specified (Continued)

Small Signal Step

Response

(A

Large Signal Step

Response

(A

10006431

10006432

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:

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

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 10M

Ω and a gain of 100, one needs to come up with a

feedback resistor of 1000M

Ω -an expensive task.

An alternate solution is to use a “T” Network in the feedback

path, as shown in Fig. 2.

Closed loop gain, A

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)

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

by A

and then determine R1 based on this selection:

10006435

FIGURE 1. A

10006436

FIGURE 2. “T” Network Used to Replace High Value

Resistor

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