Table of Contents
Comparator, 1. Non-inverting Comparator, 2. Inverting Comparator
We will also examine integrated devices created especially as comparators and converters in an effort to achieve better performance. A comparator, as its name suggests, compares a voltage present on one input of an op-amp with a reference voltage present 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 Vref of 1 V is applied to the negative terminal and time varying signal voltage Vin is applied tot the positive terminal. When Vin is less than Vref the output becomes V0 at –Vsat [Vin < Vref => V0 (-Vsat)]. When Vin is greater than Vref, the (+) input becomes positive, the V0 goes to +Vsat. [Vin > Vref=> V0 (+Vsat)]. 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 Vid 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 Rcomp = R is connected between the (-ve) input and Vref.
Input and Output Waveforms
2. Inverting Comparator
The reference voltage Vref is applied to the (+) input terminal of an inverting comparator, and Vin is applied to the (-) input terminal. A 10K potentiometer is used in this circuit to create a voltage divider with the dc supply voltages +Vcc and -1 and the wiper connected to the input. Vref increases as the wiper is moved closer to +Vcc. As a result, by simply adjusting the 10k potentiometer, a Vref of the desired amplitude and polarity can be obtained.
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 Vref is set to Zero. (Vref =0V).
This Fig illustrates when an input signal Vin crosses zero volts in which direction. When the input signal Vin crosses zero in a positive direction, the o/p V0 is driven into negative saturation. Similar to how the output V0 switches and saturates positively when Vin passes through Zero in the negative direction.
Drawbacks of Zero- crossing detector:
In some applications, the input Vin might be a low frequency signal with a waveform that changes slowly. Since Vin will take longer to cross 0V, V0 might not quickly switch from one saturation voltage to the next. The output V0 may alternate between the two saturation voltages, +Vsat and -Vsat, as a result of the noise at the op-input amp’s terminals. Regenerative or positive feedback, which causes the output V0 to change more quickly and gets rid of any false output transitions brought on by noise signals at the input, can solve both of these issues. a comparator that inverts and has favourable feedback. This is referred to as a “Schmitt Trigger.”
Schmitt Trigger: [Square Circuit]
This circuit transforms a waveform with an irregular shape into a square wave or pulse. Schmitt Trigger or squaring circuit are two names for the circuit. Every time the input voltage Vin exceeds the upper threshold Vut and lower threshold voltage, the o/p V0 is triggered (its state is changed). The voltage divider R1 – R2 is used to calculate these threshold voltages, with the voltage across R1 serving as feedback to the (+) input.
The voltage across R1 is variable reference threshold voltage that depends on the value of the output voltage. When V0 = +Vsat, the voltage across R1 is called ―upper threshold voltage Vut. The input voltage Vin must be more positive than Vut in order to cause the output V0 to switch from +Vsat to –Vsat. As long as Vin< Vut , V0 is at +Vsat, using voltage divider rule,
V0 is at –Vsat. Vlt 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 +Vsat and –Vsat. Resistance Rcomp
The offset issues are minimised by using R1 || R2. A dead band condition called hysteresis is said to be present in the comparator receiving positive feedback. In other words, the output of the comparator changes from +Vsat to -Vsat when the input exceeds Vut and returns to +Vsat when the input falls below Vlt. The difference between Vut and Vlt is equivalent to the hysteresis voltage.