CA3130, CA3130A
FN817 Rev.6.00 Page 8 of 17
Aug 1, 2005
Typical Applications
Voltage Followers
Operational amplifiers with very high input resistances, like the
CA3130, are particularly suited to service as voltage followers.
Figure 8 shows the circuit of a classical voltage follower,
together with pertinent waveforms using the CA3130 in a split-
supply configuration.
A voltage follower, operated from a single supply, is shown in
Figure 9, together with related waveforms. This follower circuit
is linear over a wide dynamic range, as illustrated by the
reproduction of the output waveform in Figure 9A with input-
signal ramping. The waveforms in Figure 9B show that the
follower does not lose its input-to-output phase-sense, even
though the input is being swung 7.5V below ground potential.
This unique characteristic is an important attribute in both
operational amplifier and comparator applications. Figure 9B
also shows the manner in which the CMOS output stage
permits the output signal to swing down to the negative supply-
rail potential (i.e., ground in the case shown). The digital-to-
analog converter (DAC) circuit, described later, illustrates the
practical use of the CA3130 in a single-supply voltage-follower
application.
9-Bit CM OS DAC
A typical circuit of a 9-bit Digital-to-Analog Converter (DAC) is
shown in Figure 10. This system combines the concepts of
multiple-switch CMOS lCs, a low-cost ladder network of
discrete metal-oxide-film resistors, a CA3130 op amp
connected as a follower, and an inexpensive monolithic
regulator in a simple single power-supply arrangement. An
additional feature of the DAC is that it is readily interfaced with
CMOS input logic, e.g., 10V logic levels are used in the circuit
of Figure 10.
The circuit uses an R/2R voltage-ladder network, with the
output potential obtained directly by terminating the ladder
arms at either the positive or the negative power-supply
terminal. Each CD4007A contains three “inverters”, each
“inverter” functioning as a single-pole double-throw switch to
terminate an arm of the R/2R network at either the positive or
negative power-supply terminal. The resistor ladder is an
assembly of 1% tolerance metal-oxide film resistors. The five
arms requiring the highest accuracy are assembled with series
and parallel combinations of 806,000 resistors from the same
manufacturing lot.
A single 15V supply provides a positive bus for the CA3130
follower amplifier and feeds the CA3085 voltage regulator. A
“scale-adjust” function is provided by the regulator output
control, set to a nominal 10V level in this system. The line-
voltage regulation (approximately 0.2%) permits a 9-bit
accuracy to be maintained with variations of several volts in the
supply. The flexibility afforded by the CMOS building blocks
simplifies the design of DAC systems tailored to particular
needs.
Single-Supply, Absolute-Value, Ideal Full-Wave
Rectifier
The absolute-value circuit using the CA3130 is shown in
Figure 11. During positive excursions, the input signal is fed
through the feedback network directly to the output.
Simultaneously, the positive excursion of the input signal also
drives the output terminal (No. 6) of the inverting amplifier in a
negative-going excursion such that the 1N914 diode effectively
disconnects the amplifier from the signal path. During a
negative-going excursion of the input signal, the CA3130
functions as a normal inverting amplifier with a gain equal to -
R2/R1. When the equality of the two equations shown in Figure
11 is satisfied, the full-wave output is symmetrical.
Peak Detectors
Peak-detector circuits are easily implemented with the
CA3130, as illustrated in Figure 12 for both the peak-positive
and the peak-negative circuit. It should be noted that with
large-signal inputs, the bandwidth of the peak-negative circuit
is much less than that of the peak-positive circuit. The second
stage of the CA3130 limits the bandwidth in this case.
Negative-going output-signal excursion requires a positive-
going signal excursion at the collector of transistor Q11, which
is loaded by the intrinsic capacitance of the associated circuitry
in this mode. On the other hand, during a negative-going signal
excursion at the collector of Q11, the transistor functions in an
active “pull-down” mode so that the intrinsic capacitance can
be discharged more expeditiously.
3
2
1
8
4
7
6
+
-
Rs
1M
47pF -7.5V
0.01
F
+7.5V
0.01F
NOISE
VOLTAGE
OUTPUT
30.1k
1k
BW (-3dB) = 200kHz
TOTAL NOISE VOLTAGE (REFERRED
TO INPUT) = 23V (TYP)
FIGURE 7. TEST-CIRCUIT AMPLIFIER (30-dB GAIN) USED
FOR WIDEBAND NOISE MEASUREMENTS