The comparator does exactly what its name implies, as it compares two signals. What effect would the sweep rate have on the ability for controlling the temperature of a heating element, such as for a plastic injection molding machine, or is this really not an issue?
Electronic Devices
10th ed.
Chapter 13
Basic Op-Amp Circuits
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Copyright © 2018 Pearson Education, Inc. All Rights Reserved
Electronic Devices
10th ed.
◆ Describe and analyze the operation of several types of
comparator circuits
◆ Describe and analyze the operation of several types of
summing amplifiers
◆ Describe and analyze the operation of integrators and
differentiators
◆ Troubleshoot op-amp circuits
Objectives:
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Electronic Devices
Comparators
A comparator is a specialized nonlinear op-amp circuit that compares two input voltages and produces an output state that indicates which one is greater. Comparators are designed to be fast and frequently have other capabilities to optimize the comparison function.
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Electronic Devices
Comparators
A comparator is a specialized nonlinear op-amp circuit that compares two input voltages and produces an output state that indicates which one is greater. Comparators are designed to be fast and frequently have other capabilities to optimize the comparison function.
An example of a comparator application is shown. The circuit detects a power failure in order to take an action to save data. As long as the comparator senses Vin, the output will be a dc level.
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Electronic Devices
Comparator with Hysteresis
Sometimes the input signal to a comparator may vary due to noise superimposed on the input. The result can be an unstable output. To avoid this, hysteresis can be used.
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Electronic Devices
Comparator with Hysteresis
Sometimes the input signal to a comparator may vary due to noise superimposed on the input. The result can be an unstable output. To avoid this, hysteresis can be used.
Hysteresis is incorporated by adding regenerative (positive) feedback, which creates two switching points: the upper trigger point (UTP) and the lower trigger point (LTP). After one trigger point is crossed, it becomes inactive and the other one becomes active.
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Electronic Devices
Comparator with Hysteresis
A comparator with hysteresis is also called a Schmitt trigger. The trigger points are found by applying the voltage-divider rule:
and
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Electronic Devices
Comparator with Hysteresis
A comparator with hysteresis is also called a Schmitt trigger. The trigger points are found by applying the voltage-divider rule:
and
Example:
What are the trigger points for the circuit if the maximum output is ±13 V?
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Electronic Devices
Comparator with Hysteresis
A comparator with hysteresis is also called a Schmitt trigger. The trigger points are found by applying the voltage-divider rule:
and
Solution:
Example:
What are the trigger points for the circuit if the maximum output is ±13 V?
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Electronic Devices
Comparator with Hysteresis
A comparator with hysteresis is also called a Schmitt trigger. The trigger points are found by applying the voltage-divider rule:
and
Solution:
Example:
What are the trigger points for the circuit if the maximum output is ±13 V?
= 2.28 V
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Electronic Devices
Comparator with Hysteresis
A comparator with hysteresis is also called a Schmitt trigger. The trigger points are found by applying the voltage-divider rule:
and
Solution:
Example:
What are the trigger points for the circuit if the maximum output is ±13 V?
= 2.28 V
By symmetry, the lower trigger point = -2.28 V
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Electronic Devices
Output Bounding
Some applications require a limit to the output of the comparator (such as a digital circuit). The output can be limited by using one or two zener diodes in the feedback circuit.
The circuit shown here is bounded as a positive value equal to the zener breakdown voltage.
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Electronic Devices
Comparator Applications
A comparator with hysteresis can produce a pulse with a variable duty cycle. For the circuit shown, Vout(max) ranges from 0 V to +5 V because of the GND and VDD connections on the LM311.
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Electronic Devices
Comparator Applications
A comparator with hysteresis can produce a pulse with a variable duty cycle. For the circuit shown, Vout(max) ranges from 0 V to +5 V because of the GND and VDD connections on the LM311.
The input is the red triangle wave (0 to 4 V). The duty cycle is varied with R2.
With R2 set to 100%, a 50% duty cycle is the result.
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Electronic Devices
Comparator Applications
A comparator with hysteresis can produce a pulse with a variable duty cycle. For the circuit shown, Vout(max) ranges from 0 V to +5 V because of the GND and VDD connections on the LM311.
The input is the red triangle wave (0 to 4 V). The duty cycle is varied with R2.
With R2 set to 5%, a short pulse is the result.
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Electronic Devices
Comparator Applications
Question:
What will happen when R2 is set to zero?
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Electronic Devices
Comparator Applications
When R2 is set at 0%, the comparator has no hysteresis, and the output is positive when the input triangle is negative.
Question:
What will happen when R2 is set to zero?
Answer:
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Electronic Devices
Comparator Applications
By changing the GND ref to -5 V, another useful circuit is formed. The input is a 4 Vp triangle wave (-4 V to +4 V). The output is a square wave that is delayed by an amount that depends on the setting of R2.
