2 – 2
Description of Operation
2.1
About Transimpedance Amplifiers
A
transimpedance amplifier
is an operational amplifier with a resistor
transimpedance amplifier
in the feedback path. More generally, the feedback impedance
Z
F
al-
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
in
is forced through the feedback
impedance, and the amplifier produces an output voltage
V
out
,
TA
=
V
bias
−
i
in
×
Z
F
.
The amplifier acts to translate the input current into a proportional
output voltage, with the feedback impedance being the coe
ffi
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.
2.1.1
Input capacitance and stability
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
1
divided by
the input current) has two other, much more significant contributions
in parallel. The first one is the e
ff
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.
F
F
in
i
in
in
(F)
out, TA
V
Bias
Bias
R
Z
Input BNC
Op-amp
C
C
Figure 2.1: A transimpedance amplifier.
The first term equals the feedback impedance divided by the open-
loop gain of the operational amplifier
2
:
Z
(F)
in
=
Z
F
/
A
OL
(2
.
1)
1
Without the action of the autozero circuit.
2
Within the current bandwidth of the transimpedance amplifier.
SIM918
Precision Current Preamplifier
Содержание SIM918
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