[sdiy] variable width pulse wave for JEFT sample and hold

Thu Jan 8 23:12:31 CET 2009

Dan,

Level shifting with inverting opamps is a little bit tricky.  It is not as
intuitive as one would hope.  For example, having a +5V reference voltage at
the (+) input terminal does not necessarily shift the output by +5V!

The best way to figure this out is actually to analyze the opamp circuit.
Draw the opamp, the input resistor and the feedback resistor, both taken as
R.  Apply the input voltage V_in to the input resistor.  Now, assume a
reference voltage V_ref at the input terminals.  What is the output voltage
at a given input voltage?

Well, the current through the input resistor is I = (V_in - V_ref)/R.  As no
current flows through the input terminals, the current through the feedback
resistor is also I = (V_in - V_ref)/R = (V_ref - V_out)/R.  Rearranging,
V_out = 2*V_ref - V_in.  Hence, to get a +5V offset, V_ref must be +2.5V,
and the voltage divider resistors must have the ratio 5:1 for a +15V rail
voltage, or 19:5 for a +12V rail voltage.

If you have a summing amp with two inputs, each with R-valued input
resistors, then things are different.  Assuming both are feeding V_in, then
the net current through both inputs (and through the feedback resistor) is I
= 2*(V_in - V_ref)/R = (V_ref - V_out)/R.  Hence, in this case, V_out =
3*V_ref - 2*V_in.  Now, 2*V_in is what you want from the summation of the
two inputs.  However, 3*V_ref is the offset.  Hence, to get an output offset
of +5V, the reference voltage must be +1.67V, and now the voltage divider
resistors must have the ratio 8:1 for a +15V rail voltage, or 31:5 for a
+12V rail voltage.

Clear as mud?  This stymied me too, until I realized that this is one of
those situations in electronics where you really must do the maths!

Cheers,
Dave

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C.  V6T 1Z4
Canada

Tel 1-604-822-3679
Fax 1-604-822-3619

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-----Original Message-----
From: Dan Snazelle [mailto:subjectivity at hotmail.com] 
Sent: Thursday, January 08, 2009 11:26 AM
To: dixon at interchange.ubc.ca; synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] variable width pulse wave for JEFT sample and hold

This statement brings up an issue i still am trying to learn..
how to level shift signals and voltages
 (or voltage shift or offset) 

it seems you are saying to do something like this:

input the square wave into a 102 cap with a 10k resistor to grnd. then the
level shift: plug the signal into the inverting
input of an opamp, which on the non inverting input has a voltage divider (
2 to 1 would that mean a 100k to + and 50k to gnd?

i would LOVE to be able to figure out this SHIFTING thing.

does anyone know of a chapter in any of the common opamp books that talks
about it? is it technically called voltage shifting? or voltage offset?

thanks

I keep on learning new junk everyday
> Why not just use an RC high-pass filter with a time constant of about 10
us
> to generate spikes from the square (0.001 uF and 10 k will do nicely), and
> then shift the pulses with a unity-gain inverting opamp with a voltage 2:1
> divider off the positive input between -15V and 0V so that the output DC
> rides at about -5V and the spikes go up to 0V to trigger the JFET (which,
I
> presume, is npn and therefore is full off at about -3.5V and full on at
> about 0V)?
> 
> 
> David G. Dixon
> Professor
> Department of Materials Engineering
> University of British Columbia
> 309-6350 Stores Road
> Vancouver, B.C.  V6T 1Z4
> Canada
>  
> Tel 1-604-822-3679
> Fax 1-604-822-3619
>  
> "PERFECTA FINGAMUS SERVIAT NATURA"
> 
> The information in this email and in any attachments is confidential and
> intended solely for the attention and use of the named addressee(s).  It
> must not be disclosed to any person without the writer's authority.  If
you
> are not the intended recipient, or a person responsible for delivering it
to
> the intended recipient, you are not authorized to and must not disclose,
> copy, distribute, or retain this message or any part of it.
> 
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Dan Snazelle
> Sent: Tuesday, January 06, 2009 9:14 AM
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] variable width pulse wave for JEFT sample and hold
> 
> 
> i am trying to find a way to get a variable pulse wave from a square so
that
> i can successfully clock a sample and hold built around a JFET (pn4391)
> 
> i am having no luck with a regular square wave.
> 
> thanks
> 
> 
> 
> 
> 
> 
> --------------------------------------------
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