A

= -

= 1 -

CL

R

F

R

1

1

F

A

= 1 +

=

CL

R

F

R

1

1

F

= 1 +

R

F

R

1

1

F

F =

-

+

-

+

LMP7707, LMP7708, LMP7709

www.ti.com

SNOSAW5B – JUNE 2007 – REVISED MARCH 2013

This section explains how decompensated amplifiers can be used in configurations requiring a gain lower than

how stability can be determined using the frequency response curve of the op amp together with the feedback

factor. Using the circuit theory, it will be explained how decompensated amplifiers can be stabilized at lower

gains.

FEEDBACK THEORY

Stability issues can be analyzed by verifying the loop gain function GF, where G is the open loop gain of the

amplifier and F is the feedback factor of the feedback circuit.

The feedback function (F) of arbitrary electronic circuits, as shown in Figure 52, is defined as the ratio of the

input and output signal of the same circuit.

Figure 52. Op Amp with Resistive Feedback. (a) Non-inverting (b) Inverting

(3)

GRAPHICAL EXPLANATION OF STABILITY ANALYSIS

Stability issues can be observed by verifying the closed loop gain function GF. In the frequencies of interest, the

open loop gain (G) of the amplifier is a number larger than 1 and therefore positive in dB. The feedback factor (F)

of the feedback circuit is an attenuation and therefore negative in dB. For calculating the closed loop gain GF in

dB we can add the values of G and F (both in dB).

One practical approach to stabilizing the system, is to assign a value to the feedback factor F such that the

constant over frequency and should intercept the open loop gain at a value in dB that is greater than or equal to

The inverse feedback factor for both configurations shown in Figure 52, is given by:

(4)

The closed loop gain for the non-inverting configuration (a) is:

(5)

The closed loop gain for the inverting configuration (b) is:

(6)

For stable operation the phase margin must be equal to or greater than 45°. The corresponding closed loop gain

(7)

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