LMP7709MTSLASHNOPB datasheet(23/41 Pages) TI1

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VOUT - VX

VX - VIN

(VIN ± VX) G = VOUT

VOUT

VIN

AOL

Gmin = 20 dB

1 +

= 6 dB

LMP7707, LMP7708, LMP7709

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SNOSAW5B – JUNE 2007 – REVISED MARCH 2013

Proper selection of the value of RC results in the shifting of the 1/F function to GMIN or greater, thus fulfilling the

condition for circuit stability. The compensation technique of reducing the loop gain may be used to stabilize the

circuit for the values given in the previous example, that is GMIN = 20 dB and RF = R1 = 2 kΩ. A resistor value of

250

Ω applied between the amplifier inputs shifts the 1/F curve to the value GMIN (20 dB) as shown by the

dashed line in Figure 55. This results in overall stability for the circuit. This figure shows a combination of the

open and closed loop gain and the inverse feedback function.

This example, represented by Figure 52 and Figure 53, is generic in the sense that the GMIN as specified did not

distinguish between inverting and non-inverting configurations.

Figure 55. Compensation with Reduced Loop Gain

The technique of reducing loop gain to stabilize a decompensated op amp circuit will be illustrated using the non-

inverting input configuration shown in Figure 56.

Figure 56. Closed Loop Gain Analysis with RC

The effect of the choice of resistor RC in Figure 56 on the closed loop gain can be analyzed in the following

manner:

Assume the voltage at the inverting input of the op amp is VX. Then,

where

•

G is the open loop gain of the op amp

(10)

(11)

Combining Equation 10, Equation 11, and Equation 9 produces the following equation for closed loop gain,

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LMP7709MT/NOPB Datasheet(PDF) 23 Page - Texas Instruments