13
Current Limit
The maximum current that can be provided by the
amplifier Model 3211 is 120 mA. For this reason,
when driving capacitive loads, the maximum
frequency is capacitance dependent. The displace-
ment current drawn by a capacitor is given by:
I
=
C
d
V
/d
t
<0.12 A
d
V
/d
t
max
=2
π
fV
p
.
Therefore,
f
max
(current limit)~0.0191/
CV
p
.
If the peak voltage into the capacitor is 200 V, then
the maximum frequency (in Hz) is given by:
f
max
(current limit/large signal)~9.55x10
-5
/
C
.
Therefore, if the capacitive load is 100 nF (0.1 µF),
the maximum frequency before current limiting
begins to distort a 200-V waveform is 955 Hz.
RC Filter
The Model 3211 has an output resistance of
approximately 25
Ω
. This resistance, along with
the capacitive load, forms a low-pass, RC filter with
the 3-dB point at a frequency of 1/(2
π
RC). This
RC filter can limit the bandwidth performance of
the Model 3211 for large capacitive loads and very
small peak voltages.
Summary
When determining the maximum bandwidth of
the Model 3211 for a known capacitive load and a
desired peak voltage, the user must calculate the
bandwidth determined by the slew rate (dominates
