Datasheet

FREQUENCY (Hz)
GAIN (dB)
1k 10k 100k 1M 10M 100M
Gain LMP7707
Phase LMP7707
PHASE (°)
100
80
60
40
20
0
-20
80
100
120
140
160
180
200
FPBW =
SR
2 V
P
í
LMP7707, LMP7708, LMP7709
SNOSAW5B –JUNE 2007–REVISED MARCH 2013 www.ti.com
Stability can be expressed in two different ways:
Phase MarginThis is the phase difference between the actual phase shift and 180°, at the point where the gain
is 0 dB.
Gain MarginThis 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
corresponding value of the open loop gain (also called G
MIN
) is 6 times.
Stability has only to do with the loop gain and not with the forward gain (G) of the op amp. For high gains, the
feedback network is attenuating and this reduces the loop gain; therefore the op amp will be stable for G > G
MIN
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.
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