1. Voltage Followers
Voltage followers (also known as buffers) do not amplify or invert the input signal, but provide isolation between the two circuits. The input impedance is high and the output impedance is low, avoiding any loading effects within the circuit. When the output is connected directly back to one of the inputs, the total gain of the buffer is +1 and Vout = Vin.
2. Amplifier Inverters
Inverters, also known as inverting buffers, are the opposite of the previous voltage followers. If the two resistors are equal, the inverter will not amplify but will invert the input signal. The input impedance is equal to R and the gain is -1, giving Vout = -Vin.
3. In-phase amplifier
The in-phase amplifier does not invert the input signal or produce an inverted signal, but amplifies it at a ratio of (RA+ RB)/RB or usually 1 + (RA/RB). The input signal is connected to the in-phase (+) input.
4. Inverting amplifier
The inverting amplifier simultaneously inverts and amplifies the input signal at a ratio of -RA/RB. The gain of the amplifier is controlled by negative feedback using the feedback resistor RA and the input signal is fed to the inverting (-) input.
5. Bridge amplifiers
The inverting and in-phase amplifier circuits above can be connected together to form a bridge amplifier configuration. The input signal is shared between the two op-amps and the output voltage signal is connected across the load resistor R L, which floats between the two outputs.
If the magnitudes of the two op-amp gains A1 and A2 are equal to each other, the output signal will be doubled, as it is effectively a combination of the two separate amplifier gains.
6. Voltage Adders
An adder, also known as a summing amplifier, produces an inverted output voltage proportional to the sum of the input voltages V1 and v2. More inputs can be summed. If the values of the input resistors are equal (R1 = R2 = R), the total output voltage is the given value and the gain is +1. If the input resistors are not equal, the output voltage is a weighted sum and becomes: Vout = - (V1 (RA / R1) + V2 (RA / R2) + etc.).
7. Voltage subtractors
A subtractor, also known as a differential amplifier, uses inverting and in-phase inputs to produce an output signal that is the difference between two input voltages V1 and V2, thus allowing one signal to be subtracted from the other. More inputs can be added to subtract it out if required.
If the resistances are equal (R=R3 and RA=R4), the output voltage is the given value and the voltage gain is +1. If the input resistances are not equal the circuit becomes amplified to produce a negative output when the difference V1 is higher than V2 and a positive output when V1 is lower than V2.
8. Voltage comparators
Comparators have many uses, but the most common is to compare the input voltage with a reference voltage and switch the output if the input voltage is higher than the reference voltage. If the input voltage is higher than the positive reference voltage Vin>Vref set by the voltage divider, the output will change state. When the input voltage drops below the preset reference voltage and Vin<Vref, the output switches back. By using positive feedback, the basic comparator circuit can be easily converted to a Schmitt trigger to reduce oscillations around the switching point.
