kh103 Fairchild Semiconductor, kh103 Datasheet - Page 4

• Manufacturer Part Number: kh103
• Description: Fast Settling, High Current Wideband Op Amp
• Manufacturer: Fairchild Semiconductor

DATA SHEET
KH103 Operation
The KH103 is based on a unique design which uses
current feedback instead of the usual voltage feedback.
This design provides dynamic performance far beyond
that previously available, yet it is used basically the same
as the familiar voltage-feedback op amp (see the gain
equations above).
Layout Considerations
To obtain optimum performance from any circuit
operating at high frequencies, good PC layout is
essential. Fortunately, the stable, well-behaved response
of the KH103 makes operation at high frequencies less
sensitive to layout than is the case with other wideband
op amps, even though the KH103 has a much wider
bandwidth.
In general, a good layout is one which minimizes the
unwanted coupling of a signal between nodes in a circuit.
A continuous ground plane from the signal input to output
on the circuit side of the board is helpful. Traces should
be kept short to minimize inductance. If long traces are
needed, use microstrip transmission lines which are
terminated in their characteristic impedance. At some
high-impedance nodes, or in sensitive areas such as
near pin 5 of the KH103, stray capacitance should be
kept small by keeping nodes small and removing ground
plans directly around the node.
The ±V
bypassed to ground with 0.1µF capacitors to provide
good high-frequency decoupling. It is recommended that
1µF or larger tantalum capacitors be provided for low-
frequency decoupling. The 0.01µF capacitors shown at
pins 18 and 20 in figures 1 and 2 should be kept within
0.1” of those pins. A wide strip of ground plane should be
provided for a signal return path between the load-resis-
tors ground and these capacitors.
4
Figure 1: Recommended Non-Inverting Gain Circuit
CC
connections to the KH103 are internally
Since the layout of the PC board forms such an important
part of the circuit, much time can be saved if prototype
amplifier boards are tested early in the design stage.
Settling Time, Offset, and Drift
After an output transition has occurred. the output settles
very rapidly to the final value and no change occurs for
several microseconds. Thereafter, thermal gradients
inside the KH103 will cause the output to begin to drift.
When this cannot be tolerated, or when the initial offset
voltage and drift is unacceptable, use of a composite
amplifier is advised.
A composite amplifier can also be referred to as a feed-
forward amplifier. Most feed-forward techniques such as
those used In the vast majority of wideband op amps
involve the use of a wideband AC-coupled channel in par-
allel with a low-bandwidth, high-gain DC-coupled amplifi-
er. For the composite amplifier suggested for use with
the KH103, the KH103 replaces the wideband AC-cou-
pled amplifier and a low-cost monolithic op amp is used
to supply high open-loop gain at low frequencies. Since
the KH103 is strictly DC coupled throughout, crossover
distortion of less than 0.01dB and 1° results.
For composite operation in the non-inverting mode, the
circuit in Figure 1 should be modified by the addition of
the circuit shown in Figure 3. For Inverting operation,
modify the circuit in Figure 2 by the addition of the circuit
in Figure 4. Keep all resistors which connect to the
KH103 within 0.2” of the KH103 pins. The other side of
these resistors should likewise be as close to U1 as
possible. For good overall results, U1 should be similar
to the LF356; this gives 5mV/°C input offset drift and the
crossover frequency occurs at about 2MHz. Since U1
has a feedback network composed of R
15kΩ resistor, which is in parallel with R
1.5kΩ feedback resistor of the KH103, R
adjusted to match the feedback ratios of the two net-
works. This in done by driving the composite amplifier
Figure 2: Recommended Inverting Gain Circuit
REV. 1A February 2001
g
and the internal
a
+ R
b
must be
b
and a
KH103