Operational amplifier

Working principle

An OPAMP has two inputs and one output, and two power supply connectors. The component is designed to maintain the same voltage at its inputs because of which it can amplify the signal. It requires a bipolar power supply (dual rail PSU), for example with +15V and -15V rails. It is usually drawn without the power inputs.

It is important to note that the OPAMP cannot amplify to a voltage higher than the supply voltage used, which means the output voltage is limited in both directions. If the maximum is reached, the amplifier will distort.

Inverting amplifier

One of the basic operational amplifier circuits is the inverting amplifier. Because the non-inverting leg is connected to ground (0V), the component is trying to establish 0V on the inverting leg, as well. There are two resistors in the circuit that form a voltage divider, and their ratio is the gain. The R₂ resistor is the feedback.

$V_{o u t} = - \frac{R_{2}}{R_{1}} \cdot V_{i n}$

Non-inverting amplifier

Another basic circuit is the non-inverting amplifier, which is almost identical to the inverting. The amplified output voltage is not opposite to the input voltage anymore, and its formula is also slightly different.

$V_{o u t} = (1 + \frac{R_{2}}{R_{1}}) \cdot V_{i n}$

Summing amplifier

The inverting circuit is the basis for the summing circuit. It is suitable for digital-to-analog conversion (DACs) using the outputs of a microcontroller with the right ratio of resistors, and many other applications, as well.

$V_{o u t} = - R_{4} \cdot (\frac{V_{1}}{R_{1}} + \frac{V_{2}}{R_{2}} + \frac{V_{3}}{R_{3}})$

Subtractor amplifier

There is a basic circuit for the subtraction operation, too. The formula can be greatly simplified if the value of the R₃ resistor is equal to the value of R₁ and the value of the R₄ is equal to the value of R₂, the feedback.

$V_{o u t} = \frac{R_{1} + R_{2}}{R_{1}} \cdot \frac{R_{4}}{R_{3} + R_{4}} \cdot V_{2} - \frac{R_{2}}{R_{1}} \cdot V_{1}$

Comparator

The comparator circuit can compare two input signals, which is the basis for the operation of the class D amplifier and the SAR type ADC. Depending on the relation of the inputs, the output voltage will be one of the supply voltages. Its applications build on this idea of detecting if a threshold was crossed.

$i f V_{r e f} > V_{i n} t h e n V_{o u t} = V_{-}$

$i f V_{r e f} < V_{i n} t h e n V_{o u t} = V_{+}$

Voltage follower (Buffer)

The circuit without a resistor on the feedback follows the input voltage at the output, so the input and output voltages are the same. This may not seem useful at first, but it is an essential element in circuits where a load has to connect to a voltage divider. Since the OPAMP inputs have huge resistance and so its inputs do not actually carry any current, they convey information rather than power. The load is able to have the same voltage as the input and the power is supplied through the OPAMP's power supply connections.