1. 1 COMPARATORS Function: Compares two input voltages and produces an output in either of two states indicating the greater than or less than relationship of the inputs.

2. 2 What is a Comparator ? The comparator is an op-amp circuit that compares two input voltages and produces an output indicating the relationship between them. The inputs can be two signals (such as two sine waves) or a signal and a fixed dc reference voltage. Often used as an interface between digital and analog signals. Problem Solution

3. 3 Symbol & Transfer Characteristics V in V out V ref Ideal transfer characteristic V out V in VHVH VLVL V ref 0 Practical transfer characteristic V out V in VHVH VLVL V ref 0

4. 4 Threshold Comparators  The voltage at which a comparator changes from one level to another is called the crossover (or threshold) voltage.  Its value can be adjusted by adding resistors, as shown in the non-inverting comparator. RFRF R1R1 R VSVS V ref V in V out V+V+ V-V-

5. 5 From the superposition theorem, the voltage at V + is given by Ideally, the crossover will occur when V + = 0. That is which gives the low threshold voltage V Lt = V in as V out VinVin VHVH VLVL 0 VLtVLt Thus, the output voltage becomes high (V H ) at the positive saturation voltage. (+V sat ) when V + > 0 (i.e. V in > V Lt )

6. 6  If the input signal is connected to the inverting terminal, the output will change from high (V H ) to low (V L ). The high threshold voltage V Ht = V in is given by V out V in VHVH VLVL 0 VHtVHt Thus, the output voltage becomes low (V L ) at the negative saturation voltage :- (-V sat ) when V in > V + (i.e. V in > V Ht )

7. 7 BASIC COMPARATOR CIRCUITS  COMPARATOR WITH ZERO REFERENCE  COMPARATOR WITH NONZERO REFERENCE  COMPARATOR WITH HYSTERESIS

8. 8 NONLINEAR CIRCUITS  Nonlinear circuits such as comparators, wave shapers and active-diode circuits.  Linear circuits like voltage amplifier, current sources, and active filters.  The output of nonlinear op-amp circuits usually has a different shape from the input signal. This is due to the op-amp saturates during part of the input cycle.

9. 9 ZERO REFERENCE  The simplest way to build a comparator is to connect op-amp without feedback resistors. -V +V V out V in V out V in -V sat +V sat 0 a) Comparator with zero reference b) Input/output response

10. 10 Because of the high open-loop gain, positive input voltage produces positive saturation (+V sat ), and a negative input voltage produces negative saturation (- V sat ). This comparator is called a zero-crossing detector. The minimum input voltage that produces saturation is: ZERO REFERENCE

11. 11 If a sinusoidal input voltage applied to the non-inverting input of this circuit, the result will look like this: V in V out t t0 0 -V sat +V sat ZERO REFERENCE

12. 12 Let V sat = 15V, A ol = 100,000. Then the input voltage needed to produce saturation is: V in > +0.015 mV  +V sat V in < -0.015 mV  -V sat ZERO REFERENCE

13. 13 The output is a two-state output, either +V sat or –V sat This comparator can be used as a squaring circuit (i.e. produce square wave from sine wave). ZERO REFERENCE

14. 14 Bounded Output The output swing of a zero-crossing detector may be too large in some applications. We can bound the output by using a zener diode. There are three types: 1.Bounded at positive value 2.Bounded at negative value 3.Double bounded ZERO REFERENCE

15. 15 1.Bounded at positive value Dz R +V -V V in V out -0.7V +V z 0 ZERO REFERENCE

16. 16 2.Bounded at negative value Dz R +V -V V in V out +0.7V -V z 0 ZERO REFERENCE

17. 17 3.Double-bounded Dz1 R +V -V V in V out Dz2 V z2 + 0.7V - (V z1 + 0.7V) 0 ZERO REFERENCE

18. 18 NON-ZERO REFERENCES In some applications a threshold voltage different from zero may be preferred. By biasing either input, we can change the threshold voltage as needed. It also known as non-zero level detection

19. 19 Positive Threshold V in -V V in V ref V out +V -V sat +V sat R2R2 R1R1 When Vin > Vref, Vout is High (+Vsat) When Vin < Vref, Vout is Low (-Vsat) NON-ZERO REFERENCES

20. 20 Negative threshold If a negative limit is preferred, connect –V to the voltage divider. +V sat V ref -V sat V in V out When Vin > Vref, Vout is High (+Vsat) When Vin < Vref, Vout is Low (-Vsat) NON-ZERO REFERENCES

21. 21 Using Zener diode V out VzVz V in V ref +V sat -V sat +V -V R t t CONDITIONS: Vref = Vz When Vin is less than Vref, the output remains at the max negative level When Vin is more than Vref, the output goes to the max positive level NON-ZERO REFERENCES