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COMPARATORS AND CONVERTERS

MODULE-3

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COMPARATORS

  • A comparator compares a signal voltage on one input of an op-amp, with a known voltage called the reference voltage on the other input.
  • A comparator is an open loop op-amp, with two analog input and a digital output.
  • The output may be positive or negative saturation voltage based on which input is greater. In short comparator is a type of analog to digital converter.
  • Comparators are used in circuits such as digital interfacing, discriminators, voltage level detectors and oscillators.

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Non-Inverting Comparator

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Non-Inverting Comparator

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Inverting Comparator

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Inverting Comparator

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Non- Inverting

Zero Crossing Detector

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Inverting Zero Crossing Detector

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COMPARATOR CHARACTERISTICS

  1. Speed of Operation
  2. The output of the comparator should switch rapidly between saturation levels and also respond instantly to any changes in the input.
  3. Bandwidth of the op-amp comparator should be wide
  4. Wider the bandwidth higher is the speed of operation
  5. Speed of operation can be improved with positive feedback

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COMPARATOR CHARACTERISTICS

2. Accuracy

  • The accuracy of the comparator depends on its voltage gain, common-mode rejection ratio, input offsets and thermal drifts.
  • High voltage gain requires a small difference voltage to cause comparator’s output voltage to switch between saturation levels.
  • High CMRR helps to reject common-mode input voltages such as noise at the input terminals.
  • To minimize the offset problem, the input offset current and input offset voltage must be negligible.

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COMPARATOR CHARACTERISTICS

3. Compatibility of Output

  • Since the output of comparator is a form of analog to digital converter, its output must swing between the two logic levels suitable for a certain logic family such as TTL.

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LIMITATIONS OF OP-AMP AS COMPARATOR

  • A general purpose 741 op-amp can be relatively less critical comparator applications where speed and accuracy are not major factors.
  • With positive feedback, the switching speed of the op-amp comparator can be improved and false transition due to noise can be eliminated.
  • An offset voltage compensating network and offset minimizing resistor can be eliminated can be used to minimize offset problem
  • To keep the output voltage swing within the specific limits, op-amps are used with externally wired components such as zeners or diodes.

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Inverting Schmitt Trigger

A Schmitt Trigger is a squaring circuit where, the output voltage is triggered by input voltage every time it exceeds certain voltage levels called the upper threshold voltage and lower threshold voltage.

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Inverting Schmitt Trigger

 

 

If threshold voltages are made greater than input noise voltages, the positive feedback will eliminate false output transition

 

Also positive feedback, because of its regenerative action will make output voltage switch faster between saturation voltages.

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Adjusting the trigger points

 

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Adjusting the trigger points

 

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Adjusting the trigger points

 

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Adjusting the trigger points

 

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5V

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- 7.5V +

- 2.5V +

7.5V

NON-INVERTING SCHMITT TRIGGER

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5V

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- 15V +

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0

NON-INVERTING SCHMITT TRIGGER

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5V

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ADJUSTING TRIGGER POINTS

0V

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5V

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ADJUSTING TRIGGER POINTS

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ADJUSTING TRIGGER POINTS

 

 

 

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Analyse the Schmitt trigger circuit to determine the UTP and LTP. Assume that op-amp is rail to rail operated and

that the diode forward voltage drop is 0.7V

 

 

 

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Voltage to Current Converter with Grounded Load

 

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Current to Voltage Converter

 

 

 

 

 

 

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