The RF Class C amplifier - basics and simulations (1/2)

Summary

In this video, the focus is on understanding Class C amplifiers, which are commonly used in radio frequency applications. The speaker explains their operation, design considerations, and limitations, such as narrow bandwidth and the role of the LC resonant circuit. The Class C amplifier's efficiency, frequency multiplication capabilities, and the importance of impedance matching for optimal performance are also discussed. Additionally, the video emphasizes the impact of the conduction angle and output impedance on efficiency, with practical examples provided through simulations. The video concludes with a preview of building and testing a Class C amplifier circuit.

Takeaways

• 📡 Class C amplifiers are nonlinear RF amplifiers that conduct for less than 180° of the input cycle but still produce a sinusoidal output through resonant filtering.
• 🎯 The LC resonant tank is crucial—it stores and releases energy like a pendulum, allowing clean sinusoidal oscillation even though the transistor conducts only briefly.
• 📏 The Q-factor of the RLC circuit determines bandwidth and oscillation quality; a loaded Q of at least 5 is typically recommended for proper Class C operation.
• 🔍 Class C amplifiers are narrowband and unsuitable for wideband applications like audio, making them ideal mainly for RF transmitters and high-frequency systems.
• 🔁 They can function as frequency multipliers (doublers, triplers, etc.) by tuning the resonant tank to a harmonic of the input signal.
• ⚡ The output peak-to-peak voltage can reach roughly twice the supply voltage due to resonant swing, and amplitude control typically requires adjusting the supply voltage rather than the input amplitude.
• 🔌 Impedance matching networks are essential to transfer power efficiently to various load impedances and to allow flexible power levels regardless of supply voltage.
• 🧹 Matching networks also act as filters that reduce harmonic distortion, with multi-stage networks offering significantly cleaner output spectra.
• 💡 Efficiency in Class C amplifiers can reach 60–90% in practice, surpassing Class A and Class B designs due to reduced conduction time and minimal overlap between voltage and current.
• 🔧 Efficiency is influenced by conduction angle and transistor output impedance; conduction angle is tuned via biasing, input drive level, or emitter feedback components.
• 🧪 Practical tuning is typically done empirically—often using simulation—to find the optimal conduction angle and bias point for maximum efficiency.
• 🏭 Class C amplifiers are commonly used in RF transmitters and industrial high-frequency applications, particularly for continuous-wave operation where efficiency is critical.

Q & A

What distinguishes Class C amplifiers from other amplifier classes like A, B, and AB?

Class C amplifiers conduct for less than 180 degrees of the input waveform, while Classes A, B, and AB are considered linear amplifiers with wider conduction angles.

Why is an LC resonant circuit important in a Class C amplifier?

Because the transistor conducts only part of the time, the LC resonator stores and releases energy, allowing the circuit to produce a continuous sine wave even with intermittent drive.

What is the Q factor, and why is it important in Class C amplifier design?

The Q factor defines how underdamped the resonant circuit is. A higher Q means narrower bandwidth and longer sustaining oscillations, which Class C amplifiers need for proper operation.

Why are Class C amplifiers usually used for RF applications rather than audio?

Class C amplifiers have narrow bandwidth due to their high-Q resonant circuits, making them unsuitable for wideband applications like audio but ideal for specific radio frequencies.

How can Class C amplifiers be used for frequency multiplication?

By tuning the resonant circuit to a harmonic of the input frequency, the amplifier can skip cycles while still sustaining oscillation at a higher multiple of the driving frequency.

What is the typical peak-to-peak output voltage of a Class C amplifier relative to supply voltage?

The output swing can reach approximately twice the supply voltage peak-to-peak due to the resonant action of the LC network.

Why doesn’t input amplitude strongly determine output amplitude in Class C amplifiers?

Because output amplitude is mainly governed by supply voltage and resonance behavior; adjusting supply voltage is the primary way to change output power.

How is impedance matching used in Class C amplifier systems?

Impedance matching networks adapt the amplifier’s designed load to the actual output load, enabling desired power transfer and filtering harmonic distortion.

How does harmonic distortion behave in Class C amplifiers, and how can it be mitigated?

Because the transistor conducts in pulses, the waveform contains many harmonics. Matching networks and filters help reduce these harmonics and clean the output signal.

What efficiency levels are typical for Class C amplifiers?

They theoretically can reach 100% efficiency, but real-world designs generally operate between 60% and 90%, outperforming Class A and B designs in efficiency.

What determines the optimal conduction angle for a Class C amplifier?

It depends on the relationship between the amplifier’s output impedance and the load. Designers often tune input drive and bias experimentally to find the maximum efficiency point.

How can bias adjustment affect conduction angle and efficiency?

Adjusting the transistor’s bias point or input signal level changes how long the device conducts, which directly influences both power output and efficiency.