Datasheet

V
OUT
V
IN
=
1
F
V
OUT
V
IN
=
A
1 + AF
F =
V
A
- V
B
V
OUT
LMV793
R
IN
R
C
C
R
F
LMV793, LMV794
SNOSAX6D –MARCH 2007–REVISED MARCH 2013 www.ti.com
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 LMV793/LMV794 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 LMV793/LMV794 have parasitic poles close to the second pole, giving a phase margin closer to 0°.
Therefore it is necessary to operate the LMV793/LMV794 at a closed loop gain of 10 or higher, or to add external
compensation in order to assure stability.
For the LMV796, 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 LMV793/LMV794, 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 LMV793/LMV794 this is a gain of 18 to 20 dB.
Input Lead-Lag Compensation
The recommended technique which allows the user to compensate the LMV793/LMV794 for stable operation at
any gain is lead-lag compensation. The compensation components added to the circuit allow the user to shape
the feedback function to make sure there is sufficient phase margin when the loop gain is as low as 0 dB and still
maintain the advantages over the unity gain op amp. Figure 44 shows the lead-lag configuration. Only R
C
and C
are added for the necessary compensation.
Figure 44. LMV793 with Lead-Lag Compensation for Inverting Configuration
To cover how to calculate the compensation network values it is necessary to introduce the term called the
feedback factor or F. The feedback factor F is the feedback voltage V
A
-V
B
across the op amp input terminals
relative to the op amp output voltage V
OUT
.
(1)
From feedback theory the classic form of the feedback equation for op amps is:
(2)
A is the open loop gain of the amplifier and AF is the loop gain. Both are highly important in analyzing op amps.
Normally AF >>1 and so the above equation reduces to:
(3)
Deriving the equations for the lead-lag compensation is beyond the scope of this datasheet. The derivation is
based on the feedback equations that have just been covered. The inverse of feedback factor for the circuit in
Figure 44 is:
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