Negative vs. positive feedback

What is the difference?

Consider what happens when there is a perturbation

Positive feedback drives op amp into saturation:

vOUT ± VS

Static Analysis of Positive Feedback Circuit

Representing dynamics of op amp: time dependence

Let us consider this circuit and analyze its dynamics to build insight

Time behavior of vO

T

t

Kev

0

Consider a Small Disturbance to v0 (Noise)

0

0

vv

vv

A

RCT

T

t

Kev

0

v- > v+

v+ > v-

v+ = v-

One use of instability: Build Comparator

Now, Use Positive Feedback

Without Hysteresis

Oscillator: Can Create a Clock

Assume v0=VS at t=0, vC=0

Clock in Digital Systems

We build an oscillator using an op amp

Why do we use a clock in a digital system?

a) 1,1,0?b) When is the signal valid?

• Common timebase – when to “look” at a signal (e.g. whenever the clock is high)

Discretization of time: one bit of information associated with an interval of time (cycle)

Physical Limitations of Op Amp

Voltage limit: VS- < vout <VS

+

Frequency Response Limits

• The product of gain and band width in any given op-amp is constant, i.e. A= K

• Increasing the gain-bandwidth product by means of amplifiers in cascade.

Offset voltage limitSlew rate limit