voltage to resistance

felix · Post by **felix** » Sun Jun 07, 2009 1:54 pm

So, other than Vactrols, how would one use voltage to alter resistance?

dougcl · Post by **dougcl** » Sun Jun 07, 2009 1:56 pm

Isn't this what transistors do?

felix · Post by **felix** » Sun Jun 07, 2009 2:13 pm

dougcl wrote:Isn't this what transistors do?

Huh...it appears they do! I thought they were only for amplification and switching, but it looks like they could work for my application too.

I want to have some voltage control over these little CMOS oscillators. Typically, their frequency is controlled by a resistor + capacitor network, and to add any voltage control you would use a vactrol in place of the resistor. I'd like to use something cheaper and potentially more precise, and if the transistor works, that would certainly fit the bill.

Thanks Doug!

flight · Post by **flight** » Sun Jun 07, 2009 2:54 pm

Bipolar transistors are really current-control and have a logarithmic transfer ratio (in/out). They are commonly used for voltage control but have some other drawbacks such as the addition of the base current to the output.
For a more precision application, FET transistors (such as the MPF102 JFET) are better. Some are even specified as Voltage-Controlled Resistors (VCR). Like bipolar transistors, FETs require a bit of calculation (or trial and error) to set up the biasing network to get the exact behavior you are looking for, but they work quite well. They have no base current effects and behave much more like a true resistance.

wetterberg · Post by **wetterberg** » Sun Jun 07, 2009 3:28 pm

Hey Flight, I wonder if you, or other brainiac geniuses, could help out in designing like a breadboard layout for this?

A voltage controlled resistor pcb is one of the most useful devices for modding just about any circuit.

- just something simple; an input, a control input and an output... boom. Could probably be laid out more or less based on the datasheet diagrams even?

flight · Post by **flight** » Sun Jun 07, 2009 4:04 pm

Sounds good to me. For now though, here's some links:

http://graffiti.virgin.net/ljmayes.mal/comp/vcr.htm

An excellent Siliconix App Note

LinearSystem's VCR11N datasheet

Vishay-Siliconix VCR2N/4N/7N datasheet

Discussions and Lab projects about VCRs:
http://www.edaboard.com/ftopic201952.html
http://www.nhn.ou.edu/~bumm/ELAB/Labs/lab11_FET_Lab.htm
http://www.electro-tech-online.com/gene ... -jfet.html

Detailed descriptions of FET operation:

http://aries.ucsd.edu/najmabadi/CLASS/E ... ES/FET.pdf
http://www.electronics-tutorials.ws/tra ... ran_5.html

Not FET-related, but damn interesting:
http://www.avtechpulse.com/papers/vres/

Aaand... that oughta be unough for y'all for the nonce!

DGTom · Post by **DGTom** » Sun Jun 07, 2009 4:24 pm

Felix,

Take a look at the 2nd schem. posted here;

http://deathlehem.com/php/viewtopic.php?f=21&t=34

Don't worry about tracking, altho I've not played with the dual transistor set-up much, you can also do it with another CMOS chip, the 4007;

Thanks for all the proper info flight!

Luka · Post by **Luka** » Sun Jun 07, 2009 6:46 pm

you can also use ota's like 3080
and transistors in current sink formation

wetterberg · Post by **wetterberg** » Sun Jun 07, 2009 7:01 pm

@flight: Most of those FETs are obsolete... I found one of them for like 20usd...

@DGtom... I've never seen a schematic like that... ever. What chip is that? Are those the pins of the 4007? I assume you're talking about the CD4007? I'd love to see a "proper" schematic for it...

@Luka I'm sorry, but I at least am going to need a little more than the mention of a methodology for doing this...

Could we n00bify this thread a bit? Cheers much

panda30y · Post by **panda30y** » Sun Jun 07, 2009 7:36 pm

Wow, great thread for us DIY'ers. Thanks for the info everyone.

Luka · Post by **Luka** » Sun Jun 07, 2009 8:05 pm

i heard about current sinks here
http://www.homebuilthardware.com/index. ... 39x-delay/

felix · Post by **felix** » Sun Jun 07, 2009 10:09 pm

Wow, cheers guys! Lots of useful stuff here!

flight · Post by **flight** » Sun Jun 07, 2009 10:12 pm

@ wetterberg:
Aside from the VCR-specific FETs, general-purpose FETs will work as replacements. Like the aforementioned MPF102.

<edited> 'cause it needed it...

flight · Post by **flight** » Mon Jun 08, 2009 10:21 am

I was too tired last night to go into any detail, so here's a bit more:

There are thousands of different transistors, bipolar and FET, and they go in and out of production on a regular basis. Many of them exist because some manufacturer wants a specific set of properties and orders a custom run (this is often due to military projects). Afterwards, the semiconductor company now has the dies and setup so they do runs of the unit to see if there is any demand. If there is, it stays as a product line. If not, it is discontinued.
Other times, a part is discontinued because of advances in manufacturing processes, sometimes re-appearing under a different part number.

Parts that are successful and not patented in time are subsequently cloned by other manufacturers, often under the same part number but also under a different part number that conforms to regional standards - hence the various prefixes such as: 2N, BC, MPF, etc..

So, when using any schematic that is more than a couple years old, you will often come across transistors that are obsolete. You then just need to find the specs for that part and select a replacement unit that has the same or similar. Since transistors usually require a biasing network of some kind, the specs aren't too restrictive. You will just need to play with the bias component values a bit.

