Voltage controlled oscillator with a sine wave output.

LvW

Joined Jun 13, 2013
1,279
indeed - they are not voltage controllable
Wrong! Of course, they are!
However, you should discriminate between (1) ability to tune the frequency with or without influence on the oscillation condition and (2) single-element control yes/no.
 

ci139

Joined Jul 11, 2016
1,696
it was a sarcastic reply to analog kid - (i don't like to repeat myself) i won't cange my statement about most sine wave oscillators being not entirely perfect ← this is what i have encountered so far and no matter what you say it does not change that experience.
 

LvW

Joined Jun 13, 2013
1,279
it was a sarcastic reply to analog kid - (i don't like to repeat myself) i won't cange my statement about most sine wave oscillators being not entirely perfect ← this is what i have encountered so far and no matter what you say it does not change that experience.
Ci 139......which circuit is "entirely pefect"? More than that, in electronics, no formula is "entirely" correct...we always have to face simplifications and neglections. And, of course, there is no linear oscillator which produces "clear" sinusoidal waves.
In this context, I like the following sentence:
When an oscillator circuit should be able to produce a high-quality sine wave it must be as linear as possible....and for this purpose it must contain a certain amount of nonlinearity !"
 

ci139

Joined Jul 11, 2016
1,696
i guess i missleaded you (all) - what i kept in mind is the circuit's "nature"/ability to do something "simultaneously" . . .
and that in the reasonably wide supply range and also with reasonably wide biasing elements' tolerances . . . and with not over-complex circuit layout = reasonable component count . . . ← also , with all of this , staying at a target frequency

another way to express ↑this↑ is : "At lower frequencies many sine wave generators do not lock into the Sine by default"

i rejected Colpitt's and other usually HF range ones from the comparison (at HF everything tends to become a sine - a least energy "state")

e.g. - - you probably can make/tune any "Sine Wave" osc. to produce "distortionless" output -- but it takes it's "price" - - e.g.² - takes an opamp with balanced output shoulders , a stable supply or complex frequency stabilization , etc.

options (extra to ring oscillator) -- to illustrate pos. feedback and a "free-wheeling" resonator
http://www.ee.bgu.ac.il/~paperno/Positive_Feedback_Oscillators.pdf
http://www.kennethkuhn.com/students/ee431/oscillators_sine_wave.pdf
 
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Thread Starter

alexmd

Joined Dec 5, 2019
16
I used the GIC-resonator, but I got stuck again on how to use the JFET as VCR, when I change the input voltage the resistance of the JFET doesn't really change, and I cannot find the maximum input voltage(Vp). If someone could help.

PS: I am a first-year student, which got a project task to make this, but I have hardly any knowledge about these things.
 

Papabravo

Joined Feb 24, 2006
16,473
I used the GIC-resonator, but I got stuck again on how to use the JFET as VCR, when I change the input voltage the resistance of the JFET doesn't really change, and I cannot find the maximum input voltage(Vp). If someone could help.

PS: I am a first-year student, which got a project task to make this, but I have hardly any knowledge about these things.
You're going to have to go back and learn the fundamentals. How can you do a complicated design if you don't know squat!!?
 

Thread Starter

alexmd

Joined Dec 5, 2019
16
You're going to have to go back and learn the fundamentals. How can you do a complicated design if you don't know squat!!?
I just don't know how to operate with a JFET and I can't find the information I need in the internet, so I decided to ask here.
 

Papabravo

Joined Feb 24, 2006
16,473
I just don't know how to operate with a JFET and I can't find the information I need in the internet, so I decided to ask here.
You said and I quote:

I am a first-year student, which got a project task to make this, but I have hardly any knowledge about these things.

The meaning of "hardly any" is functionally equivalent to "none". So you have to go back and learn what you should have learned before you got to this project. The internet is probably the very last place you should go to learn basic information unless you know where to look.

BTW this site has several pages of articles on the JFET
 
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LvW

Joined Jun 13, 2013
1,279
I just don't know how to operate with a JFET and I can't find the information I need in the internet, so I decided to ask here.
Of course, we will help you in case of a special problem - however, here we cannot teach you how to use a FET as a variable resistance. You cannot find the information you need in the internet?? I must say - I cannot believe you.
I have used the key wording "FET as variable resistor" - and I got plenty of information....
 

Papabravo

Joined Feb 24, 2006
16,473
How about showing us a schematic of your attempt to control the resistance by varying the gate voltage. Is it possible that you are applying a voltage to the Gate that is greater than the voltage on the Source. That would explain why you can't get the resistance to change. Oh, by the way you know that the change will be non-linear if you manage to do it correctly.
 

Thread Starter

alexmd

Joined Dec 5, 2019
16
I think that my Vds voltage is pretty high(-8,36V) and that's why it doesn't work. As I understood i need voltage lower than 100mV.
 

OBW0549

Joined Mar 2, 2015
3,566
I just don't know how to operate with a JFET and I can't find the information I need in the internet, so I decided to ask here.
The attached article might help; it discusses the design of a Wien Bridge oscillator with sine wave output, tuned by a pair of JFET voltage-controlled resistors. Obviously, not a project for beginners...
 

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Thread Starter

alexmd

Joined Dec 5, 2019
16
How about showing us a schematic of your attempt to control the resistance by varying the gate voltage. Is it possible that you are applying a voltage to the Gate that is greater than the voltage on the Source. That would explain why you can't get the resistance to change. Oh, by the way you know that the change will be non-linear if you manage to do it correctly.
This is how my system looks like
 

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LvW

Joined Jun 13, 2013
1,279
I think that my Vds voltage is pretty high(-8,36V) and that's why it doesn't work. As I understood i need voltage lower than 100mV.
Yes - that`s what I have told you already in my post#14. Use instead a series combination with a fixed resistor.
 

DickCappels

Joined Aug 21, 2008
7,545
In addition to OBW0549's excellent article, here is the Siliconix (now Vishay) application note that explains the use of JFETs as voltage controlled resistors.
 

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Papabravo

Joined Feb 24, 2006
16,473
One of the things that is different about a JFET is that with Vgs = 0, the channel is open and there is a drain current defined in some datsheets as Idss. This means they are normally on with no bias voltage on the gate. This is really not the stuff for 1st year students to grapple with. Something does not ring true here.
 

Papabravo

Joined Feb 24, 2006
16,473
The attached article might help; it discusses the design of a Wien Bridge oscillator with sine wave output, tuned by a pair of JFET voltage-controlled resistors. Obviously, not a project for beginners...
I noticed the Manhattan style dead bug construction of the VCO and can assure you this is way beyond the capabilities of a 1st year. The use of a 741 in the TS schematic is a dead giveaway that something is wrong because 9kHz is the top end of the frequency range for that part as @Audioguru has often observed.
 

DickCappels

Joined Aug 21, 2008
7,545
On the other hand, grab some JFETs and start trying things with them. I did when a student and was able to design an pretty good sine/cosine generator using them in sample-and-hold circuits. Saved my employer a bundle on sin/cos pots for a raster rotation application back in the days of vacuum imaging tubes. Don't be discouraged.
 

LvW

Joined Jun 13, 2013
1,279
This is how my system looks like
Note that a FET is not the only solution for your task. You also can use an OTA (integrated circuit) or a light-dependent resistor (LDR) instead. It is always good to know the alternatives...
 
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