Operational Amplifiers

This next topic focuses on the operational amplifier, which is one of the most widely used electronic components in the analog world.  The operational amplifier, also known as an op-amp, is a type of differential amplifier.  The output voltage of an op-amp is:

Op-amps are used in a variety of applications, which perform many different operations, hence the name.  External passive elements – such as resistors, caps, inductors, etc – are used to determine the end function.  Before continuing into any applications, you should consider the following properties of an ideal op-amp.

Ideal OP-AMPs have:

Infinite input impedance

Zero Output Impedance

Infinite Differential Gain

Zero Offset Voltage

Zero Input Bias Current

Zero Common Mode Gain

Recalling our previous equation and substituting Aol=∞ produces the following relationship for the non-inverting and inverting inputs. 

This equation merely demonstrates that for an ideal op-amp, both inputs are the same voltage value, so in a sense both inputs are virtually shorted to each other. 

Using these properties, you can solve any ideal op-amp circuit configuration.  To get you started in op-amp analysis, I’ve provided a simple circuit below, which is an inverting op-amp configuration.

Using KCL, let’s guess the current direction and solve for node 2.

What happens when I substitute a voltage for the V+ input?

I’ll leave this next configuration as homework: