissipated: $41.4\text{W} - 33.1\text{W} = 8.3\text{W}$

If we had biased this exact same 2SC1971 in Class AB (typical 50% efficiency) to get that same 33.1W of RF output, the DC input would need to be 66.2W, meaning the heatsink would have to dissipate 33.1W of waste heat. By using Class C, we cut the thermal load on the heatsink by 75%, allowing for a much smaller, cheaper physical design.

Where You Meet Class C in Practice

You will encounter Class C topologies almost exclusively in RF and high-frequency power applications where amplitude linearity is irrelevant. Common real-world installations include:

• Ham Radio & CB Transmitters: The final PA (Power Amplifier) stage in FM handhelds or CW beacons.
• RFID Readers: The 13.56 MHz driver stages that energize the antenna loop to power passive NFC/RFID tags.
• Induction Heaters: High-power Class C oscillators (often using IGBTs or heavy-duty MOSFETs) generating 50 kHz to 200 kHz fields to melt metal.
• FM Broadcast Exciters: Driving the carrier wave before it is mixed with the audio baseband in modern solid-state broadcast towers.

For a deeper dive into how these amplifier classes are implemented in modern silicon, the Electronics Tutorials amplifier guide provides excellent schematic breakdowns of the biasing networks required to push a BJT into deep Class C cutoff.

Frequently Asked Questions