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From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Transistor amplifiers v. frequency in SPICE
References: <3E20D1C8.166C@sneakemail.com> <firstname.lastname@example.org>
Date: Sun, 12 Jan 2003 12:50:52 -0500
NNTP-Posting-Date: Sun, 12 Jan 2003 12:50:19 EST
Organization: Bell Sympatico
Paul Burridge wrote:
> Mike Monett wrote in message news:<3E20D1C8.166C@sneakemail.com>...
> > Paul,
> > I have to admire your energy and persistence. SPICE is not easy to learn,
> > and you are doing a good job of asking the right questions.
> THanks, Mike. I can use all the encouragement I get!
You are entering a brave new world where the results can be highly
accurate, or streams of meaningless numbers. SPICE does not have an idiot
flag that tells you your model is screwed up, or you are using it in
areas where it is no longer valid. You have to figure these things out
for yourself. You have to learn a whole new way of thinking, and find out
the relationship between the sometimes obscure SPICE parameters and what
you are trying to accomplish. Most SPICE programs have quirks that are
not well explained in the help files, and you have to keep trying until
it finally works the way it should. This can be time-consuming.
On the other hand, you never have to worry about poor solder connections
or blowing up expensive parts. SPICE will let you explore operating
regions that would destroy real parts instantly, and can give you insight
on how to prevent this from happening in a real circuit.
> > A couple of quick comments. Your SPICE should be able to plot the AC
> > response curve instead of having to do each frequency manually. What
> > version are you using?
> I think the version is shown on the .ckt file I posted. However, I'll
> the instruction line for the AC sweep and maybe from that you can see
> the problem.
I replied to your post separately.
> Thanks for that. The reason for the high value resistors was that I
> trying to keep Zin as high as possible, since the real signal source
> I am modelling has a quite high Z. Yes, I know. Get myself a FET! :-)
Getting wide bandwidth from a high-z source can be difficult or
impossible. A fet may not help, since it has its own input capacity. You
can try boostrapping, but even that has limitations. Another option is to
use the virtual short of an inverting op amp. Win has posted informtion
on wide bandwidth transimpedance amplifiers for optical sensors, and
included some helpful urls. If that would be of interest, I can look them
up for you.
> > These are simple ways to see how a circuit is working, and to get a
> > feeling for the real-world effect of strays. It tells you for wideband
> > circuits, you need low impedance which means high current.
> Fascinating. Thanks for a valuable insight into the difference between
> SPICEWORLD and the real world! BTW and on this subject, would I be
> to assume that SPICE models take no account of component lead-length?
> example, a real-world coupling capacitor has an optimal value with
> impedance at the desired (radio)frequency due to the inductance of the
> creating a series tuned circuit of sorts, whereas the SPICE capacitor
> "perfect" and bigger (values)are always better and present less
There are various models for all components, such as capacitors,
resistors and inductors. The simplest use only the pure component, and
more complex models include the parasitic effects. You have to decide
when these become important enough to include in your simulation.
Generally, if the parasitic impedances are low compared to the circuit
impedance, you can often ignore them. But you have to watch out for
things like series resonances that will happen in the real world, such as
power traces resonating with the bypass caps.
You can often ignore the pin parasitics on ic's until you get to several
hundred MHz. Then you have to start thinking about including them in the
simulation. There are other cases when they become important at low
frequencies, such as the node capacitance to ground on an inverting op
amp, or stray capacitance across the bias resistor in a CMOS Pierce
oscillator at 32KHz.
> > But first thing is to get SPICE to plot the AC response curves for you.
> > Lets see if we can help!
> Many thanks again. I'm always grateful for informed input to my
> I'll establish the relevant instruction sequence and append it to this
Good show, Paul.
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