-1- DCNET 1.0 by Albert E. Pistilli DCNET is a linear DC network analysis program that can analyze circuits consisting of branch combinations of resistors, independent voltage and current sources and voltage controlled current sources. The limit of DCNET is 25 nodes and 50 branches. The voltage controlled current source counts for two branches. The 4 types of branch models are illustrated as follows: Resistor (R) Voltage source (E) | To node | V+ node | | | RS R + | | E | - | | From node | V- node Current source (I) Voltage controlled current source (GM) ________ __ ______________To node | | To node V+| | | | | | | | | | | | | I RP RI Id RP | | | | | | | | | | |________|__ From node V-| |__________|____From node For the above illustrations, and the | and _ symbols represents a segment of wire. The transfer function for the voltage controlled current source is: GM*[V+ - V-] = Id In order to use DCNET, all independent nodes of the circuit must be consecutively numbered starting from number one. The choice of the node selected to be node one is arbitrary. The ground node must be zero, and the total number of nodes entered must not include the ground node. When a specific branch is selected, DCNET will prompt the user for all the elements shown in the branch illustrations. Branches are entered by specifying the two nodes across the branch and the value of the branch. When entering this data, separate it with a blank space using the space bar, don't use a comma. For resistors, either side may be specified as the From or To node. All voltage sources must have a resistor RS in series. DCNET will accept a minimum value of 1E-30 ohms for a grounded voltage source when an "ideal" source is desired. However, for a floating voltage source, very small values of RS can cause roundoff errors. Therefore, it is recommended that RS be limited to a minimum of approximately .02 ohms for floating sources. The current sources have a resistor RP in parallel as illustrated previously. DCNET will accept a maximum value of 1E+30 ohms when an "ideal" current source is required. When specifying the independent current source I or the GM source, the current direction is specified by the From and To nodes. For the GM source, the V+ and V- nodes controlling Id require a resistor RI between them. For the voltage source, V+ and V- nodes indicate the polarity. DCNET -1.0 -2- When DCNET is run, it prompts the user for all necessary information. After the circuit has been entered, entering X (eXecute) will initiate analysis, and the program calculates the node voltages. Afterwards, DCNET allows the branch values to be changed by specifying the branch type (R,E,I or Gm), the new value and the branch location number. The branch location number was assigned by the program when the branch data was originally entered. In order to model an NPN transistor, the voltage source (E) and voltage controlled current source (GM) are connected as follows: Base________ ______________ | | | Collector V+| | From | | | | RI Id RP | | | V-| | To | |______RS_+_E_-_____|________|_____ V+ V- Emitter The V+ and V- nodes near RI are the controlling voltages for the dependent current source Id, while the V+ and V- nodes near E and RS represent the polarity of the voltage source. Also, the dependent current source Id has the current flowing into the emitter. Set E=0.6 volts and RS to about .1 ohms to simulate the base to emitter voltage drop. For the GM source, it can be shown that GM = Hfe/RI. Let RI = Rbb, which is about 50 ohms for a low current transistor, and using the desired Hfe and the given formula, calculate GM. Let RP = 1/Hoe, this is usually in the range of 100K to 1Meg. For a PNP transistor, reverse the polarity of E, the current direction of Id and the polarity of the controlling voltage of the GM source. For the operational amplifier model, the voltage controlled current source (GM) is used as follows: +Vin __________ _______________ Output voltage (Vout) |V+ | | | |To | | | | RI Id RP | | | | |From | -Vin __________|V- |________|___ | | To ground node (0) For any operational amplifier, let RI equal the differential input resistance (Rd) and RP equal the output resistance (Rout). It can be shown that the DC differential voltage gain Vout/[V+ - V-] = GM*Rout. For the uA741 opamp, the typical DC specifications are Rd = 2 Meg, Rout = 75 and differential gain = 200,000. Therefore, use the gain formula and solving for GM = 200,000/75 = 2667, enter this value of GM in the opamp model to get the proper voltage gain. The differential inputs for the amplifier are represented by +Vin and -Vin, and the amplifier output is Vout. DCNET -1.0 -3- A simple example using a uA741 operational amplifier is illustrated below. This circuit has a feedback path split in a "T" formation and a +1 volt input. 