Comparator

Comparator

To obtain  for  better  performance,  we  shall  also  look  at  integrated  designed  specifically  as comparators and converters. A comparator as its name implies, 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 Discriminato Voltage level detector and oscillators

1. Non-inverting Comparator:

A fixed reference voltage V_ref of 1 V is applied to the negative terminal and time varying signal voltage V_in is applied tot the positive terminal. When V_in is less than V_ref the output becomes V₀ at –V_sat [V_in < V_ref => V₀ (-V_sat)]. When V_in 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 protects the op-amp from damage due to the excessive input voltage Vin. Because of these diodes, the difference input voltage V_id of the op-amp diodes are called clamp diodes. The resistance R in series with Vin 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.

Input and Output Waveforms:

2. Inverting Comparator:

This fig shows an inverting comparator in which the reference voltage V_ref is applied to the (+) input terminal and Vin 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, Vref becomes more positive. Thus a V_ref of a desired amplitude and polarity can be obtained by simply adjusting the 10k potentiometer

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

One of the application 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 V_ref is set to Zero. (V_ref =0V).

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 V_in passes through Zero in negative direction the output V₀ switches and saturates positively.

Drawbacks of Zero- crossing detector:

In some applications, the input V_in may be a slowly changing waveform, (i.e) a low frequency signal. It will take V_in 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 +V_sat and –V_sat. 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 .

Schmitt Trigger: [Square Circuit]

This circuit converts an irregular shaped waveform to a square wave or pulse. The circuit is known as Schmitt Trigger or squaring circuit. The input voltage V_in triggers (changes the state of) the o/p V₀ every time it exceeds certain voltage levels called the upper threshold Vut and lower threshold voltage. These threshold voltages are obtained by using theh voltage divider R₁  – R₂,  where the voltage across R₁ is feedback to  the (+) input.

The voltage across R₁ is variable reference threshold voltage that depends on the value of the output voltage. When V₀ = +V_sat, the voltage across R₁ is called ―upper threshold voltage V_ut. The input voltage Vin must be more positive than V_ut in order to cause the output V0 to switch from +V_sat to –V_sat. As long as V_in< V_ut , V₀ is at +Vsat, using voltage divider rule,

V₀ is at –V_sat. V_lt is given by the following eqn.

Thus, if the threshold voltages Vut and Vlt are made larger than the input noise voltages, the positive feedback will eliminate the false o/p transitions. Also the positive feedback, because of its regenerative action, will make V0 switch faster between +V_sat and –V_sat. Resistance R_comp

R₁ || R₂ is used to minimize the offset problems. The comparator with positive feedback is said texhibit hysteresis, a dead band condition. (i.e) when the input of the comparator exceeds V_ut its output switches from +V_sat to –V_sat and reverts to its original state, +V_sat when the input goes below V_lt. The hysteresis voltage is equal to the difference between V_ut and V_lt.