Date post: | 12-Jul-2015 |
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Engr.Tehseen Ahsan
Lecturer, Electrical Engineering Department
EE-307 Electronic Systems Design
HITEC University Taxila Cantt, Pakistan
Basic Op-Amp Circuits
13-1 Comparators
Operational amplifiers are often used as comparators to
compare the amplitude of one voltage with another.
In this application, the op-amp is used in open-loop
configuration, with the input voltage on one input and a
reference voltage on the other.
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13-1 Comparators Continue…
Zero-Level Detection
One application of an op-amp used as a comparator is to
determine when an input voltage exceeds a certain level.
Figure 13.1 (a) next slide shows a zero-level detector. 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.
The input voltage Vin at the non-inverting (+) input is compared
with a reference voltage VREF at the inverting input (VREF = 0V).
Since VREF = 0V, this is called a zero-level detector.
Because of the high-open loop voltage gain, a very small
difference voltage Vd between the two inputs drives the
amplifier into saturation ( non-linear region).
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13-1 Comparators Continue… Zero-Level Detection
Figure 13-1 (b) shows the result of a sinusoidal input voltage
applied to the non-inverting (+) input of the zero-level
detector. When the sine wave is positive, the output is at its
maximum positive level. When the sine wave crosses 0, the
amplifier is driven to its opposite state and the output goes to
its maximum negative level.
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13-1 Comparators Continue… Zero-Level Detection
When Vin> VREF ( Sine wave is positive)
Vd = Vin- VREF
Vd > 0V
Vout = + Vout(max)
When Vin<VREF ( Sine wave is negative)
Vd = Vin- VREF
Vd < 0V
Vout = - Vout(max)
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13-1 Comparators Continue…
Nonzero-Level Detection
A more practical arrangement is shown in figure 13-2 (b) next
slide using a voltage divider to set the reference voltage VREF as
Where +V is the positive op-amp dc supply voltage.
The circuit in figure 13-2 (c) next slide uses a zener diode to
set the reference voltage (VREF = VZ).
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13-1 Comparators Continue…
Nonzero-Level Detection
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Most practically used
13-1 Comparators Continue… Nonzero-Level Detection
Voltage – divider Reference ( figure 13-2 (b) )
When Vin> VREF
Vd = Vin- VREF
Vd > 0V
Vout = + Vout(max)
When Vin<VREF
Vd = Vin- VREF
Vd < 0V
Vout = - Vout(max)
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13-1 Comparators Continue… Nonzero-Level Detection
Zener diode sets reference voltage ( figure 13-2 (c) )
When Vin> VZ
Vd = Vin- VZ
Vd > 0V
Vout = + Vout(max)
When Vin< VZ
Vd = Vin- VZ
Vd < 0V
Vout = - Vout(max)
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S
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13-1 Comparators Continue… Effects of Input Noise on Comparator Operation
In many practical applications, noise (unwanted voltage
fluctuations) appears on the input line.
This noise becomes superimposed on the input voltage as
shown in figure 13-5 for the case of a sine wave and can cause a
comparator to erratically switch output states.
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13-1 Comparators Continue… Effects of Input Noise on Comparator Operation
In order to understand the potential effects of noise voltage,
consider a low-frequency sinusoidal voltage applied to the non
inverting (+) input of an op-amp comparator used as a zero-
level detector as shown in figure 13-6 (a) next slide.
Figure 13-6 (b) next slide shows the input sine wave plus noise
and the resulting output.
As we can see 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.
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13-1 Comparators Continue… Effects of Input Noise on Comparator Operation
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13-1 Comparators Continue… Reducing Noise Effects with Hysteresis
An erratic output voltage caused by noise on the input occurs
because the output voltage switches states several times at the
same input voltage level (output voltage switches states several
times at + ve half cycle and same for –ve half cycle).
In order to make the comparator less sensitive to noise, a
technique called hysteresis with positive feedback can be
used.
Hysteresis
There exists a higher reference level i.e., + VREF when input
goes from lower to higher value.
There exists a lower reference level i.e., - VREF when the input
goes from higher to lower value.
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13-1 Comparators Continue… Reducing Noise Effects with Hysteresis
The two reference levels are referred to as the upper trigger point
(UTP) and lower trigger point (LTP).
This two-level hysteresis is established with a positive feedback
arrangement as shown in figure 13-7.
