FREQUENCY (Hz)

1k

10k

100k

1M

10M

100M

Gain LMP7701

Phase LMP7701

100

80

60

40

20

0

-20

80

100

120

140

160

180

200

SENSOR

V

+

V

-

+

-

+

-

LMP7707, LMP7708, LMP7709

www.ti.com

SNOSAW5B – JUNE 2007 – REVISED MARCH 2013

USAGE OF DECOMPENSATED AMPLIFIERS

This section discusses the differences between compensated and decompensated op amps and presents the

advantages of decompensated amplifiers. In high gain applications decompensated amplifiers can be used

without any changes compared to standard amplifiers. However, for low gain applications special frequency

compensation measures have to be taken to ensure stability.

Feedback circuit theory is discussed in detail, in particular as it applies to decompensated amplifiers. Bode plots

are presented for a graphical explanation of stability analysis. Two solutions are given for creating a feedback

network for decompensated amplifiers when relatively low gains are required: A simple resistive feedback

network and a more advanced frequency dependent feedback network with improved noise performance. Finally,

a design example is presented resulting in a practical application. The results are compared to fully compensated

amplifiers (Texas Instruments LMP7701/LMP7702/LMP7704).

COMPENSATED AMPLIFIERS

A (fully) compensated op amp is designed to operate with good stability down to gains of ±1. For this reason, the

compensated op amp is also called a unity gain stable op amp.

Figure 50 shows the Open Loop Response of a compensated amplifier.

Figure 50. Open Loop Frequency Response Compensated Amplifier (LMP7701)

This amplifier is unity gain stable, because the phase shift is still < 180°, when the gain crosses 0 dB (unity gain).

Copyright © 2007–2013, Texas Instruments Incorporated

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