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 P6

Op Amp

Comparator

 

In Brief....

The Operational Amplifier (Op Amp) has two inputs (pin 2 and pin 3). If the pin 3 input connection sees a voltage slightly larger than the pin 2 input connection, then the output of the device will swing to the positive supply rail voltage and vice versa.

More specifically, the two inputs to the OP Amp are inverting (-) on pin 2 and non-inverting (+) on pin 3. The relative input voltages on these legs determine the output voltage state on pin 6 (which can either be high or low).



Circuit Diagram


How the Circuit Works....

In this circuit, the inverting (-) input has been connected to a voltage mid-way between the two power rails. This has been achieved by using two identical 10 K resistors connected as a potential divider (see IP1). The input to the inverting input (-) is therefore half the supply voltage. So, the output of the comparator will go high (+), if the input voltage to the non- inverting input (+) goes above half the power supply's voltage. To be able to vary the input threshold, i.e. the point at which the comparator switches "on" and goes high at the output, we will need to adjust the voltage value on the inverting input. This may be achieved by varying the relative values of the two 10K resistors in series. This can be done by substituting a variable resistor, (or sensor whose resistance changes), in place of one of the fixed resistors. This is described earlier in the FastTrack programme, in section "Subsystem Circuits".

 

Additional Information....


The Op Amp is an Integrated Circuit (I.C.), with a maximum sourcing output current of 10mA. It normally comes in an 8 pin Dual- In- Line (D.I.L.) package like a 555 Timer (see P7 and P8 for details of these).

The Op. Amp. can be used to make different types of amplifier circuits, but in this circuit it is used as a "Comparator". As mentioned, its output can either be a high voltage (close to the positive supply voltage) or a low voltage (close to 0 volts). There are many different types of Op Amps and this one has been chosen as it has a near-zero output voltage in the "low", or "off" state. This is particularly useful when it is being used as part of an input sensor connected to a logic circuit or PIC device.