560K 560K ________/\/\/\____________/\/\/\_ | | | | / | | \ 560K | | / | | | | | Ground | 560K | | ____/\/\/\_____|_________********* | | -Vin * *_______________|__ | * uA741 * Vout +1 volt _______* * | | +Vin ********* | | Ground Ground In order to enter the above circuit into DCNET, we must add the expanded uA741 model. The nodes numbers are shown in parenthesis. This is illustrated as follows: 560K 560K ________/\/\/\____________/\/\/\_ | (3) | | | \ | | / 560K | | \ | | | | (4) | Ground (0) | | | 560K | | (1) ___/\/\/\_____|_________ _______________|__ Vout | (2) -Vin |-V | | +1 volt RI |To | | _______|+V Id RP | | +Vin |From | | | |________|__ | | | Ground (0) Ground (0) Ground (0) In this example, the lowest numbered node (1) is at the left and the nodes are consecutively numbered moving to the right. Although desirable, it isn't necessary to number the nodes in this fashion. For example, the nodes labeled (1) and (3) may be interchanged and the circuit reentered without problems. However, you can't define any node with a number whose value is greater than the total number of nodes in the circuit. In other words, since the above circuit has a total of four nodes, no node may be defined with the number 5 or above. The DCNET run for the above circuit is shown below with the user entering all information to the right of the colon. The program echos back the data just entered and assigns a branch location number. After initiating analysis by DCNET -1.0 -4- entering X, the results are printed out and the program then allows the branch values to be changed. In this run, the value of GM was reduced to 1333. After the change, DCNET prompts the user again if additional changes to the circuit are desired. If not, the analysis begins. Enter number of nodes: 4 Enter R,E,I,Gm or eXecute: E Enter E in volts,RS,V+,V- nodes: 1 .0001 1 0 E= 1 RS= .0001 V+= 1 V-= 0 Branch= 1 Enter R,E,I,Gm or eXecute: ? R Enter R in Ohms, From, To nodes: 560E3 1 2 R= 5.600E+05 From node= 1 To node= 2 Branch= 2 Enter R,E,I,Gm or eXecute: R Enter R in Ohms, From, To nodes: 560E3 2 3 R= 5.600E+05 From node= 2 To node= 3 Branch= 3 Enter R,E,I,Gm or eXecute: R Enter R in Ohms, From, To nodes: 560E3 0 3 R= 5.600E+05 From node= 0 To node= 3 Branch= 4 Enter R,E,I,Gm or eXecute: R Enter R in Ohms, From, To nodes: 560E3 4 3 R= 5.600E+05 From node= 4 To node= 3 Branch= 5 Enter R,E,I,Gm or eXecute: G Enter GM in Mhos,RP,From,To nodes: 2667 75 0 4 GM= 2667 RP= 75 From node= 0 To node= 4 Branch= 6 Enter Rin,V+,V- : 2E6 0 2 RI= 2.000E+06 V+=0 V-=2 Branch= 7 Enter R,E,I,Gm or eXecute: X Node( 1 )= 1.0000E+00 Volt(s) Node( 2 )= 1.4999E-05 Volt(s) Node( 3 )= -1.000E+00 Volt(s) Node( 4 )= -3.000E+00 Volt(s) Change branch value? Enter Yes or No: y Enter Branch Type,New Value,Branch number: G 1333 6 New Value of G is 1.333E+03 at Branch 6 eXecute or Change another branch value? Enter X or C: X Node( 1 )= 1.0000E+00 Volt(s) Node( 2 )= 3.0008E-05 Volt(s) Node( 3 )= -9.999E-01 Volt(s) Node( 4 )= -3.000E-00 Volt(s) Change branch value? Enter Yes or No: No Program terminated DCNET -1.0 -5- If you have any questions regarding DCNET, send them along with a self addressed stamped envelope to the address listed below. Your comments or suggestions on the program are welcome. Permission is given to freely copy this program. Future changes that will be added to later versions of DCNET include better error trapping and the addition of a diode branch. HVCC ELT Dept. Vandenburgh Ave. Troy, New York 12180 Acknowledgment is given to R. Jensen and B. Watkins, authors of "Network Analysis - Theory and Computer Methods". NOTICE: This program is provided without warranties of any kind. The entire risk as to quality and performance of this program is with the user. Should the program prove defective, the user assumes the entire responsibility. DCNET -1.0