COMPARATORS Function Compares two input voltages and produces
- Slides: 21
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. 1
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. l Often used as an interface between digital and analog signals. l Problem Solution 2
Symbol & Transfer Characteristics Vin Vout Vref Vout VH 0 VH Vref VL Ideal transfer characteristic Vin 0 Vref Vin VL Practical transfer characteristic 3
Threshold Comparators q The voltage at which a comparator changes from one level to another is called the crossover (or threshold) voltage. q Its value can be adjusted by adding resistors, as shown in the non-inverting comparator. RF Vref R 1 VS Vin V+ VR Vout 4
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 VLt = Vin as Vout VH Thus, the output voltage becomes high (VH) at the positive saturation voltage. (+Vsat) when V+ > 0 (i. e. Vin > VLt) VL t 0 Vi n VL 5
q If the input signal is connected to the inverting terminal, the output will change from high (VH) to low (VL). The high threshold voltage VHt = Vin is given by Vout VH VHt 0 VL Vin Thus, the output voltage becomes low (VL) at the negative saturation voltage : (-Vsat) when Vin > V+ (i. e. Vin > VHt) 6
BASIC COMPARATOR CIRCUITS v COMPARATOR WITH ZERO REFERENCE v COMPARATOR WITH NONZERO REFERENCE v COMPARATOR WITH HYSTERESIS 7
NONLINEAR CIRCUITS q Nonlinear circuits such as comparators, wave shapers and active-diode circuits. q Linear circuits like voltage amplifier, current sources, and active filters. q 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. 8
ZERO REFERENCE q The simplest way to build a comparator is to connect op-amp without feedback resistors. Vout +V Vin +Vsat Vout 0 -V Vin -Vsat a) Comparator with zero reference b) Input/output response 9
ZERO REFERENCE Because of the high open-loop gain, positive input voltage produces positive saturation (+Vsat), and a negative input voltage produces negative saturation (Vsat). l This comparator is called a zero-crossing detector. l The minimum input voltage that produces saturation is: l 10
ZERO REFERENCE l If a sinusoidal input voltage applied to the non-inverting input of this circuit, the result will look like this: Vin 0 Vout t +Vsat 0 t -Vsat 11
ZERO REFERENCE l Let Vsat = 15 V, Aol = 100, 000. Then the input voltage needed to produce saturation is: Vin > +0. 015 m. V +Vsat Vin < -0. 015 m. V -Vsat 12
ZERO REFERENCE l The output is a two-state output, either +Vsat or –Vsat l This comparator can be used as a squaring circuit (i. e. produce square wave from sine wave). 13
ZERO REFERENCE Bounded Output The output swing of a zero-crossing detector may be too large in some applications. l We can bound the output by using a zener diode. l There are three types: l 1. Bounded at positive value 2. Bounded at negative value 3. Double bounded 14
ZERO REFERENCE 1. Bounded at positive value Dz +V Vin +Vz R Vout -V 0 -0. 7 V 15
ZERO REFERENCE 2. Bounded at negative value Dz +V Vin R +0. 7 V 0 Vout -V -Vz 16
ZERO REFERENCE 3. Double-bounded Dz 1 Dz 2 +V Vin R Vz 2 + 0. 7 V Vout -V 0 - (Vz 1 + 0. 7 V) 17
NON-ZERO REFERENCES In some applications a threshold voltage different from zero may be preferred. By biasing either input, we can change threshold voltage as needed. l It also known as non-zero level detection l 18
NON-ZERO REFERENCES Positive Threshold +V Vout R 1 +V Vref R 2 +Vsat Vout Vin Vref Vin -V -Vsat • When Vin > Vref, Vout is High (+Vsat) • When Vin < Vref, Vout is Low (-Vsat) 19
NON-ZERO REFERENCES Negative threshold l Vout If a negative limit is preferred, connect –V to the voltage divider. +Vsat Vref Vin -Vsat • When Vin > Vref, Vout is High (+Vsat) • When Vin < Vref, Vout is Low (-Vsat) 20
NON-ZERO REFERENCES CONDITIONS: Using Zener diode • 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 +V Vref R +V Vz Vin t Vout Vin -V +Vsat Vout t -Vsat 21
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