Envelope-to: dave@picks.force9.co.uk
Delivery-date: Thu, 26 May 2005 22:17:20 +0100
From: "james moritz" <j.r.moritz@herts.ac.uk>
To: rsgb_lf_group@blacksheep.org
Date: Thu, 26 May 2005 22:17:00 +0100
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Subject: LF: RE: off topics: Inverter 12V/-12V wanted  from DK 8 ND
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Dear Hans-Albrecht, LF group,

You could bypass the problem altogether by using a single-supply op amp,
e.g. TLC271 or almost any other CMOS op-amp (but these are usually limited
to 15V total supply voltage), or LM324 / LM358 etc.

For just a few milliamps, it is very practical to use a diode/capacitor
voltage inverter to get several volts -ve by rectifying a square wave output
from, e.g. a 555 timer running at a suitable frequency. This is a
voltage-doubler type rectifier capacitively coupled to the square wave
source. If it is mains-powered, you can do the same thing using the +ve
supply transformer winding. 

Don't worry about regulating the negative supply; op-amps are very tolerant
of supply voltage variations. But do make sure you filter the negative
supply with an R-C filter.

Cheers, Jim Moritz
73 de M0BMU

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Hans-Albrecht Haffa
Sent: 26 May 2005 15:45
To: rsgb_lf_group@blacksheep.org
Subject: LF: off topics: Inverter 12V/-12V wanted from DK 8 ND

Dear Lowfers,
for a cw-shaping circuit with an op-amp (second order key-klick filter by
Jim Moritz) I need an inverter for the negative supply of the op-amp...