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Old 19th August 2012, 11:51 PM   #891
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Quote:
Originally Posted by buzzforb View Post
If drawn with the PSU, dont we see that both fets are passing signals through the PSU to the load, sort of like a circlotron. That is why Kasey got double the gain of what a single fet would yield
buzzforb: I hope that I put your words above in 3 simplified schematics of the attached file. Fig. 1 shows the direction of current flow [Io] through the load from the upper JFET only due to a positive signal at its gate. Fig. 2 shows the direction of current flow [Io] through the same load from the bottom JFET only due to the remaining [inverted] portion of the sine signal at its gate. Fig. 3 shows the integrated current through the load from the contribution of the upper JFET [shaded positive part of sine] and the contribution of the lower JFET [unshaded negative]. Its net value= 2Io. The net voltage across the load Zl is equal to 2Io multiplied by Zl.
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File Type: pdf OTLAmp7A.pdf (42.1 KB, 40 views)

Last edited by Antoinel; 19th August 2012 at 11:57 PM.
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Old 20th August 2012, 12:11 AM   #892
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Quote:
Originally Posted by buzzforb View Post
Kasey197 or others,
Is bias stablity an issue with the simple R divider version?
Not for me ... but both types of fets I tried had a postive tempco (2sk1058 and SS R100s). I presume with vertical fets IRF etc youd have to add a tehrmistor to keep things well managed but never tried it (beyond frying a pair of IRFs in complementary pp)
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Old 20th August 2012, 12:13 AM   #893
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Quote:
Originally Posted by lhquam View Post
I think I have found a simple and very stable bias circuit for the F6.

I wired it up and made measurements. From 500Hz to 20KHz the THD behaves as one would expect. Below 500Hz, the THD increases in a highly erratic manner. See the figures below.

All tests were with 8 ohm load, 1.3A bias and 25V rails.

The frequency sweep is at 1 watt.
The power sweep is at 1kHz.
The spectrum is at 1kHz and 1 watt.

I am temporarily withholding the schematic so that I do not spoil your fun.
Well done !!
How much was the gain of the complete circuit ? Im assuming your tx is set up is 1:1 right ? Edit: Also if/when you get a chance, what was the output impedance you found ?

Last edited by kasey197; 20th August 2012 at 12:34 AM. Reason: added question re output impedance
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Old 20th August 2012, 12:20 AM   #894
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ZM, lhquam and others... I am wondering what i'm missing regarding the DC through the coils issue.

I'm using this approach at the moment and tried measuring the DC current thru the coils but couldn't get anything. Was wondering if i'm oversimplifying things in my head or perhaps the current starts to appear in some cases which I haven't tested for ? Eg maybe if driving the fets >3V with SS R100s in output etc ?

Just want to make sure I understand before I order some nice input txs

Cheers all,
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Old 20th August 2012, 12:29 AM   #895
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Originally Posted by Nelson Pass View Post
Operating Common-Source, they are both voltage and current amplifiers.

Antionel,
I could be totally wrong, but I was considering this if both fets are seeing the same signal.

KAsey,
What is preventing DC through the coils? Doesnt the coil see the DC bias for the fet or is your cap stopping that?
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File Type: pdf F6.pdf (178.1 KB, 74 views)
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Last edited by buzzforb; 20th August 2012 at 12:36 AM.
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Old 20th August 2012, 12:36 AM   #896
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Quote:
Originally Posted by kasey197 View Post
Well done !!
How much was the gain of the complete circuit ? Im assuming your tx is set up is 1:1 right ? Edit: Also if/when you get a chance, what was the output impedance you found ?
The closed-loop gain was slightly less than 6x, using 50 ohms and 10 ohms for the feedback divider. Tx is 1:1, both primaries in parallel. I haven't attempted to measure output impedance.
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Old 20th August 2012, 12:38 AM   #897
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Ilquam,
What values are you using on the input jfet? I have 10Rs with 200R trimmmer across them to trim offset.
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Old 20th August 2012, 12:38 AM   #898
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Originally Posted by lhquam View Post
The closed-loop gain was slightly less than 6x, using 50 ohms and 10 ohms for the feedback divider. Tx is 1:1, both primaries in parallel. I haven't attempted to measure output impedance.
cool thanks!!
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Old 20th August 2012, 12:42 AM   #899
lhquam is offline lhquam  United States
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Quote:
Originally Posted by kasey197 View Post
ZM, lhquam and others... I am wondering what i'm missing regarding the DC through the coils issue.

I'm using this approach at the moment and tried measuring the DC current thru the coils but couldn't get anything. Was wondering if i'm oversimplifying things in my head or perhaps the current starts to appear in some cases which I haven't tested for ? Eg maybe if driving the fets >3V with SS R100s in output etc ?

Just want to make sure I understand before I order some nice input txs

Cheers all,
You shouldn't need those caps.
The main problem with this circuit is that it is sensitive to rail voltages. If the fluctuate, the bias changes. There will be only a couple of microamps through the coils due to the FET gate leakage. Otherwise there is no DC path. My R100s have Vgs=1.33V at 1.3A.

Last edited by lhquam; 20th August 2012 at 12:44 AM.
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Old 20th August 2012, 12:44 AM   #900
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Zener is easiest way to give stable voltage. Noise may be an issue, but how important that is, I don't know.
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