CS3011
CS3012
The loop gain plot shown in Figure 17 illustrates
the unity gain configuration, and indicates how this
is modified when using the amplifier in a hig her
gain configuration withcompensation. If itis config-
ured for higher gain, for example, 60 dB, the x–axis
will move up by 60 dB (line B). Capacitor C2 adds
a zero and a pole. The modified plot indicates the
effects of introducing the pole and zero due to ca-
pacitor C2 . The pole can be located at any fre-
quency higher than the hand-over frequency, the
zero has to be at a frequency lower than the hand-
over freque ncy so as to provide ade quate ga in
margin. The separation between the pole and the
zero is governed by the closed loop gain. The zero
(z1) occurs at the intersection of the –100 dB/de-
cade and –80 dB/decade slopes. The point X inthe
figure should be at closed loop gain plus 20 dB
gain margin. The value for C2 = 1/(2πR1p1). Us-
ing p1 = 500 kHz works very well and is indepen-
dent of gain. As the closed loop ga in is changed,
the zero location is also modified if R1 remains
fixed. Capacitor C2 can be incre ased in value to
limit the amplifier’s rising noise above 1 kHz.
-100 dB/dec
z1
p1
-80 dB/dec
X
Margin
B
Desired Closed
Loop Gain
-20 dB/dec
50kHz
1MHz
5MHz
25 kHz 500 kHz 2.4 MHz
FREQUENCY
Figure 17. Loop Gain Plot: Unity Gain and with Pole-Zero Compensation
12
DS597F6
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