Re: gEDA-user: New Poster - Trying to get ngspice up and running with gschem

[al davis <ad252@xxxxxxxxxxxxxxxx>]

On Tuesday 03 August 2010, Bill Gray wrote:
> To keep things as simple as possible, I am trying to simulate
> a voltage follower using an LM358 opamp.  So far, I have
> been met only with abject failure, and my ability to
> continue investing time at this rate is waning... I have to
> get this project moving forward!!!
> 
> I am running gschem, gnetlist, and ngspice on an ubuntu box.
> 
> I have attached the relevant files to this email.
> 
> As I said, I can get a simple voltage divider to work, but
> not the LM358 voltage follower circuit.

I see a few problems.

The first is "your circuit doesn't work", but I will get to that 
later.

The next one is that this particular model is not compatible 
with gnucap ..   "POLY(5)".    

For starters, I prefer to use a simple VCVS model for an opamp.  
Then when that works, I switch to a more detailed model.  Then 
you could try the generic opamp model I posted a while back, 
passing the appropriate parameters like slew rate and GBP.

Now back to "your circuit doesn't work" .....   If you want a 
voltage follower, tie the output to the inverting input, and 
drive the non-inverting input.  You have the inputs swapped.


Here's an op-amp.  To use this model, you need the development 
version of gnucap.


* Generic op-amp behavioral model
.subckt opamp (out+ out- in+ in-)
.param gain=100k sr=1e99 clip=1e99 gbp=1e99
.param cc='1/(6.283185*gbp)'
.param ilim='cc*sr'
G1 (0 1 in+ in-) pwl ('-2*ilim','-ilim','-ilim',
+ '-ilim','ilim','ilim','2*ilim','ilim')
R1 (1 0) 'gain'
Cc (1 0) 'cc'
E1 (out+ out- 1 0)  pwl ('-2*clip','-clip','-clip',
+ '-clip','clip','clip','2*clip','clip')
.ends

You can specify:
gain = open loop gain
clip = output clipping voltage
sr = slew rate (volts per second)
gbp = gain bandwidth product

There are default values, in case you don't want to specify.

To call:
X1 (out 0 in+ in-) opamp gain=10k gbp=1meg clip=13 sr='5/1e-6'


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