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CHAPTER 8:

BASIC OP-AMP CIRCUITS

D.Wilcher

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CHAPTER OBJECTIVES

•Describe and analyze the operation of several basic comparator circuits. •Describe and analyze the operation of several types of summing amplifiers.

D.Wilcher

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COMPARATORS

Operational Amplifiers are quite often used as non-linear devices to compare the amplitude of one voltage with another. In this application, the op-amp is used in the open loop configuration, with the input voltage on the one input and a reference voltage on the other.

D.Wilcher

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ZERO LEVEL DETECTION

•One application of an op-amp used as a comparator to determine when an input voltage exceeds a certain level. See Figure 8-1 (a)

•Notice that the inverting (-) input is grounded to produce a zero level and that the input signal voltage is applied to the non-inverting (+) input.

•Because of the high open-loop voltage gain, a very small difference voltage between the two inputs drives the amplifier into saturation, causing the output voltage to go its limit.

D.Wilcher

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FIGURE 8-1 The op-amp as a zero-level detector.

Thomas L. Floyd and David Buchla

Fundamentals of Analog CircuitsCopyright ©2002 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458

All rights reserved.

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ZERO LEVEL DETECTION. . .

•Since most op-amps have output voltage limitations of +/- 15V or less, the device would be driven to saturation.

•For many comparison applications, special op-amp comparators are selected.

•These ICs are generally uncompensated to maximize speed.

•In less stringent applications, a general purpose op-amp works nicely as a comparator.

D.Wilcher

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ZERO LEVEL DETECTION. . .

•When the sine wave is negative, the output is at its maximum negative level.

•When the sine wave crosses “0”, the amplifier is driven to its opposite state and the output goes to its maximum positive level. See Figure 8-1(b)

•The zero level detector can be used as a squaring circuit to produce a square wave from a sine wave.

D.Wilcher

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NON ZERO LEVEL DETECTION

•The zero level detector in Figure 8-2(a) can be modified to detect positive and negative voltages by connecting a fixed reference voltage to the inverting (-) input.

• A more practical arrangement is shown in Figure 8-2(b) using a voltage divider to set the reference voltage as follows:

EQ 1) VREF =[ R2 /(R1 + R2)] x +V

where +V is the positive op-amp supply voltage.

D.Wilcher

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FIGURE 8-2 Nonzero-level detectors.





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NON ZERO LEVEL DETECTION. . .

•The circuit in Figure 8-2(c) uses a zener diode to set the reference voltage (VREF = VZ).

•As long as the input voltage VIN is less than VREF

the output remains at the maximum negative level.

When the input voltage exceeds the reference voltage, the output goes to its maximum positive state with a sinusoidal input voltage. See Figure 8-2(d).

D.Wilcher

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EFFECTS OF INPUT NOISE ON COMPARATOR OPERATION

•In many practical situations, noise (unwanted voltage or current fluctuations) may appear on the input line.

• This noise voltage becomes superimposed on the input voltage and can cause a comparator to erratically switch output states. (See Figure 8-5)

• When the sine wave approaches 0, the fluctuations due to noise cause the total input to vary above and below 0 several times, thus producing an erratic output voltage.

D.Wilcher

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FIGURE 8-5 Sine wave with superimposed noise.





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FIGURE 8-6 Effects of noise on comparator circuit.





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REDUCING NOISE EFFECTS WITH HYSTERESIS

•An erratic output voltage caused by noise on the input occurs because the op-amp comparator switches from its negative output state to its positive output state at the same input voltage level that causes it to switch in the opposite direction from positive to negative.

• This unstable condition occurs when the input voltage hovers around the reference voltage, and any small noise fluctuations cause the comparator to switch first one way and then the other.

D.Wilcher

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REDUCING NOISE EFFECTS WITH HYSTERESIS. . .

•In order to make the comparator less sensitive to noise, a technique incorporating positive feedback, called hysteresis can be used.

• Basically, hysteresis means that there is a higher reference level when the input voltage goes from a lower to higher value than when it goes from a higher to a lower value than when it goes from a higher to a lower value.

•A good example of hysteresis is a common household thermostat that turns the furnace on at one temperature and off at another.

D.Wilcher

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FIGURE 8-7 Comparator with positive feedback for hysteresis.





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•The two reference levels are referred to as the upper trigger point (UTP) and the lower trigger point (LTP).

• This two-level hysteresis is established with a positive feedback arrangement as shown in Figure 8-7.

•The non-inverting (+) input is connected to a resistive voltage divider such that a portion of the output voltage is fed back to the input.

