FREQUENCY (Hz)

1k

10k

100k

1M

10M

100M

Gain LMP7707

Phase LMP7707

100

80

60

40

20

0

-20

80

100

120

140

160

180

200

FPBW =

SR

2

í

LMP7707, LMP7708, LMP7709

SNOSAW5B – JUNE 2007 – REVISED MARCH 2013

www.ti.com

Stability can be expressed in two different ways:

Phase Margin This is the phase difference between the actual phase shift and 180°, at the point where the gain

is 0 dB.

Gain Margin This is the gain difference relative to 0 dB, at the frequency where the phase shift crosses the 180°.

The amplifier is supposed to be used with negative feedback but a phase shift of 180° will turn the negative

feedback into positive feedback, resulting in oscillations. A phase shift of 180° is not a problem when the gain is

smaller than 0 dB, so the critical point for stability is 180° phase shift at 0 dB gain. The gain margin and phase

margin express the margin enhancing overall stability between the amplifiers response and this critical point.

DECOMPENSATED AMPLIFIERS

Decompensated amplifiers, such as the LMP7707/LMP7708/LMP7709, are designed to maximize the bandwidth

and slew rate without any additional power consumption over the unity gain stable op amp. That is, a

decompensated op amp has a higher bandwidth to power ratio than an equivalent compensated op amp.

Compared with the unity gain stable amplifier, the decompensated version has the following advantages:

1. A wider closed loop bandwidth

2. Better slew rate due to reduced compensation capacitance within the op amp

3. Better Full Power Bandwidth, given with Equation 2

(2)

Figure 51 shows the frequency response of the decompensated amplifier.

Figure 51. Open Loop Frequency Response Decompensated Amplifier (LMP7707)

As shown in Figure 51, the reduced internal compensation moves the first pole to higher frequencies. The

second open loop pole for the LMP7707/LMP7708/LMP7709 occurs at 4 MHz. The extrapolated unity gain (see

dashed line in Figure 51) occurs at 14 MHz. An ideal two pole system would give a phase margin of > 45° at the

location of the second pole. Unfortunately, the LMP7707/LMP7708/LMP7709 have parasitic poles close to the

second pole, giving a phase margin closer to 0°. The LMP7707/LMP7708/LMP7709 can be used at frequencies

where the phase margin is > 45°. The frequency where the phase margin is 45° is at 2.4 MHz. The

Stability has only to do with the loop gain and not with the forward gain (G) of the op amp. For high gains, the

and no special measures are required. For low gains the feedback network attenuation may not be sufficient to

ensure loop stability for a decompensated amplifier. However, with an external compensation network

decompensated amplifiers can still be made stable while maintaining their advantages over unity gain stable

amplifiers.

20

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