Datasheet
'
A
OL
G
min
f
d
f
1
fu
f
2
f
u
f
GBWP
UNITY-GAIN STABLE OP AMP
DECOMPENSATED OP AMP
1k
100k
100M
-20
20
100
GAIN (dB)
10M
1M
10k
80
40
0
60
FREQUENCY (Hz)
GAIN
PHASE
-20
20
100
80
40
0
60
PHASE (°)
1k
100k
100M
-20
20
100
GAIN (dB)
10M
1M
10k
80
40
0
60
FREQUENCY (Hz)
-20
20
100
80
40
0
60
PHASE (°)
LMP7717, LMP7718
SNOSAY7H –MARCH 2007–REVISED MARCH 2013
www.ti.com
Figure 46. LMP7717 A
VOL
vs. Frequency Figure 47. LMP7715 A
VOL
vs. Frequency
Figure 46 shows the much larger 88 MHz bandwidth of the LMP7717 as compared to the 17 MHz bandwidth of
the LMP7715 shown in Figure 47. The decompensated LMP7717 has five times the bandwidth of the LMP7715.
What is a Decompensated Op Amp?
The differences between the unity gain stable op amp and the decompensated op amp are shown in Figure 48.
This Bode plot assumes an ideal two pole system. The dominant pole of the decompensated op amp is at a
higher frequency, f
1
, as compared to the unity gain stable op amp which is at f
d
as shown in Figure 48. This is
done in order to increase the speed capability of the op amp while maintaining the same power dissipation of the
unity gain stable op amp. The LMP7717/LMP7718 have a dominant pole at 1.6 kHz. The unity gain stable
LMP7715/LMP7716 have their dominant pole at 300 Hz.
Figure 48. Open Loop Gain for Unity Gain Stable Op Amp and Decompensated Op Amp
Having a higher frequency for the dominate pole will result in:
1. The DC open loop gain (A
VOL
) extending to a higher frequency.
2. A wider closed loop bandwidth.
3. Better slew rate due to reduced compensation capacitance within the op amp.
The second open loop pole (f
2
) for the LMP7717/LMP7718 occurs at 45 MHz. The unity gain (f
u
’) occurs after the
second pole at 51 MHz. An ideal two pole system would give a phase margin of 45° at the location of the second
pole. The LMP7717/LMP7718 have parasitic poles close to the second pole, giving a phase margin closer to 0°.
Therefore it is necessary to operate the LMP7717/LMP7718 at a closed loop gain of 10 or higher, or to add
external compensation in order to assure stability.
For the LMP7715, the gain bandwidth product occurs at 17 MHz. The curve is constant from f
d
to f
u
which occurs
before the second pole.
For the LMP7717/LMP7718 the GBW = 88 MHz and is constant between f
1
and f
2
. The second pole at f
2
occurs
before A
VOL
=1. Therefore f
u
’ occurs at 51 MHz, well before the GBW frequency of 88 MHz. For decompensated
op amps the unity gain frequency and the GBW are no longer equal. G
min
is the minimum gain for stability and
for the LMP7717/LMP7718 this is a gain of 18 to 20 dB.
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