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Electronic Devices
Comparator Applications
By changing the GND ref to -5 V, another useful circuit is formed. The input is a 4 Vp triangle wave (-4 V to +4 V). The output is a square wave that is delayed by an amount that depends on the setting of R2.
Question:
What are the upper and lower trigger points when R2 is set to maximum?
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Electronic Devices
Comparator Applications
By changing the GND ref to -5 V, another useful circuit is formed. The input is a 4 Vp triangle wave (-4 V to +4 V). The output is a square wave that is delayed by an amount that depends on the setting of R2.
Question:
What are the upper and lower trigger points when R2 is set to maximum?
Answer:
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Electronic Devices
Comparator Applications
By changing the GND ref to -5 V, another useful circuit is formed. The input is a 4 Vp triangle wave (-4 V to +4 V). The output is a square wave that is delayed by an amount that depends on the setting of R2.
Question:
What are the upper and lower trigger points when R2 is set to maximum?
Answer:
= +3.94 V
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Electronic Devices
Comparator Applications
By changing the GND ref to -5 V, another useful circuit is formed. The input is a 4 Vp triangle wave (-4 V to +4 V). The output is a square wave that is delayed by an amount that depends on the setting of R2.
Question:
What are the upper and lower trigger points when R2 is set to maximum?
Answer:
= +3.94 V
By symmetry, VLTP = -3.94 V
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Electronic Devices
Comparator Applications
Simultaneous or flash analog-to-digital converters use 2n-1 comparators to convert an analog input to a digital value for processing. Flash ADCs are a series of comparators, each with a slightly different reference voltage. The priority encoder produces an output equal to the highest value input.
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Electronic Devices
Comparator Applications
Simultaneous or flash analog-to-digital converters use 2n-1 comparators to convert an analog input to a digital value for processing. Flash ADCs are a series of comparators, each with a slightly different reference voltage. The priority encoder produces an output equal to the highest value input.
In IC flash converters, the priority encoder usually includes a latch that holds the converter data constant for a period of time after the conversion.
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Electronic Devices
Summing Amplifier
A summing amplifier has two or more inputs; normally all inputs have unity gain. The output is proportional to the negative of the algebraic sum of the inputs.
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Electronic Devices
Summing Amplifier
A summing amplifier has two or more inputs; normally all inputs have unity gain. The output is proportional to the negative of the algebraic sum of the inputs.
Example:
What is VOUT if the input voltages are +5.0 V, -3.5 V and +4.2 V and all resistors = 10 kW?
10 kW
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Electronic Devices
Summing Amplifier
A summing amplifier has two or more inputs; normally all inputs have unity gain. The output is proportional to the negative of the algebraic sum of the inputs.
VOUT = -(VIN1 + VIN2 + VIN3)
= -(+5.0 V – 3.5 V + 4.2 V)
Solution:
Example:
What is VOUT if the input voltages are +5.0 V, -3.5 V and +4.2 V and all resistors = 10 kW?
10 kW
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Electronic Devices
Summing Amplifier
A summing amplifier has two or more inputs; normally all inputs have unity gain. The output is proportional to the negative of the algebraic sum of the inputs.
VOUT = -(VIN1 + VIN2 + VIN3)
= -(+5.0 V – 3.5 V + 4.2 V)
Solution:
Example:
What is VOUT if the input voltages are +5.0 V, -3.5 V and +4.2 V and all resistors = 10 kW?
= -5.7 V
10 kW
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Electronic Devices
Averaging Amplifier
An averaging amplifier is basically a summing amplifier with the gain set to Rf /R = 1/n (n is the number of inputs). The output is the negative average of the inputs.
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Electronic Devices
Averaging Amplifier
An averaging amplifier is basically a summing amplifier with the gain set to Rf /R = 1/n (n is the number of inputs). The output is the negative average of the inputs.
Example:
What is VOUT if the input voltages are +5.0 V, -3.5 V and +4.2 V? Assume R1 = R2 = R3 = 10 kW and Rf = 3.3 kW?
3.3 kW
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Electronic Devices
Averaging Amplifier
An averaging amplifier is basically a summing amplifier with the gain set to Rf /R = 1/n (n is the number of inputs). The output is the negative average of the inputs.
VOUT = -⅓(VIN1 + VIN2 + VIN3)
= -⅓(+5.0 V – 3.5 V + 4.2 V)
Solution:
Example:
What is VOUT if the input voltages are +5.0 V, -3.5 V and +4.2 V? Assume R1 = R2 = R3 = 10 kW and Rf = 3.3 kW?
3.3 kW
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Electronic Devices
Averaging Amplifier
An averaging amplifier is basically a summing amplifier with the gain set to Rf /R = 1/n (n is the number of inputs). The output is the negative average of the inputs.
VOUT = -⅓(VIN1 + VIN2 + VIN3)
= -⅓(+5.0 V – 3.5 V + 4.2 V)
Solution:
Example:
What is VOUT if the input voltages are +5.0 V, -3.5 V and +4.2 V? Assume R1 = R2 = R3 = 10 kW and Rf = 3.3 kW?
