RF power amplifier safety

Article By : John Dunn

RF power amplifiers can deliver enough RF output power to do biological harm to somebody. Here's a way to prevent a potentially deadly failure mode.

Radio frequency (RF) power amplifiers are all over the place. Some of them can deliver enough RF output power to do biological harm to somebody. Many such amplifiers are solid state, but many others at really high power levels use vacuum tubes. Whichever kind you may be dealing with, there is a possible failure mode which is sometimes not addressed by an amplifier’s designer. That failure mode is a capacitor short circuit failure shown in the two schematics below as “C1.”

Figure 1
Solid-state RF power amplifier

Figure 2
Vacuum tube RF power amplifier

In both cases, if C1 fails shorted, it will bring the +Vcc or the +Vplate to the RF output. If the amplifier’s load is open circuited for DC, as would be so for a half-wave straight dipole or a quarter-wave ground plane, that DC voltage will dangerously appear across that load.

Fifty volts or so in the solid-state case might do some harm, but if a multi-hundred to multi-thousand volt DC output is delivered to the RF output, somebody could be killed.

The insidious thing is that even with a failed C1, the amplifier might continue to work anyway with the C1 failure going unnoticed. To prevent this, the “something missing” in the two figures above must be replaced by the “something included” in the two figures below.

Figure 3
Solid-state RF power amplifier, corrected

Figure 4
Vacuum tube RF power amplifier, corrected

In these two new schematics, a radio frequency choke (RFC) has been added across the RF output. If C1 fails and becomes a short circuit, the +Vcc or the +Vplate will be DC short circuited to ground. This will lead to some fireworks (What’s that funny smell?), but the amplifier will clearly be malfunctioning, and the RF output will not be likely to develop a DC presence that might cause loss of life.

Just for further illustration, the following schematic shows an RFC inclusion at RFC3.

Figure 5
RFC inclusion example (A 1500W RF Amplifier Circuit, courtesy of Amplifier Circuit Design)

Yes, in this schematic, my so called “C1” is now “C4,” but you get the idea.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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