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Old 30th September 2012, 12:42 PM   #5411
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Thx Salas, Idss of 7.5mA on the K170's
on the Zobel R, could that be 0.8 as 2x 0.4R?

And the 560R on the current mirror. Would 470R suffice?

Last edited by Dr.H; 30th September 2012 at 12:53 PM.
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Old 30th September 2012, 03:55 PM   #5412
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I'll get the required components tomorrow, LOL.
What determines the range of the reg? I have built the 5V version, using 1K pot in series with a green LED. I have 18V input and yet my output range is just between 5V and 6V...
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Old 30th September 2012, 08:04 PM   #5413
Salas is offline Salas  Greece
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See #5395
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Old 1st October 2012, 07:55 AM   #5414
Phisci is offline Phisci  South Africa
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Thx Salas, saw that. Still not sure why the 1k pot would only give me a range from 5 to 6V. Even the formula would suggest a broader range.
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Old 1st October 2012, 08:37 AM   #5415
Salas is offline Salas  Greece
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The setting resistor should not be allowed to fall bellow 1/3 of its opposite 1K resistor, the one between the Mosfet's gate stopper and ground. Thus 330R run by about 4Vgs/1K=4mA mirrored, gives 1.32V. Add 2V from the LED and two 0.7Vbe from the right branch mirror BJTs. That gives you the near 5V base. I would expect 4.7-7.4V with 1K VR. Tolerances or some problem may have squeezed that to 5-6V? We should know the practical Vgs, LedVf and Vbe in a build to see if it computes.
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Old 1st October 2012, 10:10 AM   #5416
see2 is offline see2  Russian Federation
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Hello,

If placing V1.2R LV sensing part near load circuit, big in size C1 (4.7uf) becomes an issue.
Can it value be greatly lowered, to 10n, and for low output impedance R10 also lowered, from 27k to 2k, and R9 adjusted? See attachment for 5v output. It simulates ok, near DC impedance only about 1.5-2 times bigger than with big C1. Does it has negative effects?
Also, what limiting use of 2sk170 as q8 and q3? (For output 5-15v, input 12-23 v)
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Old 1st October 2012, 12:02 PM   #5417
Salas is offline Salas  Greece
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On which schematic the designations you talk refer to? If you will run a noise simulation you will see the problem with 10n. Its not only about impedance. Use a compact electrolytic 16V with very low ESR & HF impedance 1000uF if you plan for digital. That can filter deeply and have good impedance on low VrefR also. You can find a multitude of computer motherboard capacitors offered cheaply with those characteristics. For J3 in the schematic you show here K170BL can do because in low voltage its high idss will not overheat Q18. Although I would prefer you cut its idss down to 2mA with a resistor for Q18 to work in a less noisy manner. If the other position you refer to is some common gate top half of a cascode CCS to another K170 , forget it, you need double the Vgs and up, for top half. So to have some Vds for bottom. Try a J310 or BF245C.
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Old 1st October 2012, 01:11 PM   #5418
roender is offline roender  Romania
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Quote:
Originally Posted by Salas View Post
Although I would prefer you cut its idss down to 2mA with a resistor for Q18 to work in a less noisy manner.
Any degeneration resistor will increase noise not vice versa. Moreover, jfets are less noisier at higher currents
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Old 1st October 2012, 01:20 PM   #5419
Salas is offline Salas  Greece
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If he will use 100 Ohm between g&s on an average BL to go ~2-3mA, the contribution is going to be 0.18uV for 20kHz BW.The original calls for a low noise VHF 2-3mA Idss Jfet for Q18's load anyway. No resistors, low capacitance semi. Q18 is a bjt. Current noise also.
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Old 1st October 2012, 01:27 PM   #5420
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Thanks Salas, will measure Vgs, LED Vf and Vbe and see
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