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Old 10th October 2012, 10:25 PM   #1591
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Originally Posted by Waly View Post
Careful. BF545 has a minimum pinch voltage of -0.4V. If by chance you run into the worst case combination of BF862 and BF545, the BF862 may not saturate (but stay in the linear region).

Better find a n jfet with higher pinch voltage, like this. Only 25V Vds but dual, and -2V minimum pinch voltage.
Good advice, the data sheet is a little unclear but from my read the BF545C is fine. That last line of Vp at 1uA looks like it covers all the grades, not sure why it's there.
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Old 10th October 2012, 10:27 PM   #1592
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Hmmm ... in my sim I have very good agreement on the +ve PSRR figures, but the -ve PSRR is dramatically superior at lower frequencies for stinius. I do note that the -ve PSRR is highly variable, depending strongly on the choice of BJTs, so I'll try replicating stinius's selections ...

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Old 10th October 2012, 10:44 PM   #1593
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Hmmm ... in my sim I have very good agreement on the +ve PSRR figures, but the -ve PSRR is dramatically superior at lower frequencies for stinius. I do note that the -ve PSRR is highly variable, depending strongly on the choice of BJTs, so I'll try replicating stinius's selections ...

Frank
AC or DC? You can scope out the DC component by doing a DC sweep and poking around.
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Old 10th October 2012, 10:55 PM   #1594
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AC. I also note that PSRR is highly dependent on the output DC operating point; if I trim the circuit to minimise DC offset it actually worsens PSRR behaviour - I'm not sure whether this is a sim artifact so far, and I don't have anywhere near enough knowledge about amplifier tolopogies to understand behaviours in an intuitive way ...

Frank
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Old 10th October 2012, 11:29 PM   #1595
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AC. I also note that PSRR is highly dependent on the output DC operating point; if I trim the circuit to minimise DC offset it actually worsens PSRR behaviour - I'm not sure whether this is a sim artifact so far, and I don't have anywhere near enough knowledge about amplifier tolopogies to understand behaviours in an intuitive way ...

Frank
In the end this will depend on build and we are smart enough to tweak it if need be.
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Old 11th October 2012, 03:21 AM   #1596
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Better find a n jfet with higher pinch voltage, like this. Only 25V Vds but dual, and -2V minimum pinch voltage.
This thread has highlighted some very interesting small JFET dual parts. Thanks Scott, Brad, and everybody else who has made parts suggestions along the way.

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Old 11th October 2012, 06:51 AM   #1597
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Hmmm ... in my sim I have very good agreement on the +ve PSRR figures, but the -ve PSRR is dramatically superior at lower frequencies for stinius.
Mystery solved - he's added two more cascodes, Q7 and Q8. Using J111s there I'm now able to get 125dB -ve PSRR with one BJT combo, still 100dB at 20kHz ...

Have to say the sim distortion figures are truly excellent, even at gains of 100 ...

Frank
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Old 11th October 2012, 01:49 PM   #1598
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Careful. BF545 has a minimum pinch voltage of -0.4V. If by chance you run into the worst case combination of BF862 and BF545, the BF862 may not saturate (but stay in the linear region).

Better find a n jfet with higher pinch voltage, like this. Only 25V Vds but dual, and -2V minimum pinch voltage.
Just noticed this morning another AM radio FET but dual. Someone should evaluate the matching and audio frequency noise. This could be another gem like the BF862. But the user will be punished by an even smaller package.
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Old 11th October 2012, 01:54 PM   #1599
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If you refer to the PMBFJ620, then it has much higher noise and much lower Yfs than the BF862.
Idss is also way too high.
The package is a small problem. We have soldered even 0.5mm pitch 6-pin.
It is humanly possible, even by hand.



Or you are talking about another device ?


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Old 11th October 2012, 02:00 PM   #1600
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If you refer to the PMBFJ620, then it has much higher noise and much lower Yfs than the BF862.
Idss is also way too high.
The package is a small problem. We have soldered even 0.5mm pitch 6-pin.
It is humanly possible, even by hand.



Or you are talking about another device ?


Patrick
Yes, then it should stay in cascode duty. I do like the contrast between the parts we are discussing and the usual almost obsolete ones. I think one of the npn/pnp duals had 5 million and 3 million "ready to ship" at the first two distributors.
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