# Limiter + Opamp + Reverse phase on negative swing

G

#### George Jefferson

Jan 1, 1970
0
On page 239 of AOE there is a laboratory DC amp. The input uses a limiter.
It says that if Vin exceeds V- that the phase will change. I do not
understand what it means? The output of the op amp's phase will change sign?
Is this a normal property of op amps? e.g., if I put in a voltage that goes
below it's most negative rail it will invert the output?

Also, what about using fets for limiters instead of diodes? Any worth in
doing it or just adding useless complexity?

T

#### Tim Williams

Jan 1, 1970
0
Yes, it is a common feature. Typically, only amps which specifically note "no output reversal" are safe from this.

Single supply amps (including LM358, comparators like LM393, etc.) are prone to this, a small sacrifice for getting inputs down to -0.3V.

Cue larkin to talk about his new favorite FET current limiters.

Tim

K

#### Kevin McMurtrie

Jan 1, 1970
0
George Jefferson said:
On page 239 of AOE there is a laboratory DC amp. The input uses a limiter.
It says that if Vin exceeds V- that the phase will change. I do not
understand what it means? The output of the op amp's phase will change sign?
Is this a normal property of op amps? e.g., if I put in a voltage that goes
below it's most negative rail it will invert the output?

Also, what about using fets for limiters instead of diodes? Any worth in
doing it or just adding useless complexity?

http://en.wikipedia.org/wiki/Long-tailed_pair#Long-tailed_pair

Consider the case of the 'inv input' being between -v and +v and higher
than the 'non-inv input'. The output will go low as expected. Keep
raising 'inv input' and it will eventually force the output to an
incorrect high level. It gets worse if 'non-inv input' is driven too
high - the output will be an open circuit that may receive current from
outside the pair.

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