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Applications of the LM392
Comparator Op Amp IC
The LM339 quad comparator and the LM324 op amp are
among the most widely used linear ICs today. The combina-
tion of low cost, single or dual supply operation and ease of
use has contributed to the wide range of applications for
these devices.
The LM392, a dual which contains a 324-type op amp and a
339-type comparator, is also available. This device shares all
the operating features and economy of 339 and 324 types
with the flexibility of both device types in a single 8-pin
mini-DIP. This allows applications that are not readily imple-
mented with other devices but retain simplicity and low cost.
Figure 2 provides an example.
Sample-Hold Circuit
The circuit of Figure 2 is an unusual implementation of the
sample-hold function. Although its input-to-output relation-
ship is similar to standard configurations, its operating prin-
ciple is different. Key advantages include simplicity, no hold
step, essentially zero gain error and operation from a single
5V supply. In this circuit the sample-hold command pulse
(Trace A, Figure 1) is applied to Q3, which turns on, causing
current source transistor Q4’s collector (Trace B, Figure 1)to
go to ground potential. Amplifier A1 follows Q4’s collector
voltage and provides the circuit’s output (Trace C, Figure 1).
When the sample-hold command pulse falls, Q4’s collector
drives a constant current into the 0.01 µF capacitor. When
the capacitor ramp voltage equals the circuit’s input voltage,
comparator C1 switches, causing Q2 to turn off the current
source. At this point the collector voltage of Q4 sits at the
circuit’s input voltage. Q1 insures that the comparator will not
self trigger if the input voltage increases during a “hold”
interval. When a DC biased sine wave is applied to the circuit
(Trace D, Figure 1) the sampled output (Trace E, Figure 1)is
available at the circuit’s output. The ramping action of the Q4
current source during the “sample” states is just visible in the
output.
00749301
FIGURE 1.
00749302
Q1, Q2, Q3 = 2N2369
Q4 = 2N2907
C1, A1 = LM392 amplifier-comparator dual
*
1% metal film resistor
FIGURE 2.
National Semiconductor
Application Note 286
September 1981
Applications of the LM392 Comparator Op Amp IC AN-286
© 2002 National Semiconductor Corporation AN007493 www.national.com
