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Old 3rd December 2016, 05:13 PM   #21
Nelson Pass is offline Nelson Pass  United States
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The SiC depletion mode N channel JFETs have reverse temp drift as compared
to enhancement mode P channel Mosfets, so they tend to cancel.

Nicely, in my example.

Why would it have more noise?
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Old 3rd December 2016, 05:27 PM   #22
EUVL is offline EUVL  Europe
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MOSFETs not noisier than JFETs ?


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Old 3rd December 2016, 06:41 PM   #23
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I wouldn't worry for that amount of noise ,even having gargantuan compression tweet
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Old 3rd December 2016, 09:58 PM   #24
xrk971 is offline xrk971  United States
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Can we use an LU1014 for the N channel depletion mode?
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Old 3rd December 2016, 10:44 PM   #25
needtubes is offline needtubes  United States
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Quote:
Originally Posted by xrk971 View Post
Can we use an LU1014 for the N channel depletion mode?
Doubtful. With around -1Vgs at reasonable currents, it will be very difficult to find a matching P-channel MOSFET.
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Old 4th December 2016, 12:18 AM   #26
Nelson Pass is offline Nelson Pass  United States
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Quote:
Originally Posted by xrk971 View Post
Can we use an LU1014 for the N channel depletion mode?
If you cascode it you can adjust the Vgs for a given current by setting the
Vds of the JFET via the cascode. In the F3, this is only a couple of volts,
but if you raise it to, say, 4 or 5 volts you can have a higher Vgs, something
to match your P channel part perhaps.
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Old 4th December 2016, 12:20 AM   #27
Nelson Pass is offline Nelson Pass  United States
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Quote:
Originally Posted by EUVL View Post
MOSFETs not noisier than JFETs ?
I've seen some fairly noisy power Jfets....
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Old 4th December 2016, 01:07 AM   #28
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Quote:
Originally Posted by Nelson Pass View Post
If you cascode it you can adjust the Vgs for a given current by setting the
Vds of the JFET via the cascode. In the F3, this is only a couple of volts,
but if you raise it to, say, 4 or 5 volts you can have a higher Vgs, something
to match your P channel part perhaps.
I use IRFP9240 so Vgs is around 3.8v to 4.0v. Can you give an example of a cascode that will allow the LU1014D to work with the 9240? What about the resistor divider network between the output and the P gate? Could that be set to allow the LU1014D to balance with the IRFp9240?
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Old 4th December 2016, 02:39 PM   #29
Antoinel is offline Antoinel  United States
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Default Check R085 for gate leakage.

I have two R085s. One does not have a gate leakage current. The second R085 has a gate leakage current of ~15 mA in a properly biased circuit. I either messed it up in my goofy experiments or it came this way. I measured the resistance between its gate and drain and found it to be is ~ 500 Ohms versus infinite for the other one.

This leaky R085 gave me heartburn.
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Old 4th December 2016, 05:27 PM   #30
Antoinel is offline Antoinel  United States
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The attached schematic shows a DEF which uses the leaky R085 I have. A constant current source [CCS] of ~15 mA bypasses its gate current to the negative rail. The CCS also maintains a high input impedance for the follower. If I wish to use a resistor instead of a CCS, its value is 1.28K; calculated from: [23-3.8] divided by 15 mA. The value of the joint Vgs = -3.8V is arbitrary.
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