= -1.9 V
3.3 kW
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Electronic Devices
Scaling Amplifier
A scaling adder has two or more inputs with each input having a different gain. The output represents the negative scaled sum of the inputs.
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Electronic Devices
Scaling Amplifier
A scaling adder has two or more inputs with each input having a different gain. The output represents the negative scaled sum of the inputs.
Example:
Assume you need to sum the inputs from three microphones. The first two microphones require a gain of -2, but the third microphone requires a gain
of -3. What are the values of the
input R’s if Rf = 10 kW?
10 kW
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Electronic Devices
Scaling Amplifier
A scaling adder has two or more inputs with each input having a different gain. The output represents the negative scaled sum of the inputs.
Solution:
Example:
Assume you need to sum the inputs from three microphones. The first two microphones require a gain of -2, but the third microphone requires a gain
of -3. What are the values of the
input R’s if Rf = 10 kW?
10 kW
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Electronic Devices
Scaling Amplifier
A scaling adder has two or more inputs with each input having a different gain. The output represents the negative scaled sum of the inputs.
Solution:
Example:
Assume you need to sum the inputs from three microphones. The first two microphones require a gain of -2, but the third microphone requires a gain
of -3. What are the values of the
input R’s if Rf = 10 kW?
5.0 kW
10 kW
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Electronic Devices
Scaling Amplifier
A scaling adder has two or more inputs with each input having a different gain. The output represents the negative scaled sum of the inputs.
Solution:
Example:
Assume you need to sum the inputs from three microphones. The first two microphones require a gain of -2, but the third microphone requires a gain
of -3. What are the values of the
input R’s if Rf = 10 kW?
5.0 kW
10 kW
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Electronic Devices
Scaling Amplifier
A scaling adder has two or more inputs with each input having a different gain. The output represents the negative scaled sum of the inputs.
Solution:
Example:
Assume you need to sum the inputs from three microphones. The first two microphones require a gain of -2, but the third microphone requires a gain
of -3. What are the values of the
input R’s if Rf = 10 kW?
5.0 kW
10 kW
3.3 kW
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Electronic Devices
Scaling Amplifier DAC
An application of a scaling adder is the D/A converter circuit shown here. The resistors are inversely proportional to the binary column weights. Because of the precision required of resistors, the method is useful only for small DACs.
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Electronic Devices
R/2R Ladder DAC
A more widely used method for D/A conversion is the R/2R ladder. The gain for D3 is -1. Each successive input has a gain that is half of previous one. The output represents a weighted sum of all of the inputs (similar to the scaling adder).
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Electronic Devices
R/2R Ladder DAC
A more widely used method for D/A conversion is the R/2R ladder. The gain for D3 is -1. Each successive input has a gain that is half of previous one. The output represents a weighted sum of all of the inputs (similar to the scaling adder).
An advantage of the R/2R ladder is that only two values of resistors are required to implement the circuit.
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Electronic Devices
The Integrator
The ideal integrator is an inverting amplifier that has a capacitor in the feedback path. The output voltage is proportional to the negative integral (running sum) of the input voltage.
Ideal
Integrator
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Electronic Devices
The Integrator
The ideal integrator is an inverting amplifier that has a capacitor in the feedback path. The output voltage is proportional to the negative integral (running sum) of the input voltage.
Op-amp integrating circuits must have extremely low dc offset and bias currents, because small errors are equivalent to a dc input. The ideal integrator tends to accumulate these errors, which moves the output toward saturation. The practical integrator overcomes these errors– the simplest method is to add a relatively large feedback resistor.
Ideal
Integrator
Practical
Integrator
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Electronic Devices
The Integrator
If a constant level is the input, the current is constant. The capacitor charges from a constant current and produces a ramp. The slope of the output is given by the equation:
220 kW
10 kW
0.1mF
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Electronic Devices
The Integrator
If a constant level is the input, the current is constant. The capacitor charges from a constant current and produces a ramp. The slope of the output is given by the equation:
220 kW
10 kW
0.1mF
Example:
Sketch the output wave:
Vin
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Electronic Devices
The Integrator
If a constant level is the input, the current is constant. The capacitor charges from a constant current and produces a ramp. The slope of the output is given by the equation:
220 kW
10 kW
0.1mF
Solution:
Example:
Sketch the output wave:
Vin
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Electronic Devices
The Integrator
If a constant level is the input, the current is constant. The capacitor charges from a constant current and produces a ramp. The slope of the output is given by the equation:
220 kW
10 kW
0.1mF
Solution:
Example:
Sketch the output wave:
Vin
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Electronic Devices
The Integrator
If a constant level is the input, the current is constant. The capacitor charges from a constant current and produces a ramp. The slope of the output is given by the equation:
220 kW