Reply-To: "fred bartoli"
From: "fred bartoli"
References: <3D760B64.CC303206@scs.uiuc.edu> <3D763949.4B6D795E@scs.uiuc.edu> <email@example.com>
Subject: Re: High impedance low noise
Date: Wed, 11 Sep 2002 15:07:19 +0200
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NNTP-Posting-Date: 11 Sep 2002 15:05:45 MEST
Win Hill a écrit dans le message :
> "fred wrote...
> >Win Hill wrote...
> >> Hi Jim. On Wednesday, September 4, 2002, you wrote:
> >>> Win wrote,
> >>>> In one opamp, yes, but that's not necessary a good approach. When
> >>>> measuring voltage, the reason for considering an FET opamp operated
> >>>> as unity-gain follower is to drive a second high-bandwidth low-Zin
> >>>> amplifier stage that uses conventional components, such as current-
> >>>> feedback opamps that have low noise compared to the FET, obviating
> >>>> the need for the FET stage to have any gain. Well, a gain of 2x in
> >>>> the FET stage would be nice, and has over 1MHz bandwidth with any of
> >>>> the parts below. The follow-on G = 50 low-noise 1MHz stage is easy.
> >>> Well, this was exactly my thinking too, but I keep hearing from others
> >>> that "all (or most of) the gain should be in the first stage, for max
> >>> sig/noise. We were pondering the trick of multiple fet op amp inputs
> >>> all paralleled, then summed. I read somewhere that this is a trick for
> >>> cancelling random noise. I think another engineer here tried that a
> >>> while back and it did not work well. We are all having a meeting
> >>> tomorrow, and I should know more then. I just don't have enough
> >>> details yet.
> >> Hi Jim, best wishes in your meeting. To explain the issue of one
> >> vs. several stages. The situation is accurately understood by
> >> the effect of each stage on the effective input noise. It's useful to
> >> use input-noise-density squared, for a simple easy-to-understand
> >> The formula below is for a three-stage amplifier. Please rewrite it
> >> paper, to see it easier without the poor ASCII-character limitations.
> >> v_n(in)^2 = v_n1^2 + (v_n2/G1)^2 + (v_n3/G1G2)^2
> >> We can see that if the gain G1 of the 1st-stage in a 2-stage amplifier
> >> is 1, and both stages have the same noise level v_n, the effective
> >> noise will be v_n(in)^2 = v_n1^2 + v_n2^2 = 2 * v_n^2, which means
> >> V_n(in) is sqrt 2 = 1.4 times the noise of the 1st stage alone.
> >> where the common wisdom comes from, assuming that the 1st stage uses
> >> lowest-noise amplifier, and saying that G1 = 1 is a bad thing.
> >> However, we know that high-Z JFET amplifiers are much noisier than
> >> low-Z wideband amplifiers. For example, an AD8011 has 2nV of noise,
> >> compared to a JFET opamp with say 6nV of noise. Using the formula, we
> >> see that the overall noise is sqrt(6^2 + 2^2) = sqrt 40 = 6.32nV.
> >> is so close to 6nV we can ignore the extra noise from the 2nd stage.
> >> If the 1st stage has a gain of 2x, the effect is more striking, 6nV vs
> >> 6.08nV. Clearly the old rubric is wrong under these conditions.
> >> . f_T e_n Cin max Vcc
> >> . (MHz) (nV) (pF) (+/-V)
> >> . ----- ----- ---- ------
> >> AD743 4.5 2.9 18 18
> >> LT1792 5.6 4.2 14 20
> >> AD711 4 16 5.5 18
> >> LT1057 5 13 4 20
> >> OPA655 240 6 1.0 5.5
> >> AD8011 * 2 low-Z 6.3 current-feedback, 57MHz at G = 10.
> >> Thanks,
> >> - Win
> > Also don't forget the TI winners
> > f_T e_n Cin max Vcc
> > (MHz) (nV) (pF) (+/-V)
> > ----- ----- ---- ------
> > THS4021 350 1.5 1.5 +/-16 stable G>10
> > THS4031 100 1.6 1.5 +/-16 stable G>2
> > THS4011 290 7.5 1.2 +/-16
> Indeed those are nice amplifers, but they aren't JFET input
> oamps, and they have an input current of up to 6uA, which
> would be a killer for Jim, I suspect!!
I just found those amps for my VNA IF amplifier and their nice perfs made me
forget this point.
One not so nice point is that the lowest noise amps aren't unity gain
stable, and that seems to be a constant, at least for the TI amplifiers.
Do you have any idea about the reason(s) ?
That's a too bad point for me because this prevent me from using them in the
> - Win
> - Win