Spice simulation

[JonHarrison]

Try simulating this ...

[keantoken]

The placement of the 33R resistor, R13, and Q5's base are wrong.

I recommend the second picture you posted, it's design makes the most sense to me.

- keantoken

[Buckeye]

Wow, that's very cool Jon. It works! The big difference I see is the value of the Rbe in the darlington pair. Although it looked fishy, I plugged it in as shown in the Multicomp datasheet, http://www.farnell.com/datasheets/296716.pdf. Now I see that STMicro has a different opinion, one that aligns with yours, http://www.st.com/stonline/books/pdf/docs/4132.pdf.

Other than moving that pesky capacitor, did you do anything else?

Thanks tons!

[Buckeye]

Oh, I see... you also changed the driver to be a common emitter as opposed to an emitter follower. That's probably the big change.

[Buckeye]

Yeah, this corrected circuit works in all three simulators I was trying. Silly me. I built this amp and it worked perfectly. I just couldn't see my drawing error. Thank goodness I'm a little better with the soldering pencil. I am really grateful for your help.

Hey, this software simulation stuff is really cool! Someone ported Falstad's circuit simulator to a smart phone. How cool is that!?

[JonHarrison]

The main problem with your simulations was the VAS connection.

Also the power transistors you'd chosen aren't a complimentary pair which resulted in asymmetry in the output so I just left them as generic npn and pnp. Some of the capacitors values appeared with some extended character set characters instead of u in the uf so I corrected these and changed the input filter to a non-polar 100pF. I connected input and output test points and change the simulation parameter to do a 1 second transient run instead of a frequency sweep. That's pretty much the extent of my changes.

I didn't change the topology of the output stage but I would recommend you use some different transistors for the VAS and the output stage and then run the simulation again. There is some crossover distortion at the moment because the biasing of those 3 diodes isn't correct.

Oh and if you need help with LTSpice and can't get it here there is a good group on Yahoo.

[Buckeye]

Right, I knew the transistors weren't right. The standard libraries have so little. I joined the Yahoo group & learned how to add the SUBCKT darlngtn.lib library so now I can use the TIP142/147 pair. Also updated the standard.bjt & standard.dio.

Still, the transistor library is missing many power and most Japanese transistors. I probably don't totally understand the LTspice library structure, but it seems kludgy. It's a bit difficult adding entire libraries. When you do, you don't get to pick your part, you have to type it in, which means you have to memorize everything that's in the new library. To get the selection option, you have to add the parts one by one or risk duplication. Looks like library maintenance will be a big part of the effort with this simulator.

The character set problem is maybe because I'm running LTspice on Linux under a WINE environment. It does not appear to cause any problems so LTspice must be aware of the Linux character set.

As for topology, I'm not working on the amp, I'm working on short circuit protection schemes. So I wanted a simple amp to start with.

Anyway, thanks for getting me unstuck. Now I'm off & running, albeit at a snail's pace.

[teodorom]

LTSpice is a great product ! and the support you can get (thanks Helmut !) is even greater.

[keantoken]

I run LTSpice on Linux too.

Have you tried winetricks? Try the "allfonts" package.

- keantoken

[JonHarrison]

The choice of transistor models in LTSpice may be limited but don't forget Linear produced the tool to assist with simulation of their opamps and regulators etc. not as a general purpose simulator (although this is how many of us it - and thanks to Mike for that) !

If I want to simulate a complex circuit with many new parts I'll create a sub file and include that on the schematic but more often than not I simply add a spice directive 'S' and put a model for a transistor I want to use directly onto the schematic. If you can only find subckt definitions than create a new assembly and use that to reference the part.