1. Comparator What is a Comparator?
2nd most widely used building block after Op Amp
The comparator is essentially a 1-bit analog to digital converter.
Input is analog
Output is digital
Types of comparators:
Open-loop (op amps without compensation)
Regenerative (use of positive feedback - latches)
Combination of open-loop and regenerative comparators

2. Circuit symbol:
Ideal I-O transfer curve
Actual I-O transfer curve

3. Static Specs Static Characteristics Gain Output high and low states
Slope of the linear region
=(Voh-Vol)/(Vih-Vil)
Output high and low states
Voh, and Vol
Needs to be clearly defined as digital 1, 0
Input resolution
Vih – vil
Vi_range / (Vih-Vil), can be expressed as bits
Offset
(Vih+Vil)/2
Noise

4. Comparator Noise
Noise of a comparator is modeled as if the comparator were biased in the transition region.
Noise leads to an uncertainty in the transition point which causes jitter or phase noise.

5. Offset cancellation 1_advanced
Switches off slightly ahead of Φ1 to minimize charge injection.

6. Offset cancellation
Chapter 10 Figure 06

7. Must have stability during phi_1
Book’s solution:
compensate during phi_1
Chapter 10 Figure 05

8. Alternate solution: Offset cancellation by stage
VDD
VDD vs
Vin+
vin2

9. Propagation Time Delay
Rising propagation delay time:
Propagation delay =(Rising delay + Falling delay)/2
or = Rising delay + Falling delay

10. Single pole response of amplifier

11. Time delay due to dynamic response

12. Slew Rate of a Comparator
If the rate of rise or fall of a comparator becomes large, the dynamics may be limited by the slew rate.
Slew rate comes from the relationship,
i = Cdv/dt
where i is the current through a capacitor and v is the voltage across it.
If the current becomes limited, then the voltage rate becomes limited. Therefore for a comparator that is slew rate limited we have,
tp = ∆T =∆V/SR =(VOH- VOL)/2·SR
where
SR = slew rate of the comparator.

13. Find the propagation delay time of an open loop comparator that has a dominant pole at 103 radians/sec, a dc gain of 104, a slew rate of 1V/µs, and a binary output voltage swing of 1V. Assume the applied input voltage is 10mV.
Solution
The input resolution for this comparator is 1V/104 or 0.1mV. Therefore, the 10mV input is 100 times larger than vin(min) giving a k of 100. Therefore, we get
tp =1/103 ln(2·100/(2·100-1))
= 10-3 ln(200/199) = 5.01µs
For slew rate considerations, we get
tp =1/(2·1x106) = 0.5µs
Therefore, the propagation delay time for this case is the larger or 5.01µs.
But for Vin=Vin(min), tp =1/103 ln(2/(2-1)) = 693µs.

14. Two-Stage Comparator
An important category of comparators are those which use a high-gain stage to drive their outputs between VOH and VOL for very small input voltage changes.
The two-stage op amp without compensation is an excellent implementation of a high-gain, open-loop comparator.

15. Delay time  1/(√Av(0))

16. Push-Pull Comparators
Push-pull output  higher slew rate

17. Driving Large Capacitive Load
Inverters
clean up
output
Typically added at latch output.

19. Chapter 10 Figure 07

22. Chapter 10 Figure 10