Winding RF Choke for HF linear amplifier with FT50-43 toroid using 18 SWG enameled copper wire
In a Radio Frequency (RF) linear amplifier, the RF Choke (RFC) is a specialized inductor designed to act as a “frequency-selective gatekeeper.” Its primary role is to provide a path for DC power while blocking high-frequency RF signals from escaping into the power supply or other sensitive parts of the circuit.
1. The “Choking” Mechanism
The RF choke works on the principle of inductive reactance (XL). The opposition it offers to a signal is defined by:
XL = 2πfL
- For DC (f = 0): The reactance is nearly zero, allowing the DC bias or supply current to pass to the transistor or vacuum tube with minimal resistance.
- For RF (f is high): The reactance becomes very high (effectively an open circuit), which “chokes” or stops the RF signal from flowing past it.
2. Key Functions in a Linear Amplifier
A. DC Biasing and Power Feeding
The most critical role of the RFC is to safely inject DC voltage into the output stage (the collector of a BJT, the drain of a MOSFET, or the plate of a vacuum tube). Without the choke, the DC supply would be directly connected to the RF output, creating a short circuit for the signal you are trying to amplify.
B. Isolation of the Power Supply
Linear amplifiers are sensitive to feedback. If RF energy leaks into the DC power supply lines:
- Oscillations: It can cause the amplifier to become unstable and oscillate, potentially destroying the components.
- Interference (EMI): The power cables could act as antennas, radiating unwanted signals that interfere with other electronic devices.
- Loss of Efficiency: RF energy that “leaks” back into the power supply is energy that isn’t reaching your antenna, reducing the overall gain and power output.
C. Maintaining High Output Impedance
In many amplifier designs, the RFC is placed in parallel with the load. Because the choke has high impedance at the operating frequency, it ensures that the RF signal “sees” the intended load (like a 50-ohm antenna) rather than the low-impedance DC power supply. This is crucial for proper impedance matching and maximum power transfer.
3. Comparison: Ideal vs. Real-World RFC
| Feature | Ideal RF Choke | Real-World RF Choke |
| DC Resistance | 0 Ω (No voltage drop) | Small but present (Causes heat) |
| RF Reactance | Infinite (Total block) | Finite (Some leakage exists) |
| Parasitics | None | Has “Self-Resonant Frequency” (SRF) |
Pro Tip: Designers must choose an RFC whose Self-Resonant Frequency (SRF) is higher than the amplifier’s operating frequency. Above the SRF, the choke starts acting like a capacitor and loses its ability to block RF effectively.