Find Thévenin and Norton Equivalent Circuits Using Source Transformation
Find the Zero-Input and Zero-State Responses of a Series RC Circuit
Relate the Current and Voltage of a Capacitor

Analyze Inverting Op Amp Circuits

An inverting amplifier takes an input signal and turns it upside down at the op amp output. When the value of the input signal is positive, the output of the inverting amplifier is negative, and vice versa. Here is an inverting op amp. The op amp has a feedback resistor R2 and an input resistor R1 with one end connected to the voltage source.

image0.jpg

The other end of the input resistor is connected to the inverting terminal, and the noninverting terminal is grounded at 0 volts. The amount of amplification depends on the ratio between the feedback and input resistor values.

Because the noninverting input is grounded to 0 volts, you have

image1.jpg

For ideal op amps, the voltages at the inverting and noninverting terminals are equal and set to zero. The inverting terminal is connected to a virtual ground because it’s indirectly connected to ground by vP.

Apply Kirchhoff’s current law (KCL) to Node A to get the following:

image2.jpg

Simplify the equation with the following constraints for an ideal op amp:

image3.jpg

The constraints make the KCL equation simpler:

image4.jpg

You wind up with the following relationship between the input and output voltages:

image5.jpg

Again, the amplification of the signal depends on the ratio of the feedback resistor R2 and input resistor R1. You need only external components of the op amp to make the signal way bigger. The negative sign means the output voltage is an amplified but inverted (or upside down) version of the input signal.

For a numerical example, let R2 = 10 kΩ and R1 = 1 kΩ. In that case, the inverted output voltage vO is ten times as big as the input voltage vS. In nothing flat, you just made a weak signal stronger. Nice work — you deserve a raise!

Resistors should be around the 1 kΩ to 100 kΩ range to minimize the effects of variation in the op amp characteristics and voltage sources.

  • Add a Comment
  • Print
  • Share
blog comments powered by Disqus
How to Use Sine Substitution to Integrate
How to Work with Voltage Sources in Node-Voltage Analysis
How to Solve Differential Equations Using Op Amps
Analyze a Series RC Circuit Using a Differential Equation
How to Use Trig Substitution to Integrate
Advertisement

Inside Dummies.com