Engineering Notes

POWER RF AMPLIFIERS TECHNICAL REFERENCE MATERIAL

Graceful Degradation

Solid state power amplifiers contain many isolated transistors in parallel at the output. There is a significant advantage in "graceful degradation" compared to tube amplifiers. A failure of the tube results in the total loss of output power while the failure of a parallel transistor still yields significant output power. Another significant advantage is that transistors do not change output power as a function of time whereas tubes continue to drift.

For N combined devices, the RF power output as a function of device failures is obtained by the following formula:

P_o = P_t (( N - N_f ) / N)²

where: P_o = Output Power
P_t = Total P_o without transistor failures
N = Total number of devices
N_f = Total number of failed devices

N REDUCTION IN OUTPUT POWER
WITH A SINGLE DEVICE FAILURE

dB %

2
4
8
16
32
64 -6.02
-2.50
-1.16
-0.56
-0.28
-0.14 75
44
23
12
6
3