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APPLICATION BULLETIN
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
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MACROMODELS FOR RF OP AMPS ARE A
POWERFUL DESIGN TOOL
By Christian Henn, Burr-Brown International GmbH
Simulation is a particularly effective way to design and test
high-frequency circuits. Increasingly powerful computers
and the wide use of the circuit simulator, PSpice
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, make
simulation with PSpice a practical method to test the various
parameters of a circuit. Good simulation models are essen-
tial, however, to achieve realistic results. Burr-Brown now
offers PSpice models for a number of new RF circuits that
far exceed the usual simulation standards in scope and
details. The macromodel for the Diamond Transistor OPA660
gives an example of the structure and performance of these
new PSpice models.
A prerequisite for PSpice simulation is an exact model of the
internal structure of the component. These models are usu-
ally called macromodels and have differing levels of com-
plexity. A type of simulation using transistors to reproduce
the real performance of an analog circuit with a reasonable
simulation time offers a good deal of accuracy and has
become the 鈥渋ndustry standard鈥? Many models, however,
fall far short of this standard; they are so simplified that they
can not come close to predicting the exact performance of a
circuit. A number of mathematical models, for example, are
able to reproduce the exact gain-phase relation ratio of an op
amp, while providing only poor simulation of the amplifier鈥檚
noise and distortion. Thus such models are often insufficient
for an engineer鈥檚 need.
In contrast, the model for the OPA660, which is made up of
transistors, out performs even the industry standard. PSpice
models are also available for the high-speed components
BUF600/1, MPC100, OPA622, and OPA623, each with the
same structure as that for the OPA660. Each model contains
a simplified simulation option (S) and a comfortable option
(C) on a 5-1/4" disk (or 3-1/2" disk upon request). These
simulation versions, which differ in their simulation accu-
racy and required simulation times, deliver enough flexibil-
ity and accuracy to analyze a large number of circuit char-
acteristics, while remaining simple enough to ensure fast
simulations.
THE DIAMOND TRANSISTOR OPA660
Analog circuit technology often uses integrated circuits such
as op amps to solve a circuit problem. Because they require
feedback, op amps generally fall behind open-loop amplifi-
PSpice
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,
Micro Sim Corporation
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ers in processing steep input pulses. Open-loop amplifiers,
however, do not always provide the required DC accuracy
and are susceptible to aging effects and temperature varia-
tion. The OPA660 unites the advantages of both methods.
This operational transconductance amplifier contains the
Diamond Transistor (DT) and Diamond Buffer (DB) in an
8-pin SO or DIL package together with the biasing circuitry
(BC). With these independent macroelements, the OPA660
can be used in circuits designed to process high-frequency
signals of up to 500MHz. Figure 1 shows a block diagram of
the OPA660, while Figure 2 shows a more detailed circuit
diagram. The numbering of the inner and outer connection
pins shown in the figures matches that used in the
macromodel.
The Diamond Transistor is a voltage-controlled, wide-band
current source, which functions as an ideal transistor. It
contains a high-impedance input (base), a low-impedance
input/output (emitter), and a high-impedance current source
output (collector). In contrast to a bipolar transistor, this
amplifier allows operation over all four quadrants. All of its
pins are at 0V with no input signal; all working points are
fixed and stable over temperature; the emitter circuit func-
tions in inverting mode and the base circuit in noninverting
mode; the transconductance, hence the ratio between output
current variation and input current variation, is extremely
constant over the entire input voltage range and can be
programmed by an external resistor.
V
CC
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7
13
BC
2
14
4
1
R
qc
V
EE
3
X2
DT
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8
5
X1
DB
6
FIGURE 1. Block Diagram of the OPA660.
1993
Burr-Brown Corporation
AN-189
Printed in U.S.A. December, 1993
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