DGTom · Post by **DGTom** » Mon Jun 08, 2009 3:52 pm

wetterberg wrote:@DGtom... I've never seen a schematic like that... ever. What chip is that? Are those the pins of the 4007? I assume you're talking about the CD4007? I'd love to see a "proper" schematic for it...

Its just a snippet really, fairly transportable. If you replace the 2N3904 in the synthmonger 'Simple VCO' schem it'll work, differant to the transistor, but, it'll work.

Likewise if you just bolt that onto the front of a Collins-esque 40106 oscillator it will give you VC, I'd stick a diode in front - CD4007

doesn't like negative V's much.

I think there is a way to CV a 40106 with a 4016 switch as well, but I've not had that on the breadboard, my first few modules are starting to be built so I'll have to pick up where I left that soon.

I got this to work w/ the CD4007 as well;

Just wired the pins as per the 4007 datasheet IIRC.

Navs · Post by **Navs** » Wed May 23, 2012 2:59 am

Bumping this thread for help with a specific circuit.

I've breadboarded an all-pass filter which I'd like to VC at audio rates (i.e. vactrols out of the running):

I bought what I thought was a JFET - the BSN10A. Can this also be used as a resistor and, if so, how do I hook it up? Do I need further resistors on the input and output?

I've tried using a CD4049 as per the ETI 447 phaser, but this was noisy. Can I expect better performance from the discrete transistor or is the filter circuit itself just not capable of being VC-d at audio rate?

Help a noob to trannies please!

Thanks, Navs

Refs:
http://www.pa.msu.edu/courses/2007fall/ ... s/P1_3.pdf
http://synthdiy.com/show/?id=1068
http://ecircuitcenter.com/Circuits/JFET ... e_Res1.htm
http://english.electronica-pt.com/db/cr ... =BS&page=9

frijitz · Post by **frijitz** » Wed May 23, 2012 3:15 am

wetterberg wrote:@Luka I'm sorry, but I at least am going to need a little more than the mention of a methodology for doing this...

It's in the data sheet.

Ian

diablojoy · Post by **diablojoy** » Wed May 23, 2012 3:54 am

I've breadboarded an all-pass filter which I'd like to VC at audio rates (i.e. vactrols out of the running):

check the MFOS site look in the section for WSG mods
there's an easy VC jfet cct there you can try

Navs · Post by **Navs** » Wed May 23, 2012 10:52 am

Thanks, diablojoy, I'll take a look.

Any other thoughts on the suitability of the circuit and BSN10A for audio-rate VC?

Navs · Post by **Navs** » Thu May 24, 2012 9:38 am

The Ray Wilson circuit worked, thanks. It's just as noisy as the CMOS trick, tho, so will have to find another alternative.

DGTom · Post by **DGTom** » Thu May 24, 2012 9:58 am

Navs, have you seen Neil Johnsons All Pass?

http://www.milton.arachsys.com/nj71/ind ... bsubmenu=3

I'd bet the 2164 will be alot cleaner & easier than a FET.

In my experiance starting with something that sounds the way you want & adding VC can be a PITA, better to start with something that is VC'able & get it to sound the way you want.

Navs · Post by **Navs** » Thu May 24, 2012 11:15 am

Yes! Found that today after I changed my usual google search to 'voltage controlled all-pass'

Way too much information for me - I'll have to mail you for help!

Dr. Sketch-n-Etch · Post by **Dr. Sketch-n-Etch** » Thu May 24, 2012 11:24 am

I use the 2164 quad exponential VCA almost exclusively as a voltage-control element. It takes a current in (at virtual ground) and attenuates it (with positive voltages) or amplifies it (with negative voltages) depending on the voltage at the control pin. Hence, the ratio between input and output current is determined by a voltage. It is a "voltage-controlled current source" -- in other words, a voltage-controlled resistor.

2164 VCAs must terminate to virtual ground, which means that you must feed each 2164 VCA into the inverting terminal of an opamp with its positive terminal grounded. An opamp wired as an inverter with a resistor in the feedback loop, but with no input resistor, is a "current-to-voltage" converter, and this is what typically follows a 2164 VCA. However, you can also put a capacitor in the feedback loop of the opamp, and now you have an integrator. 2164 + resettable integrator = VCO. 2164 + integrator with feedback to the 2164 input = 6dB LPF. Neither of these require exponential converters, since the 2164 does that already.

lysander · Post by **lysander** » Thu May 24, 2012 11:32 am

Dr. Sketch-n-Etch wrote:I use the 2164 quad exponential VCA almost exclusively as a voltage-control element. It takes a current in (at virtual ground) and attenuates it (with positive voltages) or amplifies it (with negative voltages) depending on the voltage at the control pin. Hence, the ratio between input and output current is determined by a voltage. It is a "voltage-controlled current source" -- in other words, a voltage-controlled resistor.

2164 VCAs must terminate to virtual ground, which means that you must feed each 2164 VCA into the inverting terminal of an opamp with its positive terminal grounded. An opamp wired as an inverter with a resistor in the feedback loop, but with no input resistor, is a "current-to-voltage" converter, and this is what typically follows a 2164 VCA. However, you can also put a capacitor in the feedback loop of the opamp, and now you have an integrator. 2164 + resettable integrator = VCO. 2164 + integrator with feedback to the 2164 input = 6dB LPF. Neither of these require exponential converters, since the 2164 does that already.

Great tips , thanks !

Navs · Post by **Navs** » Tue May 29, 2012 10:43 am

Thanks, Dave! Now, where do I get 2164s in Europe?

MOD WIGGLER

MOD WIGGLER

voltage to resistance

voltage to resistance