Forums » General Electronics Discussion

Hello. I'm really enjoying what the circuit lab has to offer. As a beginner in the area and hobbyist, I decided to use circuitlab to learn more, instead of using spice programs already established. Mainly because of the circuitlab inteface is the best I've ever seen. very user friendly. I think it attracts many people who had no contact with programs like spice. I'm an example. = P

Most of the sections of the program allows intuitive use for hobbyists. Other, with a read in the documentation, FAQ, sample circuits, or in forum threads.

Only problem I found to users like me, is a section to explain how to model components dfe according to the datasheets. I tried to use some information parameters of other programs I found spice but they do not have many equivalents to the circuitlab (JFETs, for example)

I have this fet boost. Originally it has 2N5484 JFETs. I tried to edit the individual parameters, to approach the original component, but can not find the equivalent parameters datasheets.

[circuitlab]cb27r5n[/ circuitlab]

Could anyone help me and explain how to do it?

So I would suggest, also, a section for beginners and hobbyists with such explanations.

Thanks in advance. Obrigade since j

Sorry

image:

As a general rule, in any well-designed circuit, it's designed with a lot of DC and AC feedback, so it's not so dependent on the exact device parameters.

Also, it's helpful to note that FETs come with a WIDE tolerance on their parameters, unless they're specially selected FETs. For instance the zero-bias current can vary by a factor of 4 or more. So MOST circuits will tolerate you plugging in another FET, in real life or in simulation, as long as you choose a FET of roughly the same voltage and current ballparks.

For example you could use the 2N3819 model in CL and be close enough.

Note that this is not quite true if you need it to work exactly as designed, within a tenth of a dB, at 100MHz. But close enough for audio work for sure.

If your circuit doesn't work with a close but different FET, it's maybe a clue that the circuit was not designed with very good DC stability. There is a fair amount of this out there in example circuits-- they work fine with a certain FEt at room temp, but not so swell when things diverge a bit.

@Leirbag

SPICE parameters for devices have little to do with datasheet specifications. They are much more to do with semiconductor physics and the particular process under which a given device is manufactured at the foundry.

Consequently it is not always a simple task to map datasheet information onto the model parameters.

If you want to try to understand more about what the SPICE - and hence CL - parameters mean in diodes, bipolar transistors and diodes, then you might like to have a look at:

http://www3.imperial.ac.uk/pls/portallive/docs/1/56133736.PDF

with individual slide sets:

http://www3.imperial.ac.uk/pls/portallive/docs/1/7292571.PDF

http://www3.imperial.ac.uk/pls/portallive/docs/1/7292572.PDF

http://www3.imperial.ac.uk/pls/portallive/docs/1/7292573.PDF

For more detailed information about bjt's in particular, have a look at this book:

http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=01480193

available from:

http://www.lulu.com/spotlight/iangetreu

and

http://www.amazon.com/Modeling-Bipolar-Transistor-Ian-Getreu/dp/B000EYPQLU

Thank you both for the info. Very helpful. Good news for me, then. =p

Thank you again.

FYI:

Another excellent (and free) book about transistor modelling, go to:

http://www.aeng.com/spice_modeling.htm

and register to get a copy of:

Definitive Handbook of Transistor Modeling

What are you guys talking about?

@tube66677

CircuitLab (CL) is not just a way to draw circuit diagrams. It is also a circuit simulator similar to SPICE and other simulators.

A circuit simulator is basically a specialised mathematical program, optimised to construct and then solve the equations that describe the behaviour of circuit you have described to it in your circuit diagram.

In order to simulate the behaviour of the individual components, they have to be described mathematically. The underlying equations that describe the behaviour of a component are written into the simulator program (sometimes they can be added by the user). The coefficients of those equations are the parameters that are given in device models such as for diodes, bipolar or JFET transistors.

These are the parameters that you can see listed for CL components and more generally in SPICE models that component manufacturers may make publicly available.

Models of more complex devices are often made up by connecting lower level devices and saving them as what are called subcircuits. The low level components in subcircuits are described by the same sort of models (those lists of parameters or coefficients) as for the basic diodes etc., already referred to.

However, the equations used to describe the components and therefore the models for them, bear little relationship to the sort of information that is given in typical discrete component datasheets.

Several of the links referred to earlier in this thread are to information about how those models can be extracted from measurements of real devices. There is some information about how to extract model parameters from device datasheets but care must be exercised in building models this way.

Using Behavioral Voltage and Current Sources:

https://www.circuitlab.com/docs/circuit-elements/

and expressions:

https://www.circuitlab.com/docs/expressions/

it is possible to create your own what are called behavioural models of components. These are models that behave like a device but which have little of the actual underlying realistic circuit defined and are mostly - or perhaps completely - described by explicitly defined expressions (equations). Most opamp models are largely behavioural.

There are many examples of behavioural models publicised here in CL.

For more see:

http://en.wikipedia.org/wiki/Electronic_circuit_simulation

and:

http://en.wikipedia.org/wiki/SPICE