Path: news.eternal-september.org!eternal-september.org!feeder3.eternal-september.org!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail
From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: Designs of voltage/current controlled resistance
Date: Wed, 21 May 2025 18:10:24 +0100
Organization: Poppy Records
Lines: 43
Message-ID: <1rcp2tg.6422euemg000N%liz@poppyrecords.invalid.invalid>
References: <d469c760bbcd14a1f148c0e1b29653f7@www.novabbs.com>
X-Trace: individual.net gGd6S112RPmFG2qEBXJVYQ9R8fSZsbz04Kltc/bV1PLROfr4vJ
X-Orig-Path: liz
Cancel-Lock: sha1:86buuMPvkWYGVOK5xCED59LXi2U= sha256:67scqqs0FI75HRT+NyEiw2YzOxCC095jQQUJuLUYk1s=
User-Agent: MacSOUP/2.4.6

RodionGork <rodiongork@github.com> wrote:

> Hi Friends!
> 
> What approaches exist of creating voltage or current-controlled
> resistance?

[...]

> May I be missing some fine schematic with a handful BJT or something
> like this?

There are also thermal devices like tungsten-filament lamps,
iron-hydrogen barretters and thermistors.  These have a relatively slow
reponse but are less likely to distort the waveform.  They have fallen
out of popularity in recent years and may not suit your application
because of the power they consume and the difficulty of obtaining the
particular type you may need.  

They are two-terminal devices, so the heating power has to be fed into
the same terminals as the signal.  It can sometimes be difficult to
prevent interaction between them.

Photoconductive cells have a faster (but slightly strange) response and
can be illuminated by LEDs, tungsten lamps or gas-discharge tubes.
Their linearity is quite good enough for domestic audio purposes.  They
are disliked by environmental campaigners because many of them contain
cadmium (in insignificant quantities).

Another method is to use a fast-acting CMOS switch in series with a
resistor.  By controlling the on/off ratio of the switch, the resistor
can appear to be a higher value.  This is particularly useful for making
resistors in different parts of the circuit track accurately together,
such as in an audio filter.  The disadvantage is the need for a
switching frquency which is much higher than the highest frequency
signal and the necessity for a filter on the output to remove that
switching frequency from the signal.


-- 
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk