How to Plan Essential Protection Circuitry (Overdrive, SWR, Overheat) for HF Linear Amplifier?

Johnson Francis | February 10, 2026 | Amateur Radio (Ham Radio) | No Comments

Building a reliable HF linear amplifier, especially one utilizing robust components like the IRFP150, requires more than just a solid RF deck. The protection circuitry acts as the “brain” that prevents expensive silicon from turning into expensive smoke. For a 100W+ build, you need to monitor three critical failure points: Overdrive, High SWR, and Overheat. As I have already faced an overdrive problem and luckily escaped without ‘expensive smoke’, I am wondering how I can implement the following possible protections!

1. Input Overdrive Protection

The gates of MOSFETs are incredibly sensitive to voltage spikes. If your transceiver accidentally spikes or you forget to dial back the power, the IRFP150 can fail instantly.

The Mechanism: Use a sampling coupler or a simple resistive tap at the input. This RF is rectified into a DC voltage.
The Circuit: A fast-acting comparator (like the LM311 or LM339) compares this sampled voltage against a preset reference (set by a multi-turn trimpot). LM311 is a single, high-speed precision comparator while LM339 is a “quad” comparator.
The Action: The comparator output triggers a high-speed relay or an electronic attenuator to disconnect the input or shunt it to ground.

2. SWR (Reflected Power) Protection

If a wrong antenna is accidentally connected, high reflected power creates high voltage peaks at the MOSFET drains, leading to breakdown.

The Mechanism: A Bruene Bridge is an option which can be used at the amplifier output. Bruene Bridge is a classic and highly reliable circuit used in amateur radio to measure Forward and Reflected power. Unlike a simple SWR bridge, the Bruene design uses a combination of a capacitive voltage divider and a current transformer to provide accurate readings across a wide frequency range.
The Logic: * Forward power and Reflected power are converted to DC.
- If the reflected DC voltage exceeds a threshold (typically representing an SWR > 2.0:1 or 3.0:1), the protection kicks in.
Integration: This should trigger a “Latching” fault. Once SWR protection trips, the amplifier should stay off until you manually reset it, preventing it from “chattering” under high SWR.

3. Thermal Protection (Overheat)

Since I am working with an aluminum heat sink for IRFP150s, managing the thermal mass is vital. MOSFETs exhibit “thermal runaway” if they get too hot, where they draw more current, leading to even more heat.

The Sensor: Mount a 10k NTC (Negative Temperature Coefficient) Thermistor or a dedicated IC like the LM35 directly onto the heat sink, as close to the MOSFET casing as possible. LM35 outputs exactly 10 mV per degree Celsius
Two-Stage Logic:
- Stage 1 (Fan Control): At 45°C, trigger the cooling fans to high speed.
- Stage 2 (Emergency Cutoff): At 75°C–80°C, the comparator should disable the PTT line or cut the Bias voltage to the MOSFETs to stop amplification immediately.

VU3GZR has already implemented a thermal sensor cutoff in his build of VU2EVQ linear amplifier.

Essential Components for the Protection Board

To build this into a single “Supervisory Board,” you will need:

Component	Function
LM339/LM324	Quad Op-Amp/Comparator to handle all three inputs.
2N2222 or equivalent	Transistors to drive the protection relays.
12V DPDT Relay	To interrupt the PTT line and/or the VCC supply.
LED Indicators	Red for “Fault,” Green for “Ready,” and Yellow for “High Temp.”

Another option is to ensure the bias circuit is also tied to the protection. If any fault is detected, dropping the gate bias to 0V is the fastest way to “shut down” the MOSFETs while the slower mechanical relays are still moving.