Some amplifier applications
require clamping or limiting
due to large, sporadic signals
appearing at the amplifier
input. Clamping these errant
signals protects the amplifier
and other sensitive downstream circuitry. It also
improves overdrive recovery
time and can lower distortion.
At the heart of the clamp circuit described here is the AD8099 high-speed, low-noise,
externally compensated amplifier. The device consists of a
single-stage amplifier followed by a unity-gain buffer. External
compensation is required in low-gain applications, but not with gains
of greater than 10. The clamper exploits
that feature by setting the amplifier for a
gain of 15, eliminating the need for a
compensation network. The compensation pin can then be used in an unconventional manner to provide clamping.
The clamping circuit employs two
1N5711 high-speed Schottky barrier
diodes connected to the compensation pin through resistor R3, which limits the
current in and out of the compensation
pin (). In this application, the current is approximately 6 mA. It should be
kept below 10 mA for safe operation.
The diodes perform the lion's share of
the clamping function. They feature very
low capacitance and picosecond switching, thus maintaining high-frequency performance and crisp transition edges.
The drop across R3 is about 0.6 V, which combines with
another 0.4 V across D1 or D2
to make the total drop approximately 1 V. With ±2-V references, the amplifier output
clamps at ±3 V.
Without the clamping network
in place, overdriving the amplifier's input with a 1-V, 5-MHz
square wave causes the amplifier to rail. Asymmetry, distortion,
and other signs of overdriving
are clearly visible at the amplifier's output (). With the
clamping circuit incorporated,
the output is clamped to 3 ±V,
and the asymmetry and distortion effects of overdriving have been
eliminated ().