Out-of-the-loop compensation method

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8/22

3

Out-of-the-loop compensation method

3.1

Theoretical overview

A simple compensation method, using only one extra component, consists in adding a

resistor in series between the output of the amplifier and its load (see Figure 13). It is often

referred to as the out-of-the-loop compensation method because the additional component

network from the capacitive load.

From Figure 14, the loop gain with this compensation method is:

This compensation introduces a zero in the loop gain, just after the pole caused by the

capacitive load, at:

This pole is also unfortunately shifted to lower frequencies at:

However, due to the zero, the effect of the pole is minimized and the stability is improved. To

occurs at least one decade before unity-gain frequency. It then allows a significant shift of

the phase and therefore increase phase and gain margins.

accuracy).

Finally, this compensation method is effective, but the drawback is a limitation on the

Figure 13.

Out-of-the-loop compensation

schematics

Figure 14.

Out-of-the-loop equivalent

schematics for loop gain

calculation

V

r

V

s

------

–

A

ω

()

1

R

OL

R

L

-----------

jR

++

⎝⎠

⎛⎞

⋅

1

R

OL

Z

o

+

R

L

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

jZ

o

R

OL

+

()C

++

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

=

1

2

π R

OL

⋅

R

||

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

1

2

π

Z

o

R

OL

+

()

⋅

R

||

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