From: Terry Pinnell
Subject: Re: Transistor amplifiers v. frequency in SPICE
Date: Tue, 21 Jan 2003 20:06:42 +0000
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NNTP-Posting-Date: Tue, 21 Jan 2003 20:07:15 +0000 (UTC)
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Paul Burridge wrote:
>Okay, back again, despite the barrier of computer technology. :-(
>I've been unable to quote your post this time since it - along with
>God knows how many others - has failed to appear on my news server.
>I'll therefore have to address your points in a necessarily buggered
OK, let me know if you want a copy. Or use google news to retrieve.
>Firstly, you say (in relation to AC analysis): "All that's required is
>that at least one independent source is specified with an AC value"
>The question arises as to which parameter that value should relate?
>(amplitude, frequency, phase, whatever)
The AC Analysis will use your existing AC source. As I said, it will
simulate small sinusoidal variations in the linear region of your
circuit. Also as mentioned, it's independent of your Transient
Analyis. It's calculated regardless of what type of AC signal you're
*actually* using (pulse, exponential, sine, and so on).
>Secondly, I'm aware the load is "crucially important" but I think
>you've taken my remark out of context with the original point. I'm
>trying to get a better understanding of SPICE here within this thread,
>not trying to solve any real-world problem. All I need to know is that
>whatever the load resistance is, we are both modeling for the *same*
>value of it! At this stage, I'm simply trying to get my simulations to
>agree with yours; that's all.
But earlier in the thread weren't you quoting gain figures? Were those
Spice results or real life results, or what? In either case, what load
were you using or assuming? I didn't know, so I used an arbitrary 1 M.
>You give six different values of load and gain figures for each, for
>which I'm very grateful. Can you confirm that the component values you
>are using in this sim. are the same as those appearing on your web
>site in the schematic entitled (something like) "PaulAmp"?
Yes. (It was called
>To answer your last question, B2 Spice is the applicable s/ware, as
>listed on your site so I assume you're familiar with it.
That's a wild and illogical assumption! No, I'm not 'familiar' with
the 60 or so packages I've listed.
As it happens, I do have a copy of B2 Spice Version 2 though. If
that's what you have, or similar, have you read its manual? In
particular, have you stepped through Tutorial 2 on page 18, covering
an AC Analysis?
There's also a *lot* of material online about Spice in general and AC
Analysis in particular. One reference I recommend is the Spice User's
Manual, which I recall was edited for the web by Charles Williams to
support his teaching. I think it was originally based on text from
Kevin Thomas's html version of the SPICE3 Version 3f User's Manual.
Here's a quote:
1.1.2 AC Small-Signal Analysis
The ac small-signal portion of SPICE (.AC) computes the ac output
variables as a function of frequency. The program first computes the
dc operating point of the circuit and determines linearized,
small-signal models for all of the nonlinear devices in the circuit.
The resultant linear circuit is then analyzed over a user-specified
range of frequencies. The desired output of an ac small- signal
analysis is usually a transfer function (voltage gain, transimpedance,
etc). If the circuit has only one ac input, it is convenient to set
that input to unity and zero phase, so that output variables have the
same value as the transfer function of the output variable with
respect to the input.
But, if you're using B2 Spice's GUI interface, I wouldn't go overboard
on studying the text-based approach. Better to learn how to use the
package's facilities first, *then* study the pure stuff. (And even
better, try a decent Spice package; I found B2 Spice Workshop pretty
awkward. Doesn't come close to CircuitMaker.)
And if this is your first exposure to Spice, why not start off with
something less ambitious than even a 1-stage transistor amplifier? Try
say a signal source in series with a 10k resistor and 1 uF cap and get
comfortable with the tools.
Hobbyist, West Sussex, UK