Op-amp as a comparator

op-amp as a comparator

The op-amp in open loop configuration can be used as a basic comparator. When two inputs are applied to the open loop op-amp then it compares the two inputs. Depending upon the comparison, it produces output voltage which is either positive saturation voltage (+ V_sat) or negative saturation voltage (- V_sat).

A comparator is a circuit which compares a signal voltage applied at one input of an op-amp with a known reference voltage at the other input and produce either a high or a low output voltage, depending on which input is higher. As comparator output has two voltage levels, either high or low, it is not linearly proportional to input voltage.

There are two types of comparator circuits which are,

1. Non-inverting comparator 2. Inverting comparator.

1. Basic Non-inverting Comparator

In this comparator, the input voltage is applied to the non-inverting terminal and no reference voltage is applied to other terminal. So inverting terminal is grounded. The input voltage is denoted as V_in while the voltage applied to other terminal with which is compared is denoted as V_ref. In the basic comparator, V_ref = 0 V. The basic non-inverting comparator is shown in the Fig. 3.12.1.

In the non-inverting comparator, if Fig. 3.12.1 Basic non-inverting comparator is greater than Vref then output is + V_sat i.e. almost equal to + V_CC. While if V_in is less than V_ref then output is - V_sat i.e. almost equal to - V_EE.

Thus for Fig. 3.12.1, as V_ref = 0 V when V_in is positive then V_o = + V_sat ≈ + V_CC while when is negative then V_o = - V_sat ≈- V_EE. This is because, as open loop gain op-amp (A_OL) is very very high even for very small the op-amp output saturates.

Thus the two possible output levels of the comparator are + Vsat and - Vsat, indicating whether the input voltage is greater than or less than the reference voltage. Such type of the comparator, in which the operation is at saturation level is known as saturating type of comparator. Assuming symmetrical conditions, the two possible output levels of the saturating type comparator are + V_sat and - V_sat.

Note that no feedback is applied to the op-amp and it is operated in open loop conditions, because of which the op-amp is operating in saturating conditions.

The input and output waveforms for a basic non-inverting comparator, for sinusoidal input are shown in the Fig. 3.12.2.

The op-amp differential voltage gain A_OL is very large. So when inverting input is grounded, very small input voltage in the range of microvolt is enough to saturate the op-amp. The ±V_sat , the saturation voltage levels of op-amp are mentioned in the data sheet. Hence knowing V_sat  and differential     non-inverting comparator voltage gain A, we can determine the minimum input voltage level required to saturate op-amp as,

V_{in (min)} = V_sat / A_OL          for saturation

Now the transfer characteristics is the graph of and Vout. As AOL is very large hence for very very small positive or negative V^, the output saturates. Hence at = 0, the transfer characteristics is almost a straight line as shown in the Fig. 3.12.3 (a). For example, for 741 C op-amp, A_OL is 100,000 while ± Vsat levels are ± 13.5 V for supply of ±15 V.

Vin = V_sat / A_OL = ± 13.5 / 100000 = ± 135 µV

Thus for + 135 µV of V_in, output saturates to + V_sat while -135 µV of is enough to saturate output at - V_sat. Hence region -135 µV to + 135 µV of the graph of and Vout is linear. But this range is so small that near V_in = 0 practically we get a straight line transfer characteristics.

The Fig. 3.12.3 (a) and (b) shows the ideal and practical transfer characteristics of a basic non-inverting comparator.

The point at which the transfer characteristics is straight line is called a trip point. The trip point is the input voltage at which the output changes its states from low to high or high to low. In the basic comparator this trip point is zero as at = 0, the output changes its states.

Key Point So we can say that when Vin is greater than trip point, the output is high while if Vin is less than the trip point the ouput is low.

As this change over occurs at = 0, the basic comparator can be used to detect occurrence of zero in the input voltage. Hence this circuit is called zero crossing detector. But in practice it is possible to change the trip point from zero to other voltage. This is achieved by some modifications in the basic comparator circuit.

Moving a Trip Point

By application of a reference voltage to the inverting input rather than grounding it, the trip point can be moved.

The Fig. 3.12.4 shows the application of reference voltage to the inverting input of a basic comparator using a potential divider consisting of resistors R₁ and R₂.

The reference voltage V_ref is derived using supply + V_CC and potential divider R₁ and R₂. Mathematically V_ref is expressed as, 

V_ref  = (+V_CC / R₁ + R₂) R₂

Now as long as input voltage is less than V_ref, the output is low i.e. - V_sat. When becomes slightly greater than V_ref, the op-amp output becomes high i.e. + V_sat. Thus the trip point is moved from = 0 to = V_ref due to reference voltage applied to the inverting input terminal.

