Comparator Behaviour

A comparator compares two input voltages. These are usually a reference voltage and a signal from a sensor. The output switches state when the the signal input crosses the reference voltage.

The comparator relies on the very high open loop gain of the op' amp'.
For a real-life op' amp' this gain will be between 10⁵ and 10⁷.
This means that a potential difference between V₁ and V₂ of only a few microvolts is sufficient to saturate the op' amp'.

• If V₁ is greater than V₂ then V_out will go high (close to the + supply voltage)
• If V₁ is less than V₂ then V_out will go low (close to the - supply voltage).

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Comparator Example

The task is to have an indicator light come on when the light level drops below some chosen value. The circuit could be scaled up to turn on room, car or street lights when it gets dark.

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Here is a system diagram of the system.

• Input - The light dependent potential (input from from an LDR).
• Process - The comparator
• Output - The driver and output to the indicator light.

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If the op amp needs to drive a load, a driver circuit might be needed. Here is an example.

• R₁ limits the trasistor base current to a safe level.
• R₂ limits the LED current to a level less than 20mA
• Since V_out might never go below 2V (MOSFET Op Amp), the two diodes are needed to ensure that the transistor turns right off when the comparator output is low.
• Each diode has 0.7V across it. The transistor needs 0.7 volts to turn on. (2.1V total).

V₁ is a reference voltage. This can be provided with a potential divider. (A potentiometer could be used to make the reference voltage adjuatable.) R₃ and R₄ form a potential divider.

V₂ is the measured voltage from the light sensor. This is also a potential divider circuit. R₅ and the LDR form a potential divider. As the light level increases, the LDR resistance decreases. This reduces V₂. Once V₂ is less than V₁, the op amp output goes high.

 

Explanation

• The resistor R₅ and the light dependent resistor R_LDR form a potential divider. The potential V₂ depends on the current flowing through the LDR and R₅. It is safe to assume that the current flowing into the op amp is negligable. The value of R₅ might need to be changed depending on the properties of the LDR.
• The resistors R₃ and R₄ form another potential divider. V₁ is the voltage produced here. V₁ will be half the power supply voltage. This is assuming R₃ and R₄ are equal. Again it is safe to assume that the current flowing into the op amp is negligable.
• If V₁ is greater than V₂, the op amp will turn on and V_Out will be just less than the PSU supply voltage. (PSU voltage - 2 Volts)
• If V₁ is less than V₂, the op amp will turn off and V_Out will be close to zero. (+2 Volts)
• If V_Out is high the NPN transistor will turn on. If V_Out is low (0 to 2 volts), the NPN transistor will turn off. If V_Out is high, just under 1mA will flow through the resistor R₁. This is sufficient to saturate the transistor so the light emitting diode will turn fully on. The 741 op amp can easily provide 1mA to drive the transistor.
• R₁ limits the current through the LED and transistor to a safe level less than 20 mA.

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Tasks

Think of ways to improve this circuit.

• Could the threshold light level be made adjustable? How? Draw the circuit diagram.
• Could the power consumption be reduced? How?
• Instead of truning on an indicator light, could a high power circuit be turned on? How? Draw the circuit diagram.
• When driving room lights, the circuit flashes the lights on and off rapidly. How could this be prevented?