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:-( This Demo works only with old Java-Workshop (JDK1.0).


At the beginning of a design process the designer has to verify which circuit structure is suitable for a given purpose. At this stage numerical and symbolic analysis are of little use. There are no numerical values for conventional simulation with programs like SPICE and a Symbolic Analysis would produce large symbolic expressions. No further approximation of these expressions can be performed due to the lack of numerical values.

An experienced designer however, is able to analyze the behavior of an unknown circuit. The Qualitative Analysis reproduces this ability on computers by introducing qualitative states such as positive, zero, or negative.

Client, Server. The applet is a graphical user interface to the QA Server. The QA Server performs the computations and runs on the same computer (www.nst.ing.tu-bs.de, SUN Ultra450) as the WWW Server.

With this demo version DC and Small-Signal Analyses of the following circuit structure can be performed:

Astable Multivibrator

The QA Server uses this netlist:

Astable Multivibrator
R1 1 2 +
R2 2 0 + 
R3 4 0 + 
R4 5 0 + 
R5 1 3 +
C1 4 5 + IC = -
Q1 3 2 4 QN
Q2 1 3 5 QN
VS 1 0 +

QaSimDemo Applet

The main window can be opened again after quitting by pressing the button.

If your browser recognized the applet tag, you would see something like this:

Mini Screendump

Options & Defaults:
Expand Node and Loop Equations
Generate additional node and loop equations
Display System of Equations
Display the qualitative system of equations
Display Solved Equations
Show the equation numbers while solving the system of equations
Vector Pointer / Vector Array
Switches between different internal data structures
Start DC Analysis
Start a DC Analysis. You must set the states of the nonlinearities (transistors) with Set Nonlinears before. With Use Initial Conditions the initial conditions of the dynamic elements are taken into account.
Start Small-Signal Analysis
Start a Small-Signal Analysis. You must set the states of the nonlinearities (transistors) with Set Nonlinears and the small-signal models of the dynamic elements (capacitors) with Set Dynamics before. With Check Operating Point an additional DC Analysis validates the operating point.
Console Messages:
You can watch the solving process here. If the window Error Message appears displaying an equation, this equation leads to a contradiction.

Example (DC Analysis):
  1. Choose Analysis
  2. Click on Set Nonlinears, the window States of Nonlinearities appears
  3. Click on Apply
  4. Click on Done
  5. Click on Start DC Analysis, the window Results appears
  6. Click on Select All
  7. Click on Display
  8. The result will be shown

Interpretation of the Result:
The choosen states of the transistors are valid. There are three possible states of the circuit caused by the uncertain branch voltage U(C1).

Bipolar Transistor States:

FON Forward On
RON Reverse On
FST Forward Saturation
RST Reverse Saturation

Small-Signal Models of Capacitances:

OL Open Loop
RS Resistance
SC Short Circuit

Hints & FAQ:

The FAQ (hopefully) answers the most common questions.
Please direct additional notices, questions, and suggestions to webmaster@nst.ing.tu-bs.de!

  1. M. Thole, J. Meise, E.-H. Horneber,
    Qualitative Analysis of Nonlinear Dynamic Circuits,
    Proceedings of 2nd International Conference on Intelligent Systems Engineering, Hamburg, September 1994
  2. M. Thole, J. Meise, E.-H. Horneber,
    Evaluating Possible States of Nonlinear Dynamic Networks by Use of Qualitative Analysis
    Proc. 3rd Int. Workshop on Symbolic Methods and Applications to Circuit Design (SMACD'94), Seville, October 1994
  3. M. Thole, E.-H. Horneber,
    Qualitative Analysis of Analog Circuits with Expanded Node and Loop Equations,
    Proc. 3rd IEEE Int. Conf. on Electronics, Circuits, and Systems ICECS '96, Rodos, October 1996

aktualisiert: 13.10.2010

Verantwortlich: Michael Hinz
Feedback an: m.hinz@tu-braunschweig.de