How to Write Node Equations for Active Circuits
by James Bainter 2009 - 2012
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  1. Introduction:
    1. How to Write the Equations:
    2. Filter Schematic:
    3. Filter Node Schematic:
    4. Filter Node Equations:
    5. Filter Spec's (Component values and frequency sweep range)
    6. Filter Program Test Results:


I got interested in the Python computer language and started making multi-dimensional lists (matrix). This lead me to wonder what application I could write using Python. I decided to try and solve a set of linear equations using Matrix Algebra. I designed a Python program for analyzing both DC and AC circuits using Mash and Node analysis. All went will until I wanted to analyze an Active Circuit implemented with Operational Amplifiers (Op-Amps).  I searched the Web looking for information on writing Node Equations for Active Circuits. I did find some papers written by several professors saying that superposition could be used when current and voltage generators in a given node/mash where a functions of another node/mash. These professors also noted that many electronics book authors have stated that superposition could not be used in this situation. So, with this information, I decided to figure out how to apply my program to anlalyze an active filter I invented many years ago called the "Bainter Filter" by some people on the Internet. See article published in the October 2, 1975 issue of Electronics magazine.

How to Write the Equations:

op-amp drawing
An Op-Amp  Fig. 1  

Voltage Generator
Op-Amp Thevenin equivalent voltage generator  Fig. 2

Current Generator
Op-Amp Norton equivalent current generator  Fig. 3

All Op-Amps need to be converted to their Norton equivalent current generators as per Fig. 3 where e1 is the node voltage driving the Op-Amp, A is the open loop gain, and Ya is the output admittance of the amplifier.

Filter Schematic:

Bainter Filter Schmatic

This filter contains three op-amps. Op-Amp K1 in an inverter (gain = -1.0). Op-Amp K2 is a unity gain buffer (gain = +1.0). The center Op-Amp is the one of interest. It has an open loop gain of -A as it is connected as an inverting amplifier.

We will next convert each Op-Amp to it's Norton current generator in order to write the node analysis equations.

Filter Node Schematic:

NOTE: All current generators point into their respective nodes. The sign of the Op-Amp connection is determined by the value of the symbols K1, A, and K2. For this filter K1 = -1.0, A (open loop gain) = - 100,000, and K2 = 1.0. Op-Amps K1 and A are inverting and K2 is non-inverting.


Filter Node Equations:

NOTE: Each value of  capacitance must be multiplied by "s" where s = j*2*PI*f where "j" is used in Python to indicate a complex number and "f" is the frequency. The s's have been left out in order to simplify the equations below  because the program I wrote did this operation for each frequency as the program runs.


The voltage E4 at node 4 is a function of the voltage E3 at node 3; therefore the entry of -ya*A-c1 in cell 4-E3 in the above matrix. In this example, the amplifer gain has been defined as a negitive number. The mutual node four has a negative value because it is connected to the inverting (negative) input of the amplifer. If there would have been a mutual node for the positive input of the amplifer, it's repective equation would have a positive sign. If the gain is defined with a positive value the mutual node equations will have signs opposite from this example.

Gain sign negative: mutual equation sign same as amplifer input sign.
Gain sign positive: mutual equation sign opposite of amplifer input sign.

Filter Spec's (Component values and frequency sweep range)


Filter Program Test Results:

Following is a graph of the filter tuned to 1000 Hz. The plot was made by the Python program I wrote using the equations in this document.

Last updated:  6 June 2012        Contact:  Jim Bainter