A bypass capacitor is used on the inverting input to reduce the amount of power supply ripple and noise appearing at the inverting input of op-amp. For effective bypassing of ripple and noise, the critical frequency of the bypass circuit must be much lower than the ripple frequency of power supply.

The transfer characteristics of such a comparator is shown in the Fig. 3.12.4 (b) which indicates positive trip point. Such a comparator is also called a limit detector as it detects the particular positive level of input beyond which output goes high. The resistances R₁ and R₂ can be used to set the trip point anywhere between 0 and + V_CC.

It is possible to obtain a negative trip point by providing a negative reference voltage to the inverting input. This is achieved by using a supply - V_EE to the potential divider of R₁ and R₂.

V_ref = (-R₂ / R₁ + R₂) V_EE

This is shown in the Fig. 3.12.5 (a). When V_in is positive than -V_ref , error voltage is positive which drives op-amp into positive saturation. When is more negative than -V_ref, error voltage is negative which drives op-amp into negative saturation producing low output. The transfer characteristics is shown in the Fig. 3.12.5 (b).

The Fig. 3.12.6 (a) shows input and output waveforms with positive reference voltage while the Fig. 3.12.6 (b) shows input and output waveforms with negative reference voltage. 

2. Inverting Comparator

The Fig. 3.12.7 shows inverting comparator in which the reference voltage V_ref is applied to the non-inverting (+) input and signal voltage (V_in) is applied to the inverting (-) input of the op-amp. The V_ref can be set using a battery and potential divider as discussed earlier for non-inverting comparator.

When V_in is less than V_ref, the output voltage V_o is at + V_sat (= +V_CC) because the voltage at the inverting input (-) is less than that at the non-inverting (+) input. On the other hand, when V_in is greater than V_ref, the non-inverting (+) input becomes negative with respect to the inverting (-) input and Vo goes to -V_sat (≅ -V_EE). The Fig. 3.12.8 shows the input and output waveforms for inverting comparator.

Transfer characteristics for inverting comparator with +Vref is shown in the Fig. 3.12.9.

3. Practical Comparator

The Fig. 3.12.10 shows practical comparator circuit. It consists of protective    diodes and potentiometer to adjust the reference voltage.

As shown in Fig. 3.2.10 diode D₁ and diode D₂ are connected to protect the op-amp from damage due to excessive input voltage V_in. Because of these diodes, the difference input voltage V_id is always less than 0.7 V or -0.7 V. In case of excess input voltage, the difference input voltage Vid °f op-amp is clamped to either 0.7 V or -0.7 V due to the forward biasing of one of the diodes. Hence, these diodes are also called as damp diodes. Some op-amps have built-in input protection circuitry; they don't require external clamp diodes. The resistance R in series with V_in is used to limit the current through D₁ and D₂- The potentiometer acts as a voltage divider and allows reference voltage to set any value between + V_CC to -V_EE Some of the applications of comparator are zero crossing detector, level detector, window detector, duty cycle controller and pulse generator. 

4. Applications of Comparator

The various applications of comparator are,

1. Zero crossing detector 2. Level detector 3. Window detector 4. Duty cycle controller 5. Pulse generator 6. Time marker generator .

a. Zero Crossing Detector

The basic comparator can be used as a zero crossing detector. It answers the questions: Is the input signal greater than or less than zero ? A typical circuit for such a detector is shown in Fig. 3.12.11 (a). It is a noninverting comparator circuit with V_ref = 0V.

During the positive half cycle, the input voltage is positive i.e. above the reference voltage. Hence the output voltage is V_sat⁺. During negative half cycle, the input voltage Vin is negative, i.e. below the reference voltage. The output voltage is then V_sat .Thus the output voltage switches between V_sat+ and V_{sat -} whenever the input signal crosses the zero level. This is illustrated in Fig. 3.12.11 (b).

Looking at the waveform shown in Fig. 3.12.11 (b) we realize that zero crossing detector can be used as sine to square wave converter.

b. Time Marker Generator

In this circuit, the output of the zero crossing detector is differentiated using RC circuit. This produces a train of positive and negative pulses denoted as V_o1 in the Fig. 3.12.12 (a). Then the negative pulses can be eliminated using a diode. Thus output is a train of positive pulses separated by T as shown in the Fig. 3.12.12 (b). Such signal can be used as triggering signal for other devices such as monostable multivibrator, SCR etc.

Review Question

1. Discuss the comparator using op-amp and its applications.