About Transimpedance Amplifiers; Input Capacitance And Stability - Stanford Research Systems SIM918 Operating And Service Manual

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2.1 About Transimpedance Amplifiers
transimpedance amplifier

2.1.1 Input capacitance and stability

A transimpedance amplifier is an operational amplifier with a resistor
in the feedback path. More generally, the feedback impedance Z
ways has a resistive and a reactive component. The two inputs to the
operational amplifier have a very high impedance; and the feedback
in the amplifier acts to keep the two inputs at the same electric poten-
tial. Therefore, the input current i
impedance, and the amplifier produces an output voltage
V
The amplifier acts to translate the input current into a proportional
output voltage, with the feedback impedance being the coe cient of
proportionality. Hence the term transimpedance.
The main amplifier of the SIM918 Precision Current Preamplifier is a
transimpedance amplifier based on a composite, JFET-input design.
The input impedance of JFET devices is extremely large. However,
the impedance experienced, as a whole, by a current input to the
feedback amplifier (i.e. the change in the input voltage
the input current) has two other, much more significant contributions
in parallel. The first one is the e ect of the output on the input
via feedback due to finite open-loop voltage gain of the operational
amplifier, and the second one is input capacitance.
Input BNC
i
in
Figure 2.1: A transimpedance amplifier.
The first term equals the feedback impedance divided by the open-
loop gain of the operational amplifier
1
Without the action of the autozero circuit.
2
Within the current bandwidth of the transimpedance amplifier.
Description of Operation
is forced through the feedback
in
V i Z
out TA bias in F
C
in
Op-amp
Bias
(F)
Z
in
Bias
2
:
(F)
Z Z A
F OL
in
SIM918 Precision Current Preamplifier
al-
F
1
divided by
C
F
R
F
V
out, TA
(2 1)