[sdiy] Ofsetting LFO output

[Michael Ruberto]

wouldn't it be simpler to just use a single transistor as a switch? clamp 
the negative peak off of it's base with a diode. I used something like this 
once to convert the sync out from a Juno 6 so it would work with the gate in 
on the Realistic MG1. it used 3 parts - 1 transistor, 1 resistor, 1 diode. 
worked great too. another good idea is to use the 555 timer in 1 shot mode 
if you need a specific duty cycle for the clock input. plenty of 555 
schematics on the web too...

M.A. Ruberto

>From: Seb Francis <seb at burnit.co.uk>
>To: Steve Lenham <lenham at clara.co.uk>
>CC: synth-diy at dropmix.xs4all.nl
>Subject: Re: [sdiy] Ofsetting LFO output
>Date: Wed, 01 Jun 2005 15:21:15 +0100
>
>I'm glad someone has a clearer head on than me today ;)
>
>Seb
>
>
>Steve Lenham wrote:
>
>>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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