Note that the noninverting (+) input is connected to the a resistive
voltage divider such that a portion of the output voltage is fed back to
the input. The input signal is applied to inverting input (-) input in
this case.
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This configuration is also called Schmitt Trigger
13-1 Comparators Continue… Reducing Noise Effects with Hysteresis
The basic operation of the comparator with hysteresis is illustrated in
figure 13-8 next slide(s). Assume that the output voltage is at its
positive maximum + Vout(max). The voltage fed back to the non inverting
input is VUTP and is expressed as
When Vin exceeds VUTP , the output voltage drops to its negative
maximum, -Vout(max) as shown in part (a). Now the voltage fed back to
the non inverting input is VLTP and is expressed as
A comparator with hysteresis is also called Schmitt trigger. The
amount of hysteresis can be found as
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13-1 Comparators Continue… Reducing Noise Effects with Hysteresis
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13-1 Comparators Continue… Reducing Noise Effects with Hysteresis
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13-1 Comparators Continue… Output Bounding
In some applications, it is necessary to limit the output voltage levels
of a comparator than that provided by the saturated op-amp.
A single zener diode can be used as shown in figure13-10 to limit the
output voltage to the zener voltage in one direction and to the
forward drop in other. The process of limiting the output is called
bounding.
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13-1 Comparators Continue… Positive Value Output Bounding
When anode is connected to a negative terminal.
In positive half cycle of output voltage, the zener diode gets reverse-
biased and the limits the output voltage to the zener voltage i.e., +VZ
In negative half cycle of output voltage, the zener diode gets forward
biased and behaves as a normal conventional diode with a drop of 0.7
V across it ( -0.7 V) . It is shown in figure 13-11 (a) below
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13-1 Comparators Continue… Negative Value Output Bounding
When cathode is connected to a negative terminal.
In positive half cycle of output voltage, the zener diode gets forward
biased and behaves as a normal conventional diode with a drop of 0.7
V across it ( + 0.7 V) .
In negative half cycle of output voltage, the zener diode gets reverse-
biased and the limits the output voltage to the zener voltage i.e., it is
shown in figure 13-11 (b) below
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13-1 Comparators Continue… Double Bounded Comparator
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13-2 Summing Amplifiers
Summing Amplifier with Unity Gain
A summing amplifier has two or more inputs and its output
voltage is proportional to the negative of the algebraic sum of
its input voltages.
A two-input summing amplifier is shown in figure 13-20, but
any number of inputs can be used.
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13-2 Summing Amplifiers Continue…
Summing Amplifier with Unity Gain
The operations of the circuit and derivation of the output
expression are as follows:
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13-2 Summing Amplifiers Continue…
Summing Amplifier with Unity Gain
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13-2 Summing Amplifiers Continue…
Summing Amplifier with Gain Greater Than Unity
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13-2 Summing Amplifiers Continue…
Averaging Amplifier
Averaging amplifier is a variation of summing amplifier.
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13-2 Summing Amplifiers Continue…
Scaling Adder
Scaling Adder is also a variation of summing amplifier.
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13-3 Integrators and Differentiators
The Op-Amp Integrator
An ideal integrator is shown in figure 13-31. Notice that the feedback
element is a capacitor that forms and RC circuit with the input
resistor.
Practical integrators often have an additional resistor Rf in parallel
with the feedback capacitor to prevent saturation. However we will
consider the ideal integrator for the purpose of our analysis as it does
not affect the basic operation.
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13-3 Integrators and Differentiators Continue…
The Op-Amp Integrator
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13-3 Integrators and Differentiators Continue…
The Op-Amp Integrator
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13-3 Integrators and Differentiators Continue…
The Op-Amp Integrator
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13-3 Integrators and Differentiators Continue…
The Op-Amp Integrator
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13-3 Integrators and Differentiators Continue… The Op-Amp Integrator
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13-3 Integrators and Differentiators Continue…
The Op-Amp Differentiator
An ideal differentiator is shown in figure 13-37. Notice how the
placement of the capacitor and resistor differ from the integrator. The
capacitor is now the input element and resistor is the feedback
element. A differentiator produces and output that is proportional to
the rate of change of the input voltage.
Practical differentiators may include a series resistor Rin to reduce
high frequency noise.
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13-3 Integrators and Differentiators Continue…
The Op-Amp Differentiator
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13-3 Integrators and Differentiators Continue…
The Op-Amp Differentiator
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13-3 Integrators and Differentiators Continue…
The Op-Amp Differentiator
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