Re: gEDA-user: Barrie Gilbert

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

On Wed, Sep 06, 2006 at 06:55:35AM -0400, Dan McMahill wrote:
> Bob Paddock wrote:
> >On Wednesday 06 September 2006 03:36, Karel Kulhavy wrote:
> >
> >>Anyone knows what exactly is called Gilbert cell?
> >
> >
> >The Gilbert Cell is named after Barrie Gilbert of Analog Devices,
> >invented in 1968.
> >
> >http://www.ieee.org/organizations/pubs/newsletters/sscs/jan03/jssc_classic.html
> >
> >The Gilbert Cell has become common in RF designs, used as a double balanced
> >mixer.   It is a four quadrant multiplier.  Somewhere in my files I have a 
> >paper
> >by Gilbert  where he states that he never really meant it to be used the 
> >way,
> >that has been the most common usage.  He recommended an obscure
> >division technique instead.  I'll dig the paper up this evening.
> 
> Bonus points to anyone who can name the real inventor of the mixer in 
> question here.  Hint:  It wasn't Gilbert even though it's called a 
> Gilbert cell.
> 
> >>How many transistors does it actually have?
> >
> >
> >http://rfdesign.com/mag/503rfdf1.pdf
> >
> >
> >>Is it possible to make a well working Gilbert cell with ordinary 
> >>non-matched
> >>transistors? 
> >>And btw do you know what translinear mean?
> >
> >
> >One set of frequencies is translated linearly to an other set of 
> >frequencies.
> >Using non-matched transistors will not be linear, resulting in spurious 
> >outputs.
> 
> no.  That is not what translinear means.  Translinear circuits in this 
> context refers to the class of circuits where you find a loop consisting 
> just of bipolar junctions and have an equal number in each direction. 
> The idea is that  Ic = Is * exp(Vbe/Vt) where Is depends on the device, 
> Vt is the thermal voltage (kT/q), Ic is collector current, and Vbe is 
> the base-emitter voltage.  If you write out KVL around this loop of 
> base-emitter junctions you get:
> 
> sum( Vbe_cw ) = sum( Vbe_ccw)
> 
> where Vbe_cw = junctions where the voltage is positive in the clockwise 
> direction and Vbe_ccw = junctions where the voltage is positive in the 
> counter clockwise direction.
> 
> Now assume all the Is are the same and some simple math shows that
> 
> product( Ic_cw ) = product( Ic_ccw )

Exactly, like MOSFET is a silicon analogue of electron tube,
BJT is a silicon analogue of slide ruler.

> 
> For example, you can build a circuit where I1 * I2 = I3 * I4
> 
> and you can build a squaring circuit or a square root circuit.
> 
> These circuits work on large signals.

What if spurious resistances kick in? Or the signal gets so large
that one transistor heats up? Doesn't it crook then?

CL<

> 
> I'll try to find a reference to post tonight and I'll sketch out a more 
> concrete example.
> 
> -Dan
> 
> 
> 
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