MCP660 datasheet(24/68 Pages) MICROCHIP

24 / 68 page

MCP660/1/2/3/4/5/9

DS20002194E-page 24

 2009-2014 Microchip Technology Inc.

4.4.2

GAIN PEAKING

Figure 4-8 shows an op amp circuit that represents

non-inverting amplifiers (VM is a DC voltage and VP is

the input) or inverting amplifiers (VP is a DC voltage

and VM is the input). The capacitances CN and CG

represent the total capacitance at the input pins; they

include the op amp’s common-mode input capacitance

(CCM), board parasitic capacitance and any capacitor

placed in parallel.

FIGURE 4-8:

Amplifier with Parasitic

Capacitance.

CG acts in parallel with RG (except for a gain of +1 V/V),

which causes an increase in gain at high frequencies.

CG also reduces the phase margin of the feedback

loop, which becomes less stable. This effect can be

reduced by either reducing CG or RF.

CN and RN form a low-pass filter that affects the signal

at VP. This filter has a single real pole at 1/(2RN/CN).

The largest value of RF that should be used depends

on the noise gain (see GN in Section 4.4.1

“Capacitive Loads”), CG and the open-loop gain’s

phase

shift.

Figure 4-9

shows

the

maximum

recommended RF for several CG values. Some

applications may modify these values to reduce either

output loading or gain peaking (step response

overshoot).

FIGURE 4-9:

Maximum Recommended

RF vs. Gain.

Figures 2-35 and 2-36 show the small signal and large

signal step responses at G = +1 V/V. The unity-gain

buffer usually has RF =0 and RG open.

Figures 2-37 and 2-38 show the small signal and large

signal step responses at G = -1 V/V. Since the noise

gain is 2 V/V and CG  10 pF, the resistors were

chosen to be RF =RG =401 and RN = 200.

It is also possible to add a capacitor (CF) in parallel with

RF to compensate for the destabilizing effect of CG.

This makes it possible to use larger values of RF. The

conditions for stability are summarized in Equation 4-6.

EQUATION 4-6:

VOUT

MCP66X

1.E+02

1.E+03

1.E+04

1.E+05

110

100

Noise Gain; GN (V/V)

GN > +1 V/V

100

10k

100k

CG = 10 pF

CG = 32 pF

CG = 100 pF

CG = 320 pF

CG = 1 nF

We need:

Given:

-------

R

FCF

---------------------

----------





GBWP

---------------,





GBWP

---------------,





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