Re: analog multiplier help

On 20 Dec 2005 07:17:31 -0800, bill.sloman@xxxxxxxx wrote:

>
>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.

I know that, but they have also been used for lower frequencies in
radios ... from the 60s till today . I 've already said that before.
>
>> >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?

Yes they are superior to analog multipliers too. I wrote to you the
Mathematical reasons why thats so down there . Check up the
references.


>
>> >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.

Well you were talking about a Gilbert cell versus a dual gate Mosfet .
>
>http://www.analog.com/UploadedFiles/Data_Sheets/66167428AD734_c.pdf

You just have to open up a radio to see a dual gate mosfet being used
.... Not a data***.
>
>> >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,
Did he say he was building a scientific instrument ? No he did not.
So maybe you are talking about a subject completely new to both of
you.
You don't seem to be aware of the fact that broadcast transmissions
cover 10 Mhz.
> where the OP's low frequency
>interests are much better represented.
>


Did you read those articles? NO I guess because they mention use of
the MOS at lower frequencies.
..
>------------------
>Bill Sloman, Nijmegen



"Go easy on the whisky"

theJackal
.

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