Stability in voltage follower configuration

AN2653

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Stability in voltage follower configuration

This section examines a voltage follower configuration because it is the worst case scenario

for stability (compared with a gain configuration).

In voltage follower configuration, the loop gain is:

The capacitive load adds a pole to the loop gain that impacts the stability of the system. The

higher the frequency of this pole, the greater the stability. In fact, if the pole frequency is

lower than or close to the unity gain frequency, the pole can have a significant negative

impact on phase and gain margins. It means that the stability decreases when the

capacitive load increases.

oscillations due to instability with and without an AC input signal for a capacitive load of

550 pF. The oscillation frequency is in line with the peaking frequency observed in a closed

loop gain configuration (approximately 1.9 MHz according to Figure 10).

Figure 9.

Voltage follower configuration

Figure 10.

Closed loop gain measured for a

voltage follower configuration

Voltage Follower Configuration - Closed Loop Gain

-40

-30

-20

-10

0

10

20

Frequency (Hz)

Gain without CL

Gain with CL = 550 pF

TS507 :

Vcc = 5 V

Vicm = 2,5 V

T = 25 °C

RL = 10 kΩ

Without CL

CL=550 pF

V

r

V

s

------

–

A

ω

()

1

Z

o

R

L

-------

jZ

++

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=