LMC660 Datasheet, PDF (6/14 Pages) National Semiconductor (TI) – CMOS Quad Operational Amplifier

Typical Performance Characteristics V_S = ±7.5V, T_A = 25˚C unless otherwise specified (Continued)
CMRR vs Frequency
Open-Loop Frequency
Response
Frequency Response
vs Capacitive Load
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Non-Inverting Large Signal
Pulse Response
Stability vs
Capacitive Load
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Stability vs
Capacitive Load
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Note: Avoid resistive loads of less than 500Ω, as they may cause instability.
Application Hints
Amplifier Topology
The topology chosen for the LMC660, shown in Figure 1, is
unconventional (compared to general-purpose op amps) in
that the traditional unity-gain buffer output stage is not used;
instead, the output is taken directly from the output of the in-
tegrator, to allow rail-to-rail output swing. Since the buffer
traditionally delivers the power to the load, while maintaining
high op amp gain and stability, and must withstand shorts to
either rail, these tasks now fall to the integrator.
As a result of these demands, the integrator is a compound
affair with an embedded gain stage that is doubly fed forward
(via C_f and Cff) by a dedicated unity-gain compensation
driver. In addition, the output portion of the integrator is a
push-pull configuration for delivering heavy loads. While
sinking current the whole amplifier path consists of three
gain stages with one stage fed forward, whereas while
sourcing the path contains four gain stages with two fed
forward.
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FIGURE 1. LMC660 Circuit Topology (Each Amplifier)
The large signal voltage gain while sourcing is comparable
to traditional bipolar op amps, even with a 600Ω load. The
gain while sinking is higher than most CMOS op amps, due
to the additional gain stage; however, under heavy load
(600Ω) the gain will be reduced as indicated in the Electrical
Characteristics.
Compensating Input Capacitance
The high input resistance of the LMC660 op amps allows the
use of large feedback and source resistor values without los-
ing gain accuracy due to loading. However, the circuit will be
especially sensitive to its layout when these large-value re-
sistors are used.
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