The Basic Opamp
The opamp was originally designed to carry out mathematical operations in analogue computers, such as bombsights, but was soon recognised as having many other applications.It has two inputs and one output.
The input marked with a - sign produces an amplified inverted output.
The input marked with a + sign produces an amplified but non inverted output.
The opamp requires positive and negative power supplies, together with a common ground.
Some circuits can be designed to work from a single supply.
If the two inputs are joined together, then the output voltage should be midway between the two supply rails, i.e. zero volts.
If it is not, then there are two connections for adding a potentiometer, to remove this OFFSET.
If it is not, then there are two connections for adding a potentiometer, to remove this OFFSET.
Opamp Characteristics
The opamp has a very high gain, typically (100 dB)100,000 times.Looking at the left hand diagram, an input with a swing of a fraction of a millivolt produces an output that changes between + 12 volts and - 12 volts.
In most cases this gain is excessive, and is reduced by negative feed back.
Looking at the right hand diagram we can see that the opamp amplifies right down to dc.
Gain falls quite rapidly as the frequency increases.
In fact the bandwidth (the point at which the output has fallen by 3 dB) is only 1 kHz.
This is also improved upon by the use of negative feedback.
The input impedance is high, 1M.
The output impedance is low, 150 ohms.
In most cases this gain is excessive, and is reduced by negative feed back.
Looking at the right hand diagram we can see that the opamp amplifies right down to dc.
Gain falls quite rapidly as the frequency increases.
In fact the bandwidth (the point at which the output has fallen by 3 dB) is only 1 kHz.
This is also improved upon by the use of negative feedback.
The input impedance is high, 1M.
The output impedance is low, 150 ohms.
Setting Opamp Gain
The gain of the inverting amplifier is determined by the feedback resistor R2, and the input resistor R1.To minimize temperature drift, R3 is given the value of R1 and R2 in parallel.
Unity Gain Non Inverter
This non inverting amplifier has unity gain i.e. x1.It is called a VOLTAGE FOLLOWER.
It serves the same purpose as the emitter follower.
It has a high input impedance and a very low output impedance.
It can be used for impedance matching.
It is able to drive several loads.
Non Inverter with Gain
Gain is 1+ R2/R1Comparator
A varying input voltage is compared with a fixed reference voltage.If the input voltage is higher than the reference voltage, then the output is negative.
If the input voltage is lower than the reference, then the output is positive.
The gain can be set by negative feedback.
Temperature Alarm
This circuit is configured as a comparator.R1 and R2 provide a fixed reference voltage at the non inverting input.
The inverting input voltage is set by the other two resistors.
If the voltage at the inverting input rises above the reference voltage, then the the output goes to minus 12 volts and the buzzer is energised.
The behaviour of the circuit can be changed by swapping the preset and temperature dependent resistors.
Light dependent resistors etc can replace the temperature dependent one.
Opamp as a Timer
At switch on, the voltage across the capacitor is zero and the output is at +12 volts.The buzzer is not energised.
After a time, determined by the values of C and R3, the voltage of the inverting input rises above that of the non inverting input.
The output goes to minus 12 volts and the buzzer is energised.
Opamp as an Audio Mixer
Opamp Dual Power Supplies
Most opamp circuits require two differing polarity voltages.
The upper diagram shows how the two supplies are connected together.
The bottom diagram shows how the common lead of the power supplies is connected to the input and output (and the common connection of any other associated circuitry).
The upper diagram shows how the two supplies are connected together.
The bottom diagram shows how the common lead of the power supplies is connected to the input and output (and the common connection of any other associated circuitry).
