PSI System Controls and Diagnostics
F3200E User Manual
F3200E_UM_180726 Page 67 of 107
15
Principle of Operation
15.1 Transconductance amplifiers
The F3200E uses the transconductance amplifier circuit, or I-V converter, to produce a
convenient voltage from the small signal current. You can consider that the signal current I
flows through the precision feedback resistor R
f
in the amplifier circuit, resulting in a voltage I.R
f
at the amplifier output. Provided that the output of the amplifier does not saturate, the input
impedance of the amplifier is close to zero ohms. The actual input impedance of the circuit is
determined by any series impedance R
s
that is upstream of the feedback loop.
-
+
AGND
R
f
i(t)
-V(t)
)
(
.
t
i
R
V
f
R
s
C
f
Figure 56. The basic I-V convertor circuit.
The fidelity with which the output voltage tracks the input current over all frequencies depends
upon the speed of the amplifier, and any integrating effect of capacitance in the feedback loop,
either deliberately added or stray, which will roll off the high frequency response. A small
amount of feedback capacitance C
f
is often added to ensure that the amplifier is unconditionally
stable.
