"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10

Compensated Attenuator Applied to Op Amp
1/(R F||R GC G) pole moves close to τ 2 the stage is set for instability. Reasonable component
values for a CMOS op amp are R F = 1 MΩ, R G = 1 MΩ, and C G = 10 pF. The resulting
pole occurs at 318 kHz, and this frequency is lower than the breakpoint of τ 2 for many op
amps. There is 90 of phase shift resulting from τ 1, the 1/(R F||R GC) pole adds 45° phase
shift at 318 kHz, and τ 2 starts to add another 45° phase shift at about 600 kHz. This circuit
is unstable because of the stray input capacitance. The circuit is compensated by adding
a feedback capacitor as shown in Figure 7–18.
VIN
RG
Figure 7–18. Compensated Attenuator Circuit
+
_ a
CG
RF
CF
VOUT
The loop gain with C F added is given by Equation 7–19.
A
R G
R G C G s1
RG RGCGs1
RF RFCFs1
If R GC G = R FC F Equation 7–19 reduces to Equation 7–20.
A RG RG RF K
1s 12s 1
K
1s 12s 1
Voltage-Feedback Op Amp Compensation
(7–19)
(7–20)
The compensated attenuator Bode plot is shown in Figure 7–19. Adding the correct
1/R FC F breakpoint cancels out the 1/R GC G breakpoint; the loop gain is independent of
the capacitors. Now is the time to take advantage of the stray capacitance. C F can be
formed by running a wide copper strip from the output of the op amp over the ground plane
under R F; do not connect the other end of this copper strip. The circuit is tuned by removing
some copper (a razor works well) until all peaking is eliminated. Then measure the
copper, and have an identical trace put on the printed-circuit board.
7-17