Re: analog multiplier help


theJackal wrote:
> On 19 Dec 2005 16:06:28 -0800, bill.sloman@xxxxxxxx wrote:
>
> .
>
> >
> >> >
> >> >> With an integrated product you don't know whats in there and normally
> >> >> that makes me feel uncomfortable.
> >> >
> >> >Barry Gilbert of Analog Devices has published a number of papers on
> >> >"Gilbert cell" mixer's and multipliers.
> >> >
> >> >A search on "Gilbert B." + translinear on scholar.google.com picks up
> >> >some 28 references.
> >> >
> >> >You should be able to get pretty fair idea of what goes into the Analog
> >> >Devices mulitpliers if you read a few of them. More than enough to make
> >> >you feel comfortable, in my expereience.
> >> >
> >>
> >> Ummm....
> >> Reasons why I prefer discrete components.
> >> 1) A designer should be familiar with discrete circuit disegn
> >> techniques , something he/she loses when using integrated circuits.
> >> 2)You compromise circuit flexibility when you use an integrated
> >> circuit
> >> 3)Discrete communication circuits are superior to integrated circuits
> >> in performance.
> >>
> >> ><snipped bad advice>
> >> >
> >> Your Proof?
> >> I disagree.
> >> Its a well known fact that MOS/FET are superior to BJT mixers.
> >
> >For what frequencies?
> >
> Thats a general well known consideration. Read S.A.Maas Microwave
> Mixers.
> To make a direct circuit comparison is ALWAYS dangerous because they
> are too many variables to consider.
> >> Try reading something on that.
> >
> >Such as?
> >
> Depends exactly on what you want. If you need l proof on why MOS/ FET
> Mixers are better then their bipolar counter parts Clark Hess 1971
> "Communication Circuits" is the place to go.

And he won't be much interersted in mixer performance in situations
where all of the frequencies being mixed are lower than 10MHz. Long
wave and medium wave radio transmission is still of coomercial
significance, but nobody write books about it any more.

> >My impression is that the virtues of dual gate MOSFET mixers have
> >everything to do with the good behaviour of the components at
> >relatively high frequencies.
> Well at 10 MHz they should be OK.
> They work well at both vhf and uhf I assume.

They do - that is their particular virtue.

> >The OP wants good mixing - and explicityl wants to minimise spurious
> >high order product spurs - at frequencies up to 10MHz,
>
>
> One of the 3 possible modes in which you can operate a FET as a mixer
> is in resistive mode changing the values of the resistance or more
> specifically the conditions of the MOSFET channel between the source
> and drain from complete depletion to complete invertion or infinite
> resistance to zero resistance . In these conditions the FET channel
> resistance is linear and very low intermodulation distortion results.

But not a low as a good analog multiplier in the frequency range where
the analog multiplier is good.

> The book on Clark Hess above gives a detailed analysis using Bessel
> functions on why FETs have superior distortion performance compared to
> their bipolar counterparts.

But not analog multipliers - right?

> >A well
> >compensated Gilbert cell multiplier will do much better than a MOSFET
> >for this sort of job. He also wants to use fairly high level signals,
> >which won't work well with a dual -gate MOSFET mixer.
> The Gilbert cell multiplier has a differential output current given
> by the products of the emitter current, tanh(V1/2Vt) and tanh(V2/2Vt).
> V1 and V2 being the The only way you can get an analog
> multiplication of the 2 input signals is by assuming V1 and V2 are
> SMALL. That is by carrying out the Taylor expansion of the tanh
> expression and approximating it as V1/2Vt. Thats why predistortion is
> used in Gilbert cells.
>
> Mos have much higher p-p input signal ... compare the mathematical
> expression for the output drain current in a dual gate Mosfet .
> id = ....+...+ kVg1*Vg2+.... . k is a numerical constant. The
> drain current has no limitations as is the case for the Gilbert cell.

And the analog multipliers like the AD734 are specified for +/-10V
signals. This is a different ball-park.

http://www.analog.com/UploadedFiles/Data_Sheets/66167428AD734_c.pdf

> >The numbers for the MOSFET would have come from "Transistor Circuits
> >and Applications" (Prentice-Hall series in electronic technology) by
> >Laurence G Cowles ISBN: 0139300732.
> >
> >The ISBN is for the second edition from 1974 - I had the first edition,
> >until it got soaked when a pipe froze in Cambridge U.K. some twenty
> >years ago.

> Try looking at the following .
> Tsironis Meierer and R. Stahlmann Dual Gate Mesfet mixers "IEEE
> Transactions on Microwave theory and Techniques" Vol MTT-32,No.3
> Mar.1984"
>
> S.Weaver "TV Design considerations Using Dual Gate MOSFETS "IEEE
> Transactions on broadcast and Television receivers, Vol. BTR 19 , May
> 1973"
>
> You don't seem to get the point of my original post . The Op wanted
> some advice on both discrete and integrated circuit implementation of
> a mixer. Well while everyone was saying Gilbert cell I wanted him to
> know there were other options ... None of which were mentioned by
> anybody! I admitted in my first post they weren't necessarily the
> best and it would be challenging to use them... but if I was him I'd
> have taken that road.

He wanted a low frequency mixer, and you are giving him advice derived
from microwave and broadcast journals. Win and I are coming out of the
scientific instrument literature, where the OP's low frequency
interests are much better represented.

------------------
Bill Sloman, Nijmegen

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