Download Operational Amplifier Comparator

Operational Amplifier Comparator
Introduction
• In electronics, a comparator is a device which compares two voltages or
currents and switches its output to indicate which is larger. More
generally, the term is also used to refer to a device that compares two
items of data. Output voltage will “switch” whenever the input voltage
(at the inverting input) reaches the reference voltage Vref (at the noninverting input). It is very useful for comparing signals and working
with sensors
Simple Comparator
Cont..,
•
R2 and R1 form a voltage divider. Use a potentiometer in place of R2 for an
adjustable reference voltage.
•
Comparator circuits can be built with opamps, but there are also comparator
ICs with large slew rates and short propagation delays – good for high speed
switching.
•
A dedicated voltage comparator will generally be faster than a general-purpose
op-amp pressed into service as a comparator. A dedicated voltage comparator
may also contain additional features such as an accurate, internal voltage
reference, an adjustable hysteresis and a clock gated input.
Op-amp voltage comparator
•
An operational amplifier (op-amp) has a well balanced difference input and a
very high gain. This parallels the characteristics of comparators and can be
substituted in applications with low-performance requirements.
•
In theory, a standard op-amp operating in open-loop configuration (without
negative feedback) may be used as a low-performance comparator. When the
non-inverting input (V+) is at a higher voltage than the inverting input (V-),
the high gain of the op-amp causes the output to saturate at the highest
positive voltage it can output.
•
When the non-inverting input (V+) drops below the inverting input (V-), the
output saturates at the most negative voltage it can output.
Cont..,
• The op-amp's output voltage is limited by the supply voltage.
• An op-amp operating in a linear mode with negative feedback, using a
balanced, split-voltage power supply, (powered by ± VS) its transfer
function is typically written as: Vout = Ao(V1 − V2).
•
However, this equation may not be applicable to a comparator circuit
which is non-linear and operates open-loop (no negative feedback).
•
In practice, using an operational amplifier as a comparator presents several disadvantages as
compared to using a dedicated comparator:
1)
Op-amps are designed to operate in the linear mode with negative feedback. Hence, an op-amp
typically has a lengthy recovery time from saturation. Almost all op-amps have an internal
compensation capacitor which imposes slew rate limitations for high frequency signals.
Consequently an op-amp makes a sloppy comparator with propagation delays that can be as slow
as tens of microseconds.
2)
Since op-amps do not have any internal hysteresis an external hysteresis network is always
necessary for slow moving input signals.
3)
The quiescent current specification of an op-amp is valid only when the feedback is active. Some
op-amps show an increased quiescent current when the inputs are not equal.
4)
A comparator is designed to produce well limited output voltages that easily interface with digital
logic. Compatibility with digital logic must be verified while using an op-amp as a comparator.
5)
Some multiple-section opamps may exhibit extreme channel-channel interaction when used as
comparators.
Comparator used as a temperature sensor
•
The circuit diagram shows a comparator used to determine when the temperature goes
above or below a threshold temperature
•
The potential divider fixes the non-inverting input at some pre-determined voltage. This
"reference" voltage is determined by the values of the resistors used in the potential
divider. If a variable reference voltage is required, to set a variable temperature
threshold, then a potentiometer can be used instead of the fixed potential divider.
•
The inverting input derives a voltage from a potential divider that has a thermistor as
one of its resistors and so the voltage at the inverting input varies with temperature. If
the temperature increases then the resistance of the thermistor decreases and the
voltage at the inverting input increases
•
Cont..,
• When the temperature is low, the voltage at the non-inverting input is
higher than that at the inverting input and the output is positive.
Current flows from the 741 (current sourcing) and through the green
LED causing it to light
• As the temperature rises, the voltage at the inverting input rises and
the instant it is greater than the reference voltage at the non-inverting
input, the output becomes negative. Current flows from 0v and into
the output of the 741 (current sinking) causing the red LED to light
Comparator used as a light level detector
Cont..,
•
A variable resistor is used to set the threshold voltage at the -Ve input
•
When the light level is high, the resistance of the LDR is low and so the voltage
at the +Ve i/p is low, therefore the output is low
•
Note that the op-amp uses the 12v and 0v supplies, not the ±12v supplies and
so the minimum output voltage is 0v (or 2v in reality)
•
When the light level falls the resistance of the LDR rises and so does the
voltage at the +Ve i/p. This causes the output to go high when +Ve i/p > -Ve i/p
•
The point at which the output changes state can be set using the variable
resistor
Comparator used as a range indicator
Cont..,
•
When the input voltage is low, the inverting inputs of both op-amps are below the noninverting inputs. Both outputs are positive and so there is no voltage difference across
the LED and it does not light.
•
When the input voltage is high, the the inverting inputs of both op-amps are above both
the non-inverting inputs. Both outputs are negative and again there is no voltage
difference across the LED and it does not light.
•
When the input voltage is between the two threshold voltages determined by the three
resistors in the potential divider chain, the output of the upper op-amp is positive and
the output of the lower op-amp is negative and so current flows through the LED and it
lights to show that the input voltage is within the range required.
Bridge Amplifier
• The standard Differential Amplifier circuit now becomes a differential
voltage comparator by "Comparing" one input voltage to the other.
• For example, by connecting one input to a fixed voltage reference set
up on one leg of the resistive bridge network and the other to either a
"Thermistor" or a "Light Dependant Resistor" the amplifier circuit can
be used to detect either low or high levels of temperature or light as
the output voltage becomes a linear function of the changes in the
active leg of the resistive bridge