LMP7707MASLASHNOPB datasheet(22/41 Pages) TI1

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= 1 +

R //R

VOUT

AOL

Gmin = 20 dB

= 6 dB

= 1 +

|ACL|(min) = Gmin - 1

LMP7707, LMP7708, LMP7709

SNOSAW5B – JUNE 2007 – REVISED MARCH 2013

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For an inverting configuration:

(8)

If R1 and RF and are chosen so that the closed loop gain is lower than the minimum gain required for stability,

then 1/F intersects the open loop gain curve for a value that is lower than GMIN. For example, assume the GMIN is

equal to 10 V/V (20 dB). This is shown as the dashed line in Figure 53. The resistor choice of RF = R1 = 2 kΩ

makes the inverse feedback equal 2 V/V (6 dB), shown in Figure 53 as the solid line. The intercept of G and 1/F

represents the frequency for which the loop gain is identical to 1 (0 dB). Consequently, the total phase shift at the

frequency of this intercept determines the phase margin and the overall system stability. In this system example

1/F crosses the open loop gain at a frequency which is larger than the frequency where GMIN occurs, therefore

this system has less than 45° phase margin and is most likely instable.

Figure 53. 1/F for RF = R1 and Open Loop Gain Plot

RESISTIVE COMPENSATION

A straightforward way to achieve a stable amplifier configuration is to add a resistor RC between the inverting and

the non-inverting inputs as shown in Figure 54.

Figure 54. Op Amp with Compensation Resistor between Inputs

This additional resistor RC will not affect the closed loop gain of the amplifier but it will have positive impact on

the feedback network.

The inverse feedback function of this circuit is:

(9)

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LMP7707MA/NOPB Datasheet(PDF) 22 Page - Texas Instruments