LMC6442 Datasheet, PDF (11/16 Pages) National Semiconductor (TI) – Dual Micropower Rail-to-Rail Output Single Supply Operational Amplifier

Typical Performance Characteristics V_S = 5V, Single Supply, T_A = 25˚C unless otherwise
specified (Continued)
Small Signal Step
Response
(A_V = + 1) For Various C_L
Large Signal Step
Response
(A_V = +1) (C_L= 200pF)
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Applications Information
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 1:
Closed loop gain, A_V is given by:
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FIGURE 1. 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 re-
sponse 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 invert-
ing 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 resis-
tor 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 1000MΩ -an expensive task.
An alternate solution is to use a “T” Network in the feedback
path, as shown in Fig. 2.
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FIGURE 2. “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:
BW(−3 dB) ≈ GBWP • β
In this case, assuming a GBWP of about 10 KHz, the ex-
pected BW would be around 50 Hz (vs 100 Hz with the
conventional inverting amplifier).
Looking at the problem from a different view, with R_F defined
by A_V•Rin, one could select a value for R in the “T” Network
and then determine R1 based on this selection:
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