Comparator using Operational Amplifier

Comparator

A comparator compares a signal voltage on one input of an op-amp with a known voltage called a reference voltage on the other input. Comparators are used in circuits such as,

·           Digital Interfacing

·           Schmitt Trigger

·           Discriminator

·           Voltage level detector and oscillators

Non-inverting Comparator:

A fixed reference voltage V_ref of 1 V is applied to the negative terminal and time varying signal voltage Vin is applied to the positive terminal.

When Vin is less than V_ref the output becomes V₀ at –V_sat

[V_in < V_ref => V₀ (-V_sat)].

When Vin is greater than V_ref, the (+) input becomes positive, the V₀ goes to +V_sat.

[V_in > V_ref => V₀ (+V_sat)].

Thus the V₀ changes from one saturation level to another.

The diodes D₁ and D₂ protect the op-amp from damage due to the excessive input voltage V_in. Because of these diodes, the difference input voltage Vid of the op-amp diodes are called clamp diodes.

The resistance R in series with V_in is used to limit the current through D₁ and D₂. To reduce offset problems, a resistance R_comp = R is connected between the (-ve) input and V_ref.

Inverting Comparator:

This fig shows an inverting comparator in which the reference voltage V_ref is applied to the (+) input terminal and V_in is applied to the (-) input terminal.

In this circuit V_ref is obtained by using a 10K potentiometer that forms a voltage divider with DC supply volt +V_cc and -1 and the wiper connected to the input. As the wiper is moved towards +V_cc, V_ref becomes more positive. Thus a Vref of a desired amplitude and polarity can be got by simply adjusting the 10k potentiometer.

Applications:

Zero Crossing Detector: [ Sine wave to Square wave converter]

One of the applications of comparator is the zero crossing detector or ―sine wave to Square wave Converter. The basic comparator can be used as a zero crossing detector by setting Vref is set to Zero.

This Fig shows when in what direction an input signal V_in crosses zero volts. (i.e.) the o/p V₀ is driven into negative saturation when the input the signal V_in passes through zero in positive direction. Similarly, when Vin passes through Zero in negative direction the output V₀ switches and saturates positively.

Drawbacks of Zero- crossing detector:

In some applications, the input Vin may be a slowly changing waveform, (i.e) a low frequency signal. It will take Vin more time to cross 0V, therefore V₀ may not switch quickly from one saturation voltage to the other.

Because of the noise at the op-amp‘s input terminals the output V0 may fluctuate between 2 saturations voltages +Vsat and –Vsat. Both of these problems can be cured with the use of regenerative or positive feedback that cause the output V0 to change faster and eliminate any false output transitions due to noise signals at the input Inverting comparator with positive feedback This is known as Schmitt Trigger.