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From: email@example.com (Thorsten Brandt)
Subject: Re: Variable Gain Amplifier
Date: 19 Oct 2002 05:52:28 -0700
References: <firstname.lastname@example.org> <3DB09C15.email@example.com>
NNTP-Posting-Date: 19 Oct 2002 12:52:29 GMT
Robert Baer wrote in message news:<3DB09C15.firstname.lastname@example.org>...
> Thorsten Brandt wrote:
> > Hi,
> > i'm looking for help and information in order to design a variable
> > gain amplifier with the following spec :
> > bandwidth : 1 Ghz
> > continously variable gain from 1 to 50
> > input imedance : 10kohm/2pf
> > output resistance : 50-100 ohm
> > noise density : < 5nV/Sqrt(Hz)
> > furtheron is a high flatness in the frequency range from DC to 500 MHz
> > very important ! linearity is also important - but this is left open
> > for discussion ... (in a first approach i would say HD2 and HD3 should
> > be lower -50 to -60db for a 200mVpp signal)
> > a fully differential design would be in this context very useful for
> > my application - but it's not a must ...
> > the control of the gain can be done by voltage or by current. the
> > function how the gain depends upon this voltage is not so important
> > (linear or linear-in-db or whatever ...)
> > any help is appreciated : that means discrete design ideas like in the
> > good old days of analog tektronix scopes equally to approaches with
> > more integrated circuits (if available - i haven't found yet any good
> > ones - the best one i found is the AD8367)
> > links to papers, websites and good books would be also very
> > interesting ...
> > best regards
> > Thorsten Brandt
> Firstly, you do not say the frequency range (DC-1GHz or 1-2GHZ or ??),
> but that does not matter.
> What you fail to specify is the input impedance as a function of
> It will not be possible to retain a constant input impedance over
> frequency, and your spec of Z and noise is not exactly conducive to a
> practical design; a 50 ohm or 75 ohm input impedance would be more
well, then i will put it more precisely - hope we can get a little bit
more to the design then ...
first the bandwidth is from DC-1GHz - as you might also conclude from
the the statement that the gain should be as flat as possible over the
frequency range from DC-500 Mhz ...
the noise densitity should be as white over the frequency range as
possible - apart the resulting Vpp broadband noise assuming white
noise with the specified noise densitiy is the most stringent
constraint. when you are concerned about limiting 1/f-noise effects,
then we can go into detail there - if there would be any severe
concerning the input impedance - i'm restricted to an input resistance
in the range of 10 kohm because of the characteristics of the output
of the feeding amplifier stage - this is what matters - if you prefer
something in the 50 ohm range we have to discuss an impedance
converting stage at the front end of the variable gain amplifier ...
concerning the the possibility to retain a constant input impedance,i
don't think this is severe for the specified bandwidth - i have build
amplifier with this rather low input resistance up to several 100 Mhz
(like many commercial amplifiers) that fit very well over frequency to
such a spec - please give me some good reasons why you assume severe
problems and degradation over frequency with regard to the specified
values here. i woud assume and ask at first glance for as little
parasitic as possible so that the input impedance spec ishould be
valid for DC-500MHz ...
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