linear technology spice cad

[wonder_b]

can anyone tell me how to use LT spice cad what do I need to look for in a amp schematic to tell if its working this is the amp Im testing I have a good looking output but what else do I need to look at

[wonder_b]

.

[pro]

Try to load the examples that are installed with the cad, they are very clear. I have learned therefore.
Pro.

[hjelm]

Isn't one of the q5 transistors backwards.

What is the CF there for and why is it only on one of the q5's.

[peranders]

Start with a simple transistor stage.

Get SPICE models for all parts if you not have them already

Try to get a frequency response

Try to get a step response and sinus response

(Try to get a FFT chart)

When you know this, you can draw bigger circuits but you must test something small first. Note also that big simulations demands a powerful PC. I have a P4 1.8 GHz and a bit faster wouldn't hurt.

[zinsula]

I also am a beginner with LT Spice and found some strange behavior with a BD139 Model from the OnSemi website.
There you can download Spice2 Model, Spice3 Model, PSpice Model and Saber Model. I assumed that for LT Spice, the Pspice Model would work. But in my circuit, the B-E voltage drop is around 0.3 - 0.35 Volts . So I don’t trust the simulation.
Do someone have any advise?

Thanks, Zinsula

[Christer]

Originally posted by hjelm
Isn't one of the q5 transistors backwards.

What is the CF there for and why is it only on one of the q5's.

It is not backwards. It is a quasi-complementary output
stage.

It looks like one of the designs amp_man has posted on
the forum, so the design has been discussed quite extensively
somewhere on the forum.

[Christer]

Originally posted by zinsula
I also am a beginner with LT Spice and found some strange behavior with a BD139 Model from the OnSemi website.
There you can download Spice2 Model, Spice3 Model, PSpice Model and Saber Model. I assumed that for LT Spice, the Pspice Model would work. But in my circuit, the B-E voltage drop is around 0.3 - 0.35 Volts . So I don’t trust the simulation.
Do someone have any advise?

Thanks, Zinsula

Try the Spice3 model perhaps? It is a universal problem that
it is difficult to find good transistor models for Spice. The best
is to plot they usual graphs and compare to the datasheet.
Also try different models if something seems wrong.

[janneman]

Are you sure that the circuit is OK? That low Vbe may be the result of a circuit mistake, not the model's fault. These are the basics, and I seriously doubt that the model would be that far off. In the circuit, is there any Ie? What are the node voltages for the transistor?

Jan Didden

[jcx]

the .op ( operating point ) command is really the first analysis to option to use, if you don't like reading the output text file just close the text window that pops up after the sim and cruise the schematic with the cursor, the values of node voltages and device currents appear in the lower left corner

then all of your "debugging" should be done with test sine and pulse inputs in .tran ( transient ) analysis mode

DON'T use .AC until the circuit is really working or you will get thourghly confused .AC doesn't simulate your circuit, it assumes it works and linearizes the model around the dc operating point - if Qs are reverse biased, stages clipped, ect you will have a hard time finding it with .AC analysis

many basic spice intro books exist and give you an idea where to look in Lt SwCad's really excellent help files