Re: Re: [sdiy] The sound of various Op-Amps

[Roman]

bias current is one thing - on 10k input resistance
500nA gives 5mV, although it can be trimmed.
But high bias current opams usually have high input equivalent
current noise. So if 5532 has at 30Hz 8nV/sqrt(Hz) voltage
noise density, and 2.7pA/sqrt(Hz) current noise, one can calculate that when input resistance rises above 8nV/2.7pA=3k, current noise is contributing more noise to the circuit than voltage noise.
So using input's higher than 3k means that its performance is not fully used, and at some point, probably with not much more resistance, it may become more noisy than 741.
For super lowish noise opamps, thermal noise of the resistors is also important, and some applications require input impedance in range of several hundred ohms to keep the noise down.

Roman

---- Wiadomość Oryginalna ----
Od: Andre Majorel <amajorel at teaser.fr>
Do: synth-diy at dropmix.xs4all.nl
Data: Mon, 22 Sep 2003 10:28:07 +0200
Temat: Re: [sdiy] The sound of various Op-Amps

>
<br />
>Thanks Harry, that's interesting. I understand the DC offset stuff
<br />
>but what makes you say it's only good for low input impedance ? Is
<br />
>it the high biasing current ? (500 nA for the 5532 vs. 65 pA for
<br />
>the TL072)
<br />
>