LMC6442 Datasheet, PDF (12/24 Pages) National Semiconductor (TI) – Dual Micropower Rail-to-Rail Output Single Supply Operational Amplifier

LMC6442
SNOS013E – SEPTEMBER 1997 – REVISED MARCH 2013
APPLICATIONS INFORMATION
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USING LMC6442 IN UNITY GAIN APPLICATIONS
LMC6442 is optimized for maximum bandwidth and minimal external components when operating at a minimum
closed loop gain of +2 (or −1). However, it is also possible to operate the device in a unity gain configuration by
adding external compensation as shown in Figure 35:
Figure 35. A_V = +1 Operation by adding C_C and R_C
Using this compensation technique it is possible to drive capacitive loads of up to 300 pF without causing
oscillations (see the Typical Performance Characteristics for step response plots). This compensation can also
be used with other gain settings in order to improve stability, especially when driving capacitive loads (for
optimum performance, R_C and C_C may need to be adjusted).
USING “T” NETWORK
Compromises need to be made whenever high gain inverting stages need to achieve a high input impedance as
well. This is especially important in low current applications which tend to deal with high resistance values. Using
a traditional inverting amplifier, gain is inversely proportional to the resistor value tied between the inverting
terminal and input while the input impedance is equal to this value. For example, in order to build an inverting
amplifier with an input impedance of 10MΩ and a gain of 100, one needs to come up with a feedback resistor of
1000 MΩ -an expensive task.
An alternate solution is to use a “T” Network in the feedback path, as shown in Figure 36.
Closed loop gain, A_V is given by:
(1)
Figure 36. “T” Network Used to Replace High Value Resistor
It must be noted, however, that using this scheme, the realizable bandwidth would be less than the theoretical
maximum. With feedback factor, β, defined as:
(2)
BW(−3 dB) ≈ GBWP • β
(3)
In this case, assuming a GBWP of about 10 KHz, the expected BW would be around 50 Hz (vs. 100 Hz with the
conventional inverting amplifier).
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