Choosing a Power Supply for Your Linear Amplifier

Choosing the right power supply is the single most critical decision you’ll make when building a linear amplifier. A supply that is too weak will clip, overheat, and distort; one that is oversized can waste space, generate excess heat, and introduce unnecessary cost. This guide walks you through the essential parameters—voltage, current, regulation, duty cycle, and topology—so you can match a supply to your amp with confidence.

VOLTAGE AND CURRENT MATCHING

The first step is to read the amplifier’s datasheet (or, if you’re working from a schematic, calculate the required rail voltages). Two numbers dominate:

• Supply voltage (V_CC) – the maximum voltage the output stage can tolerate without breakdown.
• Supply current (I_CC) – the continuous current the amp will draw at full output power.

For a typical class‑AB RF amp, the output power P_out relates to these values roughly as:

P_out ≈ (V_CC × I_CC) / 2

That relationship tells you that if you double the voltage you can halve the current for the same power, but the trade‑off is higher voltage stress on transistors and often more demanding regulation.

Headroom and Regulation

Even if the datasheet lists a “nominal” supply voltage, you should provide at least 10‑15 % headroom. This ensures the amp can handle transient peaks without hitting the supply rails, which would cause clipping and potentially damage the output devices.

Regulation is equally important. Linear amplifiers are sensitive to supply ripple; a ripple voltage of more than a few hundred millivolts can modulate the RF output and create spurious emissions. Aim for a regulation spec of ±1 % or better under full load. If you must use a switching supply, add a low‑ESR bulk capacitor and, if possible, a linear post‑regulator stage to tame the ripple.

DUTY CYCLE AND THERMAL CONSIDERATIONS

Unlike a continuous‑wave (CW) transmitter that runs at full power indefinitely, many amateur operations are intermittent—S‑SB, FM voice, or digital modes often have a duty cycle well below 100 %. Your power supply must be rated for the worst‑case average current, not just the peak.

Calculate the average current as:

I_avg = I_peak × Duty Cycle

For example, a 100 W amp that draws 10 A at full power but is used at a 25 % duty cycle only requires a supply capable of 2.5 A continuous. However, the supply still needs to survive the 10 A peaks without voltage sag.

Thermal Management

Both the amplifier and the supply generate heat. A supply that runs near its maximum rating will see its internal temperature rise, which can degrade regulation and shorten component life. Provide adequate airflow, heat‑sinking, or even a small fan if the enclosure is tight. Monitor the supply’s temperature during a prolonged transmission test; a rise of more than 30 °C above ambient is a red flag.

LINEAR VS SWITCHING SUPPLIES

Historically, linear (transformer‑based) supplies have been the go‑to for RF amplifiers because of their low noise and simple construction. Modern switching supplies offer higher efficiency and smaller size, but they bring challenges that must be addressed.

When to Choose a Linear Supply

• Maximum output power under 200 W, where the efficiency penalty is modest.
• Environments with strict spurious emission limits; linear supplies naturally produce less ripple.
• Projects where simplicity and ease of troubleshooting are paramount.

When a Switching Supply Makes Sense

• High‑power builds (>500 W) where linear supplies become bulky and wasteful.
• Portable or battery‑operated rigs where every watt of efficiency counts.
• Situations where weight and space are at a premium, such as mobile installations.

If you opt for a switching supply, add a LC filter and a large electrolytic capacitor bank (low ESR, high ripple current rating) directly at the amp’s input. This hybrid approach captures the efficiency of switching while delivering the clean DC that a linear amp expects.

SAFETY AND COMPLIANCE

Power supplies for RF amplifiers operate at potentially lethal voltages. Treat every high‑voltage node as live until proven otherwise. Use insulated tools, keep a clear work area, and always discharge capacitors before handling the circuit.

Remember the legal limits: CB radios in the United States are limited to 4 W carrier AM and 12 W PEP SSB, and external linear amplifiers are not legal for CB use. Amateur radio operators may transmit up to 1500 W PEP under FCC Part 97, but you must hold a valid license and use only the power necessary for your communications.

QUICK SANITY CHECKLIST

• ✓ Verify the amp’s maximum supply voltage and add 10‑15 % headroom.
• ✓ Confirm continuous current rating meets I_avg plus a safety margin (≥20 %).
• ✓ Check regulation: ≤±1 % under full load.
• ✓ Ensure ripple voltage is below a few hundred millivolts (add filtering if needed).
• ✓ Match duty cycle to average current capability.
• ✓ Provide adequate heat sinking and airflow for both amp and supply.
• ✓ Add a protective fuse or circuit breaker sized for the peak current.
• ✓ Use insulated tools and discharge capacitors before servicing.

By respecting voltage, current, regulation, and safety, you’ll select a power supply that lets your linear amplifier perform cleanly, reliably, and within the bounds of the law.

Educational reference only. Always verify all values independently against your specific device datasheet and build documentation. Working on RF and high-voltage equipment carries real risk.

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