OPAMP Circuits - Schmitt Triggers, Differentiators and Integrators

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OPAMP CircuitsPART - III

• CEC

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Contents

• Inverting and Noninverting Schmitt Triggers.
• Introduction to Astable Multivibrator.
• Ideal and Practical Differentiators.
• Ideal and Practical Integrators.

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Schmitt Trigger

• Invented by American scientist Otto H. Schmitt in
  1934.
• A comparator circuit with hysteresis.
• Positive feedback to the noninverting input of a
  comparator or differential amplifier.
• Noninverting and Inverting Configurations.
• Output retains its value until the input changes
  sufficiently to trigger a change.

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Schmitt Trigger

• Also called Regenerative Comparator.
• Converts an analog input signal to a digital
  output signal.
• Sine wave to square wave conversion.
• An active circuit made to behave as a Schmitt
  trigger by applying a positive feedback.
• Loop gain is more than unity.
• Positive feedback introduced by adding a part of
  the output voltage to the noninverting input
  terminal of the OPAMP.

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Block Diagram of a Schmitt Trigger
Symbol
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Schmitt Trigger

• Uses two different threshold voltage levels to
  avoid input noise.
• Upper and lower trigger points .
• Action due to dual-threshold known as hysteresis.

Any noise amplitude between UTP and LTP eliminated
UTP
LTP
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Typical Hysteresis Curve
Dual Threshold Action
Circuit Transfer Function
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Comparator vs Schmitt Trigger
UTP
0
LTP
Comparator (Zero Crossing)
One Threshold
Schmitt Trigger
UTP and LTP
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Threshold Points

• Values at which the output makes transition from
  one level to the other called Threshold points
  or trip points.
• Upper Threshold Point (UTP) and Lower Threshold
  Point (LTP).
• As input amplitude crosses threshold points,
  output voltage changes state.

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Inverting Schmitt Trigger

• As input voltage increases and reaches the upper
  threshold point (UTP) the output voltage goes to
  negative saturation.
• As the input voltage decreases and reaches lower
  threshold point (LTP) the output voltage goes to
  positive saturation.
• When the voltage difference between UTP and LTP
  is larger than the noise, the output remains
  stable.

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The circuit has two stable state outputs.
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Inverting Schmitt Trigger
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Biased Inverting Schmitt Trigger
Reference Voltage
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Biased Inverting Schmitt Trigger

Reference Voltage
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Schmitt Trigger with a Reference Voltage
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Noninverting Schmitt Trigger

• As input voltage increases and reaches the upper
  threshold point (UTP) the output voltage goes to
  positive saturation.
• As the input voltage decreases and reaches lower
  threshold point (LTP) the output voltage goes to
  negative saturation.
• When the voltage difference between UTP and is
  larger than noise, the output remains stable.

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Noninverting Schmitt Trigger with a Reference
Voltage
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Schmitt Trigger Oscillator Astable Multivibrator
C charges when V0 Vsat C discharges when V0
-Vsat
Positive feedback ensures saturation.
Capacitor voltage determines the output state. RF
and C are the timing components.
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Applications of Schmitt Trigger

• One bit analog to digital conversion.
• To ensure proper level detection.
• To improve the noise immunity of a system.
• To ensure that a logic output level changes only
  when data changes and not as a result of spurious
  noise that may have been picked up.

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Ideal Integrator and Differentiator
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Ideal Integrator and Differentiator
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Ideal OPAMP Differentiator
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Ideal OPAMP Differentiator
i
Current cannot flow into the OPAMP input
terminals due to very high input impedance.
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Practical OPAMP Differentiator
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Ideal OPAMP Integrator
i
Current cannot flow into the OPAMP input
terminals due to very high input impedance.
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Thank You