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Absolute Beginner to SPICE
Absolute Beginner to SPICE
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Old 6th April 2009, 07:36 PM   #1
klewis is offline klewis  United States
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Default Absolute Beginner to SPICE

Andy_C mentioned this as a possible topic in the Solid State Spice thread. So, being close to an absolute beginner and not knowing where else to post this, I thought I might start here. I'm building an amp with the LME49810 for the front end and a darlington pair configuration for the current gain section. I would like to test several combinations/configuration of the CG section. So, I need to simulate the output, (maybe without feedback) of the front end. The image below shows what I've done to date.

First, is there a better way? what would that look like?

I would also like to do square wave tests. I've done some previously on full amplifier circuits but with this partial amp, my methodology doesn't work.
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Old 6th April 2009, 07:43 PM   #2
klewis is offline klewis  United States
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Here's the spice model.
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File Type: zip darlington1.zip (1.8 KB, 25 views)
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Old 6th April 2009, 07:59 PM   #3
Mr Evil is offline Mr Evil  United Kingdom
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What are you hoping to learn from the simulation? There are things you can do to help make the results of various tests more realistic, but you need to know what you're testing for.
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Old 6th April 2009, 08:03 PM   #4
klewis is offline klewis  United States
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I'm hoping to get a relative sense of THD when comparing different transistors; seeing the relative effects of added componets and componet values;
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Old 6th April 2009, 09:15 PM   #5
Mr Evil is offline Mr Evil  United Kingdom
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You might try using a simple model of a loudspeaker as a load instead of a resistor. This can exacerbate crossover distortion by making the crossover occur at a point other than at zero output voltage.

I think you'll find that you have a DC offset at the output with the circuit as you have it. You should split the bias voltage in two and apply the input to the middle. Note that you will have to readjust the bias for each device you try, to make sure you test them all at the optimum quiescent current.

Also try measuring input impedance (just plot input current against frequency). An output stage might not add that much distortion itself, but it can cause distortion in the VAS by presenting a low impedance, or not sufficiently isolating the VAS from the load impedance.
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Old 6th April 2009, 10:07 PM   #6
klewis is offline klewis  United States
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I'll give it a try this evening and post revised tomorrow.
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Old 7th April 2009, 08:45 PM   #7
klewis is offline klewis  United States
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I don't know how to make a speaker load... can someone provide me with a sample. I've updated the schematic as suggested regarding the voltage input. See attached.
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Old 7th April 2009, 08:52 PM   #8
klewis is offline klewis  United States
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I think I've figured out how to make the square wave work. I used 56v as Vinitial, -56 volts as Von, Trise 1n, Tfall 1n, Ton 2.5e-5 and Tperiod 5e-5. Is this correct for a 10kHz square wave? Enlarged square wave output shown.
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Old 7th April 2009, 08:55 PM   #9
klewis is offline klewis  United States
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I measured the input impedance by dividing the sine voltage output - measurement location shown with the red pointer labeled V(n005) by the sine voltage source current I(V3). See schematic below. Do I have this correct?
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Old 7th April 2009, 08:56 PM   #10
klewis is offline klewis  United States
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This is the resultant impedance graph and measurement box. Does this look like enought impedance?
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