Datasheet
GAIN (dB)
20
FREQUENCY (Hz)
60
40
80
0
-20 dB/dec
1M
1/F without compensation
1/F with compensation
20 dB/dec
100
=
V
OUT
V
IN
GF
R
F
1 +
1 +
R
1
1
LMP7707, LMP7708, LMP7709
SNOSAW5B –JUNE 2007–REVISED MARCH 2013
www.ti.com
(12)
By inspection of Equation 12, R
C
does not affect the ideal closed loop gain. In this example where R
F
= R
1
, the
closed loop gain remains at 6 dB as long as GF >> 1. The closed loop gain curve is shown as the solid line in
Figure 55.
The addition of R
C
affects the circuit in the following ways:
1. 1/F is moved to a higher gain, resulting in overall system stability.
However, adding R
C
results in reduced loop gain and increased noise gain. The noise gain is defined as the
inverse of the feedback factor, F. The noise gain is the gain from the amplifier input referred noise to the output.
In effect, loop gain is traded for stability.
2. The ideal closed loop gain retains the same value as the circuit without the compensation resistor R
C
.
LEAD-LAG COMPENSATION
This section presents a more advanced compensation technique that can be used to stabilize amplifiers. The
increased noise gain of the prior circuit is prevented by reducing the low frequency attenuation of the feedback
circuit. This compensation method is called Lead-Lag compensation. Lead-lag compensation components will be
analyzed and a design example using this procedure will be discussed.
The feedback function in a lead-lag compensation circuit is shaped using a resistor and a capacitor. They are
chosen in a way that ensures sufficient phase margin.
Figure 57 shows a Bode plot containing: the open loop gain of the decompensated amplifier, a feedback function
without compensation and a feedback function with lead-lag compensation.
Figure 57. Bode Plot of Open Loop gain G and 1/F with and without Lead-Lag Compensation
The shaped feedback function presented in Figure 57 can be realized using the amplifier configuration in
Figure 58. Note that resistor R
P
is only used for compensation of the input voltage caused by the I
BIAS
current. R
P
can be used to introduce more freedom for calculating the lead-lag components. This will be discussed later in
this section.
24 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: LMP7707 LMP7708 LMP7709