Path: csiph.com!weretis.net!feeder8.news.weretis.net!reader5.news.weretis.net!news.solani.org!.POSTED!not-for-mail From: Gerhard Hoffmann Newsgroups: sci.electronics.design Subject: Re: new polymer caps Date: Fri, 5 Sep 2025 19:23:09 +0200 Message-ID: <109f69t$1663j$1@solani.org> References: <7hijbk1bljon2o3l2u8i7kjdg27ho6e898@4ax.com> <109cksm$14ddl$1@solani.org> <45vlbk1sfbopd3n2spaoiouqsas80ogpab@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Fri, 5 Sep 2025 17:23:09 -0000 (UTC) Injection-Info: solani.org; logging-data="1251443"; mail-complaints-to="abuse@news.solani.org" User-Agent: Mozilla Thunderbird Cancel-Lock: sha1:l1nyeM/MUbwOwk13yW24+B3oxsE= X-User-ID: eJwFwYkBADAEBLCVWs9hHJT9R2iijIs2gUJ0dcvCQgkdR9jWpRCT5MMt7JL5zkx7XKenKHwMdBC/ Content-Language: en-US In-Reply-To: <45vlbk1sfbopd3n2spaoiouqsas80ogpab@4ax.com> Xref: csiph.com sci.electronics.design:735654 Am 05.09.25 um 17:12 schrieb john larkin: > On Thu, 4 Sep 2025 20:13:42 +0200, Gerhard Hoffmann > Did you find that the polymers created noise? They did in my circuit. One comparison may not be statistically significant for the general case, but enough to think about if there should be problems. It seems the circuit is a magnifying glass. 16 CPH3910 in parallel for a *low* noise preamp (315 pV/rtHz), gate at GND, sources fed from a current mirror CCS, cascode, output amplifier in 10 dB steps to +80 dB. There was no 1/f corner but 1/f**2 and that gets out of hand quite fast. 40 dB/decade. Maybe a less ideal CCS would have helped. Anyway, the effect wandered clearly with capacitor type over 3 or 4 decades of noise corner frequency. Just keep it in the back of your mind. Evading the feedback somehow felt like a personal defeat so I took a bit more gain from the cascode and replaced the op amp by a BFS19S follower to drive the low impedance sources. Burns some precious battery power. But it seems to be fast enough and avoids the 1/f noise of the 3 GHz CFA. BFS19 is EOL as Digikey wrote me last week. :-( cheers, Gerhard