How to Write Node Equations
for Active Circuits

by James Bainter © 2009 - 2011

## Introduction:

## How to Write the
Equations:

An Op-Amp Fig. 1

Op-Amp Thevenin equivalent voltage generator Fig. 2

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:

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

by James Bainter © 2009 - 2011

Printable
Version

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.

An Op-Amp Fig. 1

Op-Amp Thevenin equivalent voltage generator Fig. 2

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.

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.

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.