I have been messing about with an audio spectrum analyser, comparing the simplest possible JFET amplifier stage (FET, unbypassed source resistor, drain connected to the power supply via a load resistor, FET always biassed to about half the supply voltage, measurements always made at the same AC output voltage). Now I noticed that when it comes to THD, P-channel devices always seem to be better, i.e. more linear. The difference is very pronounced, typically at least 10 dB and it is always the 2nd harmonic that is better (the 3rd is usually pretty much the same).
Am I overlooking something here or is this real?
Am I overlooking something here or is this real?
I doubt it. Make sure that the OUTPUT of the device under test is the same with N and P. P channel devices usually have lower Gm, SO, output is lower with a given input. This makes distortion appear lower.
More details
First of all, yes, I do use a different polarity for the P-channel devices. These are all proper amplifying stages, with gain. Secondly, as I mentioned all these are measured at the same OUTPUT voltages.
Here are some actual measurement data. All double checked. The output voltage was always set to 4 V pp at 1 kHz.
2N5460 (p-channel). Id = 1 mA.
2nd harm = -61 dB ; 3rd harm = -71 dB below fundamantal
BF245A (n-channel). Again, Id = 1 mA
2nd harm = -50 dB ; 3rd harm = -71 dB below fundamantal
2SK170-BL (n-channel), Id = 3 mA
2nd harm = -48 dB ; 3rd = -72 dB
There is more but these are representative.
First of all, yes, I do use a different polarity for the P-channel devices. These are all proper amplifying stages, with gain. Secondly, as I mentioned all these are measured at the same OUTPUT voltages.
Here are some actual measurement data. All double checked. The output voltage was always set to 4 V pp at 1 kHz.
2N5460 (p-channel). Id = 1 mA.
2nd harm = -61 dB ; 3rd harm = -71 dB below fundamantal
BF245A (n-channel). Again, Id = 1 mA
2nd harm = -50 dB ; 3rd harm = -71 dB below fundamantal
2SK170-BL (n-channel), Id = 3 mA
2nd harm = -48 dB ; 3rd = -72 dB
There is more but these are representative.
As Mr. Curl points out, how about the gm, must be different for these devices? Do you have the same gain for all JFET's?
I propose to a little change in a measuring: no use a source resistor, and regulate a drain resistor to at about halve of supply voltage, and shift supply voltage up to 30V. This test will be much more strict and a jfet will be forking at idssmax. Besides try analyse a complementar pairs for example j103 - k246. The Comparsion of the other types jfets might be useless. Anyway your observation is interesting!
Comparisons with no source resistor
Thanks, Padamiecki, I am going to try that. You are right: I am not comparing like with like because of the varying amount of negative feedback the source resistor introduces.
Thanks, Padamiecki, I am going to try that. You are right: I am not comparing like with like because of the varying amount of negative feedback the source resistor introduces.
Some measurement results
Here's some results at Idss, with the power supply voltage set to 30V and the drain voltage set to half the supply voltage. The input AC voltage was always adjusted so that the output was 4V pp at 1 kHz.
Device.................. 2nd / 3rd harm
2SK170................ -39 dB -70 dB
BF245B (1)........... -37 dB -86 dB
BF245B (2)........... -37 dB -82 dB
BF245A................ -40 dB -73 dB
BF244A................ -38 dB -82 dB
2N5460................ -37 dB -77 dB
One interesting thing: the overall THD hardly changes, from 1.1 to 1.4 % but the spectral distribution varies a lot, especially the 3rd harmonic. Considering the usual preference for 2nd harmonic distortion, this may be of some significance.
Here's some results at Idss, with the power supply voltage set to 30V and the drain voltage set to half the supply voltage. The input AC voltage was always adjusted so that the output was 4V pp at 1 kHz.
Device.................. 2nd / 3rd harm
2SK170................ -39 dB -70 dB
BF245B (1)........... -37 dB -86 dB
BF245B (2)........... -37 dB -82 dB
BF245A................ -40 dB -73 dB
BF244A................ -38 dB -82 dB
2N5460................ -37 dB -77 dB
One interesting thing: the overall THD hardly changes, from 1.1 to 1.4 % but the spectral distribution varies a lot, especially the 3rd harmonic. Considering the usual preference for 2nd harmonic distortion, this may be of some significance.
Re: Some measurement results
... and do not be afraid of the 2nd harmonic!
