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How to Apply Voltage offset in single supply Opamps
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thepaddiscuile



Joined: Jul 11, 2007
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PostPosted: Tue Jun 12, 2018 11:47 am    Post subject: How to Apply Voltage offset in single supply Opamps
Subject description: Voltage offset single supply how to
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Hi.. Everyone

Im designing a vco based off this classic schmitt/integrator design using Lm324 quad single supply opamps but Ive noticed that the triangle wave sits above ground. Id Like to Bias it to ground by applying an offset but for the life of me I cant find a circuit that does so..
Anybody care to school me or whip out a schematic for single supply offset ?


https://www.google.com/search?biw=1242&bih=574&tbm=isch&sa=1&ei=2MAeW6edM8iF0wLMxJX4BQ&q=lm324+vco&oq=lm324+vco&gs_l=img.3...86794283.86796929.0.86797183.9.8.0.1.1.0.261.797.2j2j1.5.0....0...1c.1.64.img..3.6.807...0j0i67k1j0i10i67k1.0.E8qIN8Mc384#imgrc=Lt1_HxNaijzJpM:

Cheers

Patrick
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JovianPyx



Joined: Nov 20, 2007
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PostPosted: Tue Jun 12, 2018 12:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

The schematic is very blurry, but if you look at the right hand opamp, there is a circle with a line going through a resistor into the + input of the LM324 with a label that looks like V+/2. That should cause the output to be centered around V+/2. You can, however, adjust it by using a pot to deliver the "offset" voltage to the opamp - in other words, it doesn't have to be V+/2. I'd supply the wiper of a pot to that point where the ends are at V+ and ground.
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thepaddiscuile



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PostPosted: Tue Jun 12, 2018 2:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

Seems to work in my simulation.. Very Happy Cheers for that..... Is that vref the trip point for the Schmidt trigger. Having trouble wondering as to why they would purposely offset the tri if they didn't have too.. or is is they try and keep the circuit simple with least amount of components to get a working vco ?
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JovianPyx



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PostPosted: Tue Jun 12, 2018 2:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hard for me to explain, but essentially, I believe the difference between the trip points (plural) of the schmitt trigger are set by the two resistors and the ref voltage moves them both up and down. Using a pot or a trimmer for that allows you to precisely set it. If you just threw two (supposedly) equal size resistors into a voltage divider, you would get close to V+/2, but perhaps not precise enough for your musical application needs. There are also likely small defects in the LM324 that could cause a bit of offset as well.

Glad to hear it worked in sim, now try it for real Cool

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thepaddiscuile



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PostPosted: Tue Jun 12, 2018 3:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

Cheers For the explanation... I will test that tomorrow on the board ;]

while were at it.. I also noticed that the triangle becomes slightly skewed below 100 Hz.. This could be another aforementioned drawback to the lm324.
I don't have a resistor that's 50k coming from the emitter of the Tranny to the inverting input of Op amp A..[ CLOSEST i HAVE ON HAND WAS 47K ]
Wondering if this makes much difference.. Ive also seen schematics based on this oscillator that use a fet instead of the BJT tranny.. Wondering if these improve the symmetry any which way ?
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JovianPyx



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PostPosted: Tue Jun 12, 2018 4:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

thepaddiscuile wrote:
Cheers For the explanation... I will test that tomorrow on the board ;]

while were at it.. I also noticed that the triangle becomes slightly skewed below 100 Hz.. This could be another aforementioned drawback to the lm324.
I don't have a resistor that's 50k coming from the emitter of the Tranny to the inverting input of Op amp A..[ CLOSEST i HAVE ON HAND WAS 47K ]
Wondering if this makes much difference.. Ive also seen schematics based on this oscillator that use a fet instead of the BJT tranny.. Wondering if these improve the symmetry any which way ?


Hmm Confused When I click the link, I get a blurry image, but the the transistor emitter goes directly to ground. Am I looking at the correct image?

As for the FET vs BJT question, I don't really know, but my method would be to breadboard it both ways and choose what works best.

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thepaddiscuile



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PostPosted: Tue Jun 12, 2018 4:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

Sorry I forgot to show a better version.. here it is

https://www.google.com/search?biw=1242&bih=574&tbm=isch&sa=1&ei=2MAeW6edM8iF0wLMxJX4BQ&q=lm324+vco&oq=lm324+vco&gs_l=img.3...86794283.86796929.0.86797183.9.8.0.1.1.0.261.797.2j2j1.5.0....0...1c.1.64.img..3.6.807...0j0i67k1j0i10i67k1.0.E8qIN8Mc384#imgrc=EoZbgNF5oAw5cM:

Also I mentioned the emitter you are correct, I meant to say the collector Confused
I think the symmetry improved when I reduced the size of the timing cap, but obviously I then got less high frequency oscillation.
Also I tacked on a buffer between the cv input and a voltage divider, this helped squeeze a few more khz from the osc.
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JovianPyx



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PostPosted: Tue Jun 12, 2018 4:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ok, that's MUCH better Very Happy

Looking at the schematic, you see the 50K resistor value and also above it, it says "R/2". Note that to the right and above there's a 100K resistor with R above it. What that is saying is that the 50K resistor needs to be 100K / 2. This kind of note means that it is important that this exact ratio is preserved. So yes, the 47K resistor is too small. Those two resistors should be precision resistors (1%) and it will affect the performance of the circuit.

Your buffering should not negatively affect the circuit and would likely improve it.

As for the capacitor, if you make it smaller it should increase the frequency range while lowering it should decrease the range. A musically interesting fact is that if you halve the capacitor value it should double the frequency for a given CV - which is one octave higher.

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thepaddiscuile



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PostPosted: Tue Jun 12, 2018 5:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

Again my apologies.. I meant that I increased the cap value to reduce the timing. with the smaller cap the pots Linearity was dicey heading towards ground, it would shift a few octaves with very little turn left.. Also like I mentioned, I think increasing the cap reduced the skewing of the tri some, but obviously my 47k resistor isn't ideal.
All this was tested quickly just to get a working circuit to solder onto perfboard, I have a test lead coming from the input of the buffer/output of triangle and square to my breadboard for further easy testing.
I will try and make up a 50k resistor just to get me in the ballpark, I don't have 1% so must order a few..

But this has potential, I ran a saw waveshaper using one more opamp and a tranny on the lm324 after you corrected the dc offset and that seems to spit out a saw that mirrors the triangles slopes no to shabby..


saw tri.jpg
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Triangle biased to ground and sawtooth shaper for comparrison
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saw tri.jpg


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JovianPyx



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PostPosted: Tue Jun 12, 2018 8:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

No apology necessary, I sorta figured that you meant it the way I said it Very Happy but just for other readers, I wrote that.

It seems that you're having fun with this and that is the good part Very Happy Smile Cool

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Grumble



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PostPosted: Tue Jun 12, 2018 11:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

When you look at the DATASHEET you can read that the minimum output voltage when powered from 5 volt is (depending on the type of 325) typ. 5 mV up to 100mV, while the maximum output voltage in this case will be typ. 3.5 volt.
So, maybe you could better look for a opamp with a smaller headroom and lower minimum output voltage.

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