Operational Amplifiers


Contents

Introduction
Terminology
Ideal OpAmp
Real OpAmp
Typical Circuit Configurations
Advanced Operational Amplifier Applications
More Information
General Introduction:
Operational Amplifiers are integrated circuits that accept an input signal and modify it depending on the configuration of the exterior 'opamp' circuit. Throughout this lecture, we will abbreviate 'Operational Amplifier' by the term 'OpAmp'.
 
Terminology
Some terminology used when speaking of Operational Amplifiers are summarized in the table below.

Input Impedance We will denote the input impedance as Zin. The input impedance of the opamp will be high if it doesn't draw alot of current.
Input Voltage The input voltage of the Operational Amplifier is the original signal accepted by the Operational Amplifier for modification. The output voltage is denoted Vo throughout this lecture.
Output Voltage The output voltage of the Operational Amplifier is the modified input signal. The modifications made to the input signal using the Operational Amplifier are and the result is referred to as the output voltage.

Ideal Operational Amplifiers
An ideal OpAmp will have a Zin that approaches infinity. This would imply that no current actually goes into the amplifier. Also, V- <= Vo <= V+ (the rails being the maximum and minimum output voltages.) An ideal OpAmp will have a Zout of zero. This would imply that the OpAmp can supply lots of current. The ideal OpAmp will have an Aol which approaches infinity and is independant of frequency.


To summarize the basic characteristics of an Ideal OpAmp:

Zin Infinity
Zout 0
Aol Infinity


Real Operational Amplifiers
In reality, OpAmps are not quite as elegant as Ideal OpAmps. Real OpAmps have predefined limits which allow limited possibilities of amplification and limits to the other various uses of OpAmps. The input impedance Zin of the Real OpAmp is typically some high resistance value. Typically, Zin is 2 MegaOhms for a Real OpAmp.


To summarize some typical characteristics of Real OpAmps, we shall use the General Purpose LM741 as an example:

Zin 2 MegaOhms
Zout 0
Aol 100000


Typical Circuit Configurations
There are various circuits that can be constructed using OpAmps. Each of these circuits has a unique function and special output characteristics.
 
The Non-Inverting Amplifier
The equations for the non-inverting are as follows:

 
General Inverting Amplifier
The equations for the inverting are as follows:

 
The Differentiator
The equations for the Differentiator will be the inverse of equations made for the integrator.

 
The Summing Circuit
The equations of the summing circuit are:
Vo=(Zf/Z1)V1 + (Zf/Z2)V2 + (Zf/Z3)V3 + ... + (Zf/Zn)Vn

 

The Integrator
The equations for the Integrator circuit are:

 

Advanced Operational Amplifier Applications
The OpAmp has numerous applications. Particularly useful applications include:
  •   Amplification of sensor output (e.g. electret mic)
  •   Voltage Regulation
  •   Reduction of Impedance on a line (commonly known as follower circuit)
  •   High/Low/Band pass frequency filters
     
    At this time we will not comment on the first three of these applications as they are quite confusng. We shall however discuss filters for a moment.
     
    Low-Pass Filters
    A low pass filter accepts any range of frequencies and only allows frequencies below some calculated upper limit to pass through it. Note that the lowest frequency possible is 0Hz so 0Hz is theoretically the lower limit of this filter. This graph might help explain this method more clearly:

     
    High-Pass Filters
    A high pass filter accepts any range of frequencies as the low-pass does, but instead has some lower limit in which it only accepts frequencies above such limit. In this instance, we can see that the OpAmp has some upper limit as frequencies approach larger values. This limit is known as the frequency rolloff, a point at which the gain reduces to zero as frequency increases.
    Again, this graph might help with your understanding:


     

    More Information
    There are many resourced on the internet that will help you in understanding the functionality of Operational Amplifiers. Some of these include:
    Williamson Labs
    Do it Yourself Op Amps
    Op Amps
    Op Amp Information
    Op Amp tutorial

     
    This page has been brought to you by:
    Greg D'Souza and Steven Pearce