Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
References: <email@example.com> <firstname.lastname@example.org> <email@example.com>
Subject: Re: How does a mixer work?
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Fri, 18 Oct 2002 08:12:44 +0100
NNTP-Posting-Date: Fri, 18 Oct 2002 08:12:46 BST
"Frank Raffaeli" wrote in message
> Jim Thompson wrote in message
> > On 17 Oct 2002 08:05:59 -0700,
> > SNIPrf_man_frTHIS@yahoo.com (Frank Raffaeli),
> > In Newsgroup: sci.electronics.design,
> > Article: <firstname.lastname@example.org>,
> > Entitled: "Re: How does a mixer work?",
> > Wrote the following:
> > |email@example.com (Asa Cannell) wrote in message
> > |> How does a mixer work? I don't understand why it needs a
> > |> element. I think examples of why a linear element (resistor) wont
> > |> would help illustrate why a non-linear element (diode) will.
> > |
> > |It doesn't need a non-linear element. Non-linear elements, like
> > |or over-driven transistors can provide a simple way to derive a
> > |product component of two signals. For a nearly linear approach,
> > |up "Gilbert cell mixer". This uses linear elements and current
> > |steering / sharing. An AC current with a DC bias is split between a
> > |differential pair and steered to one side or the other by altering
> > |bias on the bases / gates of the pair.
> > NAAAAAH! The "elements" ARE NOT LINEAR!
> Hmm ... ok, that's true. No transistor is perfectly linear, even when
> you try to operate in a linear range. I guess my point was that a
> Gilbert cell multiplier works very well when operated in the linear
> range of the devices, and the operation of the (Gilbert) multiplier is
> not dependent upon the inherent non-linearity of the transistors.
Yes it is!.
A Gilbert multiplier fundamentally works *because* of the *exponential*
behaviour of a transistor. Its re is determined by its collector
current. If re was a constant, indepandant of collector current, it
could not multiple at all. The multiplication is due to
re is a function of it own current, therefore it is non-linear, by
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