Datasheet
=
1
F
f = 0
1 +
R
F
R
IN
f
z
=
1
2SR
c
+ R
IN
|| R
F
)C
f
p
=
1
2SR
c
C
=
1
F
¨
¨
©
§
1 +
R
F
R
IN
¨
¨
©
§
¨
¨
©
§
1 + s(R
c
+ R
IN
|| R
F
) C
1 + sR
c
C
¨
¨
©
§
V
OUT
V
IN
=
1
F
V
OUT
V
IN
=
A
1 + AF
F =
V
A
- V
B
V
OUT
LMP7717
R
IN
R
C
C
R
F
LMP7717, LMP7718
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SNOSAY7H –MARCH 2007–REVISED MARCH 2013
Input Lead-Lag Compensation
The recommended technique which allows the user to compensate the LMP7717/LMP7718 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 49 shows the lead-lag configuration. Only R
C
and C
are added for the necessary compensation.
Figure 49. LMP7717 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 49 is:
(4)
where 1/F's pole is located at
(5)
1/F's zero is located at
(6)
(7)
The circuit gain for Figure 49 at low frequencies is −R
F
/R
IN
, but F, the feedback factor is not equal to the circuit
gain. The feedback factor is derived from feedback theory and is the same for both inverting and non-inverting
configurations. Yes, the feedback factor at low frequencies is equal to the gain for the non-inverting configuration.
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