Up to 10% it may be not heard!
3nd harmonic should be as low as possible.
Note that ±4V~ is not much,
maybe try ±10V~?
... and do not be afraid of the 2nd harmonic!
Up to 10% it may be not heard!
3nd harmonic should be as low as possible.
Note that ±4V~ is not much,
maybe try ±10V~?
Re: Re: Some measurement results
try 10 KHz + 11 KHz two tone sine wave test signal with your "inaudible" 10% 2nd harmonic and tell us that the 1KHz diff freq "...it may be not heard!" - i don't think so...
as for the distortion tests I think the knowing voltage gain of each fet as biased is still useful as well as the Idss of the devices you are testing given the 5:1 variation in this parameter for the same part # for some device families
padamiecki said:
try 10 KHz + 11 KHz two tone sine wave test signal with your "inaudible" 10% 2nd harmonic and tell us that the 1KHz diff freq "...it may be not heard!" - i don't think so...
as for the distortion tests I think the knowing voltage gain of each fet as biased is still useful as well as the Idss of the devices you are testing given the 5:1 variation in this parameter for the same part # for some device families
Re: Re: Re: Some measurement results
Apples and oranges. That 1kHz intermod product isn't the 2nd harmonic of either the 10kHz or 11kHz tones.
se
jcx said:
Apples and oranges. That 1kHz intermod product isn't the 2nd harmonic of either the 10kHz or 11kHz tones.
se
Ahh, but it's caused by the exact same mechanism, the squared term in the Taylor series expansion of Id vs Vgs.
Yeah, but.......
To us crusty old telecom nerds:
It is called "2A-B distortion", and therfore a third order product.
(Intermodulation to the rest of the forum.)
Jocko
To us crusty old telecom nerds:
It is called "2A-B distortion", and therfore a third order product.
(Intermodulation to the rest of the forum.)
Jocko
andy_c said:
That's fine. But it doesn't change the fact that jcx's rebuttal was completely out of context with padamiecki's original claim about 2nd harmonic distortion.
se
Steve Eddy said:
No, I don't really think so. JCX's rebuttal is quite reasonable because the same distortion mechanism producing this "supposedly" benign distortion on a single sine wave will create all kinds of additional problems with something as simple as a two tone IM test or, heaven forbid, real music.
As one friend is want to say, "One man's "air" is another man's distortion". That was said in the context of harmonic distortion in tweeters, but it's basically the same problem.
Regards,
Jon
Re: Yeah, but.......
Huh? 2A-B would be 9kHz. jcx said 1kHz, which is a second order product.
se
Jocko Homo said:
Huh? 2A-B would be 9kHz. jcx said 1kHz, which is a second order product.
se
JonMarsh said:
So? Padamiecki never said otherwise. He made a specific reference to second harmonic. Whether or not there may be other products such as intermod products is irrelevant to what he actually said. jcx replied as if padamiecki had something completely different than what he actually said.
Either that or jcx seems to think that a second order intermod product is the same thing as second harmonic, which its not.
se
JC wrote > Make sure that the OUTPUT of the device under test is the same with N and P. P channel devices usually have lower Gm, SO, output is lower with a given input. This makes distortion appear lower.
May I add that the p-channel devices also have higher pinch-off voltage? The distortion curves should be normalized on delta I drain / delta V gate-source.
lazthegaz wrote > varies a lot, especially the 3rd harmonic
Please don't forget that there exists the bulk silicon resistance/contact resistance, which serves as a source degeneration resistance thus the third order harmonic is produced as intermodulation between fundamental and the second harmonic. See Baxandall papers, which were mentioned several times.
Please stop reinvent the wheel!
Please find paper from the last 60th from 'Electronics' magazine regarding FET distortion or take the pen and write very simple formulas.
May I add that the p-channel devices also have higher pinch-off voltage? The distortion curves should be normalized on delta I drain / delta V gate-source.
lazthegaz wrote > varies a lot, especially the 3rd harmonic
Please don't forget that there exists the bulk silicon resistance/contact resistance, which serves as a source degeneration resistance thus the third order harmonic is produced as intermodulation between fundamental and the second harmonic. See Baxandall papers, which were mentioned several times.
Please stop reinvent the wheel!
Please find paper from the last 60th from 'Electronics' magazine regarding FET distortion or take the pen and write very simple formulas.
- Status
- Not open for further replies.