From: Terry Pinnell
Subject: Re: Transistor amplifiers v. frequency in SPICE
Date: Sat, 18 Jan 2003 07:34:37 +0000
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NNTP-Posting-Date: Sat, 18 Jan 2003 07:35:07 +0000 (UTC)
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Paul Burridge wrote:
>On Fri, 17 Jan 2003 17:34:42 +0000, Terry Pinnell
> opined thusly:
>>Eh, why do you think that? I suspect you've misinterpreted the Bode
>>Analysis scale. For example, '300.0 V' means a gain of 300, not an
>>output. Reference is 1 V.
>Doh! That would explain a lot. Can do an AC sweep that shows output
>voltage level as a function of source voltage frequency for a fixed
>Vin of 2mV?
A Spice AC Analysis is a 'small signal analysis', but no particular
value of source signal is used. In that respect, it's independent of
the Transient Analysis. All that's required is that at least one
independent source is specified with an ACc value.
>>>However, I've modified my circuit to be *exactly* the same as
>>>yours now (why the 1M load?)
>>As I said earlier, it was an assumption "...(with my arbitrary
>>assumption of 1M load)". What load are you using?
>Since the purpose of this stage is undefined in practical terms, it's
>not terribly important, I guess.
It's crucially important!
>Certainly if it were an RF output
>stage, I'd use typically 50, 70 or 300 ohms depending on the radiation
>resistance of the antenna the stage was intended to feed. One meg just
>seemed inordinately large; certainly for an output stage but also for
>a small signal intermediate stage, too. I can't see the Zin of a
>real-world following stage being anything like that high. But it's not
>relevant for our purpose anyway, so long as we're both modeling
To repeat, you must specify a load if you want to get realistic
results. I've repeated the AC Analysis for six different values of
load, illustrating the dramatic range:
R Max Gain
>>Not sure I follow that. Results seem as expected to me.
>I'll be in a better position to tell if you can set the sweep
>parameters as suggested above...
What Spice are you using? Have you studied its manual or Help re AC
Hobbyist, West Sussex, UK