Re: The bi-polar transistor at RF

From: Kevin Aylward (salesEXTRACT_at_anasoft.co.uk)
Date: 07/23/04 

Date: Fri, 23 Jul 2004 13:11:10 GMT

Paul Burridge wrote:
> On Thu, 22 Jul 2004 15:40:56 -0700, Roy Lewallen <w7el@eznec.com>
> wrote:
>
>> John Woodgate wrote:
>>>
>>> If you have the s-parameters (preferably at more than one
>>> frequency) you can make an equivalent circuit and use that in
>>> SPICE. OK for r.r.a.h., I expect, but maybe not for s.e.d.. (;-)
>>
>> A SPICE model constructed from s-parameter data can be useful, but
>> only under a very limited set of circumstances. First, it's valid
>> only if the signal level is small. This is often not the case with
>> an oscillator, unless external amplitude limiting is used (as it
>> should be). Also, a circuit very often can oscillate (as well as
>> amplify) at more than one frequency, and a SPICE model that's valid
>> only over a small frequency range won't predict this behavior. (A
>> single frequency SPICE model is simple to construct from s-parameter
>> data, but one covering a wide range of frequencies is just about
>> impossible.)
>
> What's the problem in making up your own (accurate) wideband Spice
> model from measurements you make yourself on a VNA?
> I'm also under the impression that a parameter such as (for example)
> the base-collector capacitance lifted from a manufacturer's data ***
> and inserted into Spice won't work accurately. Why is this?

It should work ok if you use the right value. Go and have a look at the
basic spice capacitance equations. You can read I take it?

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.