[sdiy] Ofsetting LFO output

[Steve Lenham]

If you build the circuit exactly as shown, the offset applied will be more 
like 3.9V than 5V.
Reason? The opamp inverting input is fixed at ground potential, so R5 is 
effectively in parallel with R3 and upsets the voltage divider ratio.

To fix this you can either make R3 and R4 very small compared to R5 while 
keeping the ratio the same (making the R5 loading negligible) or, better, 
take this loading effect into account when calculating the resistor values.

In the original circuit, the calculation is easy: we want a new value for R3 
which, when put in parallel with the 10K of R5, gives a combined value of 
5K. Changing R3 to 10K would achieve this.

There is an even easier way, though: dump R3, R4 and R5 entirely and simply 
connect a 24K resistor from -12V to the inverting opamp input. The -12V sees 
an inverting amplifier with a gain of -10K/24K = -0.4, and -12V * -0.4 = 
+5V. The input signal is then added to this offset as before using R1.

Hope this helps,

Steve L.

----- Original Message ----- 
From: "Seb Francis" <seb at burnit.co.uk>
To: "Mikael Mørup" <Mikael at mikmo.dk>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, June 01, 2005 1:25 PM
Subject: Re: [sdiy] Ofsetting LFO output


something like the attached circuit should do it ..

it's inverting, so -5V in will give +10V out and +5V in will give 0V
out, but this shouldn't matter for an LFO.

R3+R4 are not 'real' resistor values, but anything in the same ratio
will be fine .. even better would be to add a trimmer in series with one
of these to get exactly the right offset

seb




Mikael Mørup wrote:

>Hi list
>
>I have an LFO that puts out a -5V to +5V peak to peak
>squarewave. For clocking another module i need a 0V to 10V
>squarewave. is it possible to make a small circuit that
>would "ofset" or whatever it's called, the LFO output ?
>
>My modules are running on +-12V.
>
>In my simple mind i am thinking something like an Opamp with
>and ofset, but i don't know excatly how to design such a
>thing.
>
>Any suggestions?
>
>A little schematic would be very nice :-)
>
>Thanks
>
>Mikael
>
>
>
>



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