From ???@0x00000DD3 Tue Sep 18 13:19:12 2001 Path: pitt.edu!newsflash.concordia.ca!sunqbc.risq.qc.ca!news.maxwell.syr.edu!news.shore.net!not-for-mail Message-ID: <3BA7735C.E1D7A9BE@rowland.org> From: Winfield Hill Organization: Rowland Institute for Science X-Mailer: Mozilla 4.7 [en] (WinNT; U) X-Accept-Language: en MIME-Version: 1.0 Newsgroups: rec.audio.tech,sci.electronics.design Subject: Re: Importance of impedance matching for Jfet op-amps? References: <9o4bvt$ov6$1@usenet.Stanford.EDU> <9o5tlr$k35$04$1@news.t-online.com> <3BA68832.928C18E3@rowland.org> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 54 Date: Tue, 18 Sep 2001 12:16:28 -0400 NNTP-Posting-Host: 192.131.102.104 X-Complaints-To: abuse@shore.net X-Trace: news.shore.net 1000830120 192.131.102.104 (Tue, 18 Sep 2001 12:22:00 EDT) NNTP-Posting-Date: Tue, 18 Sep 2001 12:22:00 EDT Xref: pitt.edu rec.audio.tech:151561 sci.electronics.design:258309 Status: N Walt Jung wrote: > > Winfield Hill wrote: > >> Circuit solutions can be easily implemented that maintain >> a constant Vdg for the JFET, thereby completely eliminating >> the problem. > > It could be that doing a cascode and/or bootstrap of a > conventional IC JFET diff-pair input could completely *miss* > this distortion mechanism. The offending culprit is the JFET > gate's junction capacitance *to the substrate*. OK, I got it! Leaving the issue aside of whether an audio designer would consider using the AD549, doesn't its unique "bottom-gate" / "top gate" bootstrap design over-ride the input JFET's substrate capacitance? The common-mode capacitance is spec'd at just 0.8pF, how much of that do you think is voltage dependent? > To comment further on bootstrapping, I did write about it, in > ADI AN232, "Bootstrapped IC Substrate Lowers Distortion in JFET Op > Amps". This note shows a method of dynamically driving the -Vs rail > of an AD744 op amp, to negate the effects of the delta-C/V (Fig. 1 > of this note). > > A summary quote from this note: ".. It should be noted that > the general principles of the non-linear C/V input characteristic > distortion mechanism applies to virtually all JFET input IC op amps > available today, independent of their source. Bipolar input op amps > built on junction isolated processes can also be subject to the > phenomenon, and may also benefit from bootstrapping. However they > are not as likely to be used with large source impedances where the > distortion magnitudes become a serious problem, as is true in the > case of JFET op amps." Walt, that's a great App note! Surely that's a better solution, http://www.analog.com/techsupt/application_notes/AN232.pdf If a bootstrapped substrate isn't practical, using low-capacitance opamps can help, e.g. the OPA655 with only 1pF of common-mode cap, compared to 5.5pF for the AD744, but low-capacitance amplifiers usually suffer from more e_n noise. But not in this case, the OPA655 has only 8nV at 1kHz compared to 18nV for the AD744. Amazing, how do they do that? The AD549, which also has 1pF of input capacitance, has 35nV of input noise. I note the OPA655 is a +/-5V part, no doubt made with low-voltage JFETs. Thanks, - Win Winfield Hill Rowland Institute for Science 100 Edwin Land Blvd Cambridge, MA 02142