Inverting Op Amp

定义

An inverting op amp is an operational amplifier circuit with an output voltage that changes in the opposite direction as the input voltage. In other words, it is out of phase by 180^o。

What is an inverting input?

An amplifier’s inverting input refers to the pin configuration. The inverting input is the terminal marked with a minus (-) sign, and the non-inverting input is marked with a plus (+) sign. These can also be referred to as negative and positive terminals.

How do inverting op amps work?

Inverting op amps work following the op amp golden rules:

1. The Current Rule: No current flows into the inputs of the op amp. (I₊=I_-=0)
2. The Voltage Rule: The output of the op amp attempts to ensure that the voltage difference between the two inputs is zero (V₊=V_-)

Consider the inverting op amp circuit shown above. Since the inverting input is tied to ground, by the Voltage Rule, the non-inverting input must also be at (virtual) ground.

The current flowing through R₁ is I=V_in/R₁，and since the Current Rule states that the inputs draw no current, all of that current must then flow through R₂.

Since the inverting input is at virtual ground, the output of the inverting op amp is V_out=-IR₂=-V_inR₂/R₁.

This makes the gain of the inverting op amp circuit -R₂/R₁.The gain is negative, meaning the output is out of phase with the input.

Op amp inverter

An op amp inverter is an inverting buffer constructed with an operational amplifier. An inverting buffer changes the direction of the signal without amplifying it, so the gain of the circuit is -1. We can see above that the inverting op amp circuit has a gain of -1 when the two resistors are equal, so an op amp inverter is an inverting op amp with R₁=R₂.