Why an opamp may act like a latch or oscillator?

[wanily1983]

a latch or oscillator?

in gray's book,he said in an op amp is unstable in a givan feedback configuration,it does not act as a amplifier but a latch or oscillator. but why a latch?? hope for ur reply.

 

[elmolla]

Re: a latch or oscillator?

It'll act as a latch due to excess positive feedback that'll cause it to rest on one of the bi-stable possible positions of the poisitive or negative supply rail.

i.e. Think of the following case :
A single practical OpAmp with no feedback, any positive input above a small value, say 1 mv, will cause it to saturate to the positive supply rail and the same goes for the negative input. But for small inputs around 1mV they'll be amplified with a large gain.

Now if we do positive feedback and reach infinite gain(conceptually), won't this do the same effect but more severely. i.e. It'll saturate a much smaller input value say 10 uV. So this is similar to acting as a latch.

I hope things are clear right now.

 

[wanily1983]

a latch or oscillator?

what is the difference condition to cause latch and oscillate in feedback op amp?
in RAZAVI's book,he said if the positive feedback gain at about zero Herz, the circuit just latch not oscillate.

in the ring oscillator design, if the invert stage number is odd, if meet the the phase shift is 180° and the loop gain is larger than unit, then it will oscillate. if the invert stage number is even, the output will latch at the power supply voltage Vdd and Vss.
if i am wrong, plz correct me.

 

[sec]

Re: a latch or oscillator?

what is the difference between an op-amp and a comparator??

 

[wanily1983]

a latch or oscillator?

op amp used as amplifier must be in close loop configuration with negative feedback.
op amp in open loop configuration can use as a comparator, but it is a little slow, so in high speed application, it always use the dynamic comparator. u can search the related topic in the forum.