•The input signal is applied to the inverting (-) input in this case.D.Wilcher

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•The basic operation of the comparator with hysteresis is as follows (See Figure 8-8).

• Assume that the output voltage is at its positive maximum, +VOUT(max). The voltage fed back to the non-inverting.

•The voltage fed back to the noninverting input is VUTP and is expressed as:

EQ 2) VUTP = [R2 / (R1 + R2 )] x (+VOUT(max))

D.Wilcher

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D.Wilcher


•When the input voltage VIN exceeds VUTP, the output voltage drops to its negative maximum, -VOUT(max). Now the voltage fed back to the non-inverting input is VLTP and is expressed as:

EQ 3) VLTP = [R2 / (R1 + R2 )] x (-VOUT(max))

The input voltage must now fall below VLTP before the device will switch back to its other voltage.

A small amount of noise voltage has no effect on the output. (See Figure 8-8).

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FIGURE 8-8 Operation of a comparator with hysteresis.





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OUTPUT BOUNDING

•In some applications, it is necessary to limit the output voltage levels of a comparator to a value less than that provided by the saturated op-amp.

• A single zener diode can be used to limit the output voltage to the zener voltage in 1 one direction and to the forward diode drop in the other. (See Figure 8-10).

This process of limiting the output range is called bounding.

D.Wilcher

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FIGURE 8-10 Comparator with output bounding.





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FIGURE 8-11 Operation of a bounded comparator.





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OUTPUT BOUNDING. . .

Two zener diodes arranged in opposite ends limit the output voltage to the zener voltage plus the forward voltage drop (0.7V) of the forward-biased zener, both positively and negatively. See Figure 8-12.

D.Wilcher

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FIGURE 8-12 Double-bounded comparator.





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WINDOW COMPARATOR

•Two individual op-amp comparators stacked form a window comparator.

•The circuit detects when an input voltage is between 2 limits, an upper and lower (called the “window”).

•The upper and lower limits are set by reference voltages designated (VU) and (VL).

•These voltages can be established with voltage dividers, zener diodes, or any type of voltage source.

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D.Wilcher

WINDOW COMPARATOR. . .

•As long as VIN is within the window (less than VU

and greater than VL) the output of each comparator is at its low saturated level.

•Under this condition, both diodes are reverse-biased and VOUT is held at zero by resistor to ground.

•When VIN goes above VU or below VL, the output of the associated comparator goes to its high saturated level.

•This action forward-biases the diode and produces a high-level VOUT.

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FIGURE 8-15 A basic window comparator.





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FIGURE 8-16 Example of window comparator operation.





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FIGURE 8-20 Two-input inverting summing amplifier.





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SUMMING AMPLIFIERS

•The summing amplifier is a variation of the inverting op-amp configuration covered in Chapter 6.

•The summing amplifier has 2 or more inputs and its output voltage is proportional to the negative of the algebraic sum of its input voltages.

•The operation of the circuit is as follows.

•Two voltages, VIN1 and VIN2 are applied to the inputs and produce currents I1 and I2.

D.Wilcher

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SUMMING AMPLIFIERS. . .

•The voltage at the inverting input of the op-amp is approximately 0V, therefore there is no current at the input.

•This means that both input currents I1 and I2combine at this summing point form the total current which is through Rf.

•See page 433 for complete derivation of VOUT.

D.Wilcher

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FIGURE 8-21 Summing amplifier with n inputs.





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AVERAGING AMPLIFIER

•A summing amplifier can be made to product the mathematical average of the input voltages.

•This done by setting the ratio Rf/R equal to the reciprocal of the number of inputs (n); that is Rf/R = 1/n.

•To obtain the average of several numbers by first adding the numbers and then dividing by quantity of numbers available.

NOTE: Rf/R is the Gain of the amplifier (See Summing Amplifier with Gain Greater than Unity on page 434 and Equation 8-4).D.Wilcher

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D.Wilcher

SCALING ADDER

•A different weight can be assigned to each input of a summing amplifier by simply adjusting the values of the input resistors (See Equation 8-5 on page 435).

•The weight of a particular input is set by the ratio of Rf to the resistance for that input.

Example: If an input voltage is to have a weight of 1, then R = Rf.

If a weight of 0.5 is required, R =2Rf. The smaller the value of R, the greater the weight and vice versa.

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FIGURE 8-26 A scaling adder as a four-digit digital-to-analog converter (DAC).





SCALING ADDER Application (See page 436)

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