Appendix 1:
Bandwidth
Limitations
When driving capacitive loads with a high-voltage
amplifier, such as the Model 3211, there are several
factors which determine the bandwidth of the system.
Closed-Loop Bandwidth
The closed-loop bandwidth of the high-voltage
operational amplifier in the Model 3211 deter-
mines the maximum bandwidth attained by the
Model 3211; this bandwidth is at least 1 MHz.
Other factors, such as slew rate, current limiting
and RC filtering can limit the bandwidth even fur-
ther, as described below.
Slew Rate
The slew rate of the Model 3211 is at least 700 V/µs.
Thus, the maximum frequency of operation is
dependent on the peak voltage. With a peak voltage
of 200 V, the maximum frequency, and therefore
the largest signal bandwidth, is given by:
V
out
=
V
p
sin(2
π
ft
)
d
V
out
/d
t
=2
π
fV
p
cos(2
π
ft
)<700 V/µs
f
max
(large signal)=111
×
10
6
/
V
p
,
where
V
p
is the peak voltage,
f
is the frequency in
Hz,
t
is time, and
V
out
is the output voltage.
Therefore, when the peak voltage is 200 V and the
amplifier is not current limited as described in the
section below, the maximum frequency on the
high-voltage output is 555 kHz.
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