JFet- Distorted sine wave at the output. Why?

Audioguru again

Joined Oct 21, 2019
4,541
An attenuator is a series input resistor driving a resistor to ground. Since you do not have a series input resistor then you can reduce the input signal level by overloading the signal source with a low value input resistor to ground but it is not recommended.
 

Thread Starter

DaniKowa

Joined Sep 23, 2020
175
Just change the amplitude of the sinewave generator
With a sine wave generator is certainly possible but what if the source signal is fixed or can't be lower?
An attenuator is a series input resistor driving a resistor to ground. Since you do not have a series input resistor then you can reduce the input signal level by overloading the signal source with a low value input resistor to ground but it is not recommended.
My preference is do not use in serie resistor due to noise generated for the component if i'm not mistaken. I'm right?
 

Audioguru again

Joined Oct 21, 2019
4,541
If you do not use a series resistor in an attenuator then the signal level will not be attenuated.

Do you know about adding an unbypassed source resistor to produce some negative feedback that reduces the level and reduces distortion? It is shown in posts #6 and #10.
 

Papabravo

Joined Feb 24, 2006
18,159
You could also put the 3 pole Butterworth Low Pass filter on the input side which would attenuate the input signal and knock down the 3rd harmonic content the amplifier is so eager to pump up. You could also continue on your present path and you might discover an alternative.
 

Bordodynov

Joined May 20, 2015
2,988
Guys. You made me laugh! You got so carried away that you forgot that the amplifier is supposed to amplify! Your output is less than the input of the amplifier.
 

Papabravo

Joined Feb 24, 2006
18,159
You could also put the 3 pole Butterworth Low Pass filter on the input side which would attenuate the input signal and knock down the 3rd harmonic content the amplifier is so eager to pump up.
Guys. You made me laugh! You got so carried away that you forgot that the amplifier is supposed to amplify! Your output is less than the input of the amplifier.
I do not disagree with you, but sometimes it is a bad idea to question the original motivation or intent behind a circuit or a design. Doing that can be more trouble than it is worth. It can lead to getting banned and who wants that to happen? It is much easier to lead a horse to water and ask if he wants to drink.
 

Thread Starter

DaniKowa

Joined Sep 23, 2020
175
I apologize if sometimes silly or repetitive questions are asked. I have recently started using this type of software which is complementary to studies. If I have asked any questions you do not like, I ask to be notified. For my part, I don't think there is any. Regarding the scheme proposed by @Bordodynov in post # 19 seems like a good solution, but I confirm that as already said, the output circuit instead of having a gain, has an attenuation. I have modified the circuit for modest gain, and it seems to work fine. The only problem is the measurement of the signal on the base of the collector, which is modestly distorted. If I increase the value of R3 the signal normalizes, but I lose the gain. This thing, I could not correct it. I have attached a new schematic and relative model of the transistor if necessary. Thanks.

a.JPG
 

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Audioguru again

Joined Oct 21, 2019
4,541
Your new circuit wrongly has the Jfet gate with a floating DC voltage. Usually the gate has a single resistor to ground so that it has a DC voltage of 0V.
Also you wrongly have R1 and R2 biasing the signal source at a positive voltage that is impossible and not wanted.
 

Thread Starter

DaniKowa

Joined Sep 23, 2020
175
Yes I was wrong and I corrected. I must have some other problem because the scheme I posted by myself works anyway. Also now the scheme posted at post 19 works correctly. I am using a very old PC which has virtualization limitations. Do you know if this can cause problems with this type of software? These things are confusing me.
 

Bordodynov

Joined May 20, 2015
2,988
Your new circuit wrongly has the Jfet gate with a floating DC voltage. Usually the gate has a single resistor to ground so that it has a DC voltage of 0V.
Also you wrongly have R1 and R2 biasing the signal source at a positive voltage that is impossible and not wanted.
I don't agree with you. Yes, one resistor is usually used. But if you use a voltage divider, you get a more stable circuit. There is less influence of the cutoff voltage and the initial drain current on the transistor mode. And the higher the supply voltage, the more you can increase the gate voltage. If you want I can show you this with examples.
 

Audioguru again

Joined Oct 21, 2019
4,541
I see that at the very high frequency of 10MHz the Jfet needs to have a fairly high current then it must have its gate biased at a positive voltage since its source voltage is fairly high.
I also see that the Jfet has negative gain but the transistor at the output has fairly high AC and DC gain that adds plenty of AC and DC negative feedback for increased stability of the operating point.
 

Thread Starter

DaniKowa

Joined Sep 23, 2020
175
I see that at the very high frequency of 10MHz the Jfet needs to have a fairly high current, then it must have its gate biased at a positive voltage since its source voltage is fairly high.
Could it be due to the characteristics of the component? I have seen another does not have this problem or haves less. I've seen another Jfet newer has different behavior at the similar or same characteristic.
 

Audioguru again

Joined Oct 21, 2019
4,541
Of course each Jfet part number has its own range of specs. Some need much more or much less Vgs than others to perform the same.
Digikey shows that the MPF102 is obsolete and is not available.
 
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