From: "John Jardine"
Subject: Re: Limitations of the BC337
Date: Fri, 10 Jan 2003 19:30:21 -0000
NNTP-Posting-Date: 10 Jan 2003 19:16:25 GMT
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Paul Burridge wrote in message
> On Thu, 9 Jan 2003 21:19:39 -0000, "John Jardine"
> opined thusly:
> >That 200kHz will be due to, too high a gain, capacitive loading effects,
> >high source resistance...
> [comprehensive discourse snipped]
> Well, you've obviously amassed a formidable expertise in RF circuit
> behavior judging by that posting! I'm grateful for the tips from you
> and others who've contributed here. One point arises, however: is the
> chief determining parameter of the maximum frequency a transistor can
> handle its base/collector capacitance (both intrinsic and external)?
> I mean, could one get a good idea of how fast an RF tranny is just by
> the manufacturer's figure for intrinsic b/c capacitance? I'm assuming
> such a figure is also a SPICE parameter?
> "What is now proved was once only imagin'd"
> - William Blake, 1793
To be honest, I've only casual knowledge in this interesting area of circuit
design!. I've noticed though, a couple of this group's posters are -real- RF
men and should have some good angles. As Jeff V mentioned, fluency with it
comes from actually doing it and as is usually the case, the best learning
come from figuring out why the stuff won't play ball.
To a first guess the Ccb value is a very good pointer to a fast transistor
but it is one of a host of other internal transistor factors that are
waiting in the wings and ready to pounce. All the factors come into play at
some point as the frequency moves upwards.
For HF use it's usually best in the first instance, to select a transistor
from the lists specifically marked as 'RF' transistors. This ensures that
most of the major, internal limiting factors, (particularly internal
capacitances), will have been characterised and optimised to allow more
predictable operation at the higher frequencies.
Yes, the Ccb is an essential spice model value.
Incidentally ... What was your rough gain and source impedance?.