Re: Voltage Gain Switch (Design Help)


leo2100@xxxxxxxxx wrote:
bill.sloman@xxxxxxxx ha escrito:

leo2100@xxxxxxxxx wrote:
John Woodgate ha escrito:

In message <1158262688.314408.170030@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
dated Thu, 14 Sep 2006, leo2100@xxxxxxxxx writes

The Gains are: 100, 500, 1000 and 5000
The other circuit has a pre-amplifier with a gain of 100, and the the
other four amplifiers with gains of 1, 5, 10 and 50 respectively. It
uses standard Op-Amps I don`t remember which model, all I know is that
there are no calculation's done about them, except for the
negative-feedback gain.

Doing it that way, no input signal must overload your high-gain
preamplifier. I would reduce that gain to 20, and follow it with four
op-amps as you have already. Then I would finish with an output stage
that has a gain of 5 and will produce +/- 10 V output. That way, your
switch never has to see more than 2 V.

That uses one more op-amp, but they are cheap. Almost all 'cooking'
op-amps will produce +/-10 V up to 200 Hz if you have +/- 15 V supplies
(maybe less) and a load on the output greater than 1 kohm. But avoid
LM324 and LM358, which need subtle treatment that will bother you.


--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
There are benefits from being irrational - just ask the square root of 2.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

The Op-Amps I`m gonna stick with are OP07s, unless there is a major
problem with them. I like the idea that the switch doesn`t have to be
the final stage, somehow I took that idea off my mind. Still I have a
problem using 4051s or 4066 because none of those would allow +-12V
power supplys, so I will have to lower the voltage to power them, I
will probably use some zenner diode.

Use three terminal regulators - much better output impedance than zener
diodes, less wasted current, and you can usually get the voltage you
want, or set it up with an LM317 (or any of its many variants).

The OP-07 is a very nice amplifier, but slow, and doesn't give all that
much gain.

The ua725 and PMI's OP-06 had DC gains in excess of 120dB but both
parts seem to be obsolete.

--
Bill Sloman, Nijmegen

I`m not sure about it, but is an LM317 really neccesary ? I thought
Ken's idea of a voltage divider was great :P

A voltage divider can sometimes do the job, with decent by-pass
capacitors, but mostly a three-terminal regulator does a better job.
The LM317 and LM337 would be an over-kill in most cases - fixed
regulators in TO-92 and surface mount packages are available off the
shelf for all of the voltages thst you are likely to lead.

You could improve on Phil Hobb's 2N7002 switches with SD214 parts - the
"on" resistance isn't as good - at 50R to 75R - but the capacitances
are a lot lower.

http://www.linearsystems.com/datasheets/SD214.pdf#search=%22SD214%22

--
Bill Sloman, Nijmegen

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