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Old 20th October 2009, 07:47 PM   #1401
Salas is offline Salas  Greece
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Resistors yes. MPSA42 no. Its NPN too low hfe.
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Old 20th October 2009, 10:13 PM   #1402
massimo is offline massimo  Italy
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Κατάλαβα! Ευχαριστώ πολύ.

(I undestood! Thank you very much)
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Old 20th October 2009, 10:50 PM   #1403
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Is there any formula to calculate the Vref based on the Idss of the njfet and the reference resistor in v1.1? For the same Vref, higher Idss requires higher resistor value, right?

Anyone has the figure of the input impedance of the Q3 BC550C?

Has anyone thought of using a JFET instead of bipolar in place of Q3? lower noise, higher impedance? What is the problem with a JFET?

Last edited by HiFiNutNut; 20th October 2009 at 10:54 PM.
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Old 21st October 2009, 12:22 AM   #1404
iko is offline iko  Canada
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Quote:
Originally Posted by HiFiNutNut View Post
Is there any formula to calculate the Vref based on the Idss of the njfet and the reference resistor in v1.1? For the same Vref, higher Idss requires higher resistor value, right?

Anyone has the figure of the input impedance of the Q3 BC550C?

Has anyone thought of using a JFET instead of bipolar in place of Q3? lower noise, higher impedance? What is the problem with a JFET?
V_out should be about 0.6 + Idss_Q6 * (R7 + Rtrim)

Yes, for same Vref, higher Idss requires higher Rtrim

Q3 vs jfet: I certainly thought of it, but found other areas of improvement so I addressed those first.
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Old 21st October 2009, 12:41 AM   #1405
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Ikoflexer,

Yes I derived the same formula but the result did not match, hence the question.

In my working example, Idss=7.3mA, so 0.6 + 0.0073 * (2200 + 200) = 18.12 (V). I only got 13V.

By the way, if that is the formula, for a given voltage, the higher the Idss would require lower Rtrim.

Edit: The above figures are approximate. I have not measured the actual current in this leg. The Idss was measured independantly, i.e. outside the working circuit. Perhaps that is where things can go wrong.


Regards,
Bill

Last edited by HiFiNutNut; 21st October 2009 at 12:45 AM.
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Old 21st October 2009, 12:47 AM   #1406
iko is offline iko  Canada
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What's the voltage across R5?
What's the current through Q4? Insert a small resistor (1R) between drain of Q4 and GND and measure the voltage across it.
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Old 21st October 2009, 12:52 AM   #1407
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Yes, I will do those. A bit difficult to do the Q4 one. I think that while there is no load the current going through Q4 has to be very close to that of Q1, just minus the one going through R5.
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Old 21st October 2009, 12:54 AM   #1408
Salas is offline Salas  Greece
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Quote:
Originally Posted by HiFiNutNut View Post
Ikoflexer,

Yes I derived the same formula but the result did not match, hence the question.

In my working example, Idss=7.3mA, so 0.6 + 0.0073 * (2200 + 200) = 18.12 (V). I only got 13V.

By the way, if that is the formula, for a given voltage, the higher the Idss would require lower Rtrim.

Edit: The above figures are approximate. I have not measured the actual current in this leg. The Idss was measured independantly, i.e. outside the working circuit. Perhaps that is where things can go wrong.


Regards,
Bill
Under just Vbe derate your 9-10V measured 2sk170 idss by 30-35% as I have wrote here some times before.
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Old 21st October 2009, 12:56 AM   #1409
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So to get the same output voltage, a lower Idss_Q6 would require a higher value R7+Rtrim, right?
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Old 21st October 2009, 12:59 AM   #1410
Salas is offline Salas  Greece
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Naturally.
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