GL-T8541 PA
Glenayre Document Number: 9110.01002
THEORY OF OPERATION
09/10/99
Page: 6-16
Copyright © 1997 Glenayre
6.7.7 PA Current Monitoring Circuit
The +28 volt supply to the driver and each power amplifier module passes through a current
monitoring circuit in the metering board. A fused +28 volt supply bus feeds the input side
of each current monitoring circuit and each amplifier board. See Table 6-3 for a listing of
metering board fuses.
Figure 6-8 shows a schematic diagram of a typical current monitoring circuit. Current
passes through R4, creating a voltage drop which is proportional to its current flow. The
voltage drop across R4 causes the output of U3-D to go negative, which causes Q1 to
conduct. U3-D sets the conduction of Q1 at a level that causes the voltage drop across R47
to equal the drop across R4. The same current flows through R28, Q1 drain, Q1 source, and
R47, therefore the voltage drop across R28 is proportional (50 times greater) than the
voltage drop across R47 or R4. The voltage drop across R28 is the output of the driver
current monitoring circuit, which is proportional to the driver amplifier board current. The
relationship to the amplifier current flow and the output of the current monitoring circuit is
given by the following formula:
Where: E
out
= current monitor output voltage sample, and
I
out
= current being monitored.
Figure 6-8 Metering Board Simplified Current Monitoring Circuits
E
out
I
o ut
R4
R28
R47
----------
=
+28 Vdc Input
R4
0.01
R47
100
R28
5000
+34V
Voltage Sample
Representing
RF Amplifier Current
(to Metering Board
+28 Vdc
Output to
Driver
Board
Fuse
U3-D
Q1
+
-
Multiplexing Circuit)
F1
