Low current source

"Fred Bloggs" a écrit dans le message de news: snipped-for-privacy@nospam.com...

In that case, if you notice that the A & B points voltages are V(W) +/- one diode drop, then the current sources can even more be simplified and reduced to a simple resistor. I'll also fixe the current divider ratio to say about 50. Then I'll delete the voltage reference and switch the input to the other opamp input. Then we arrive at this simple schematics with Vin being directly a 0/5V logic CMOS output.

+----------------[R]-----------------------------+ | 4.7M | | ___ | | .--|___|-+--+15V | | | A | | | | | | | | .-. ---' | | 100K| | | | | | | | '-' 1M | GND--[R]-+--|+\\ | W ___ C | | >--+--||-+------+------|___|-+--||--+->Vout Vin>-[R]--+-|-/ | A | | | | | | | .-. | | [R] | B | | | | | | +--|>|-+-|-+ GND .-. | +-|+/ 100K| | | | | | GND GND '-' 4.7M | ___ '--|___|-+-- -15V A | | | ---'

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Thanks,
Fred.

That is a shame- the TL082 has way too much Vio- the pot current was so linear....it can be circumvented but just more parts. Your method is better- maybe some cheap transistor cascodes and the pot at your current src: View in a fixed-width font such as Courier.

. . . . +--|

Maybe this then: View in a fixed-width font such as Courier.

. . +--|-[9.1K]-|---+-||--+ | | | | . | | | [18K] | | [1K] . CMOS | | | | | | | . IN | [1k] +-|>|-+--||-----+ . | | | | | . | | | | +-|

fixing that sink error: View in a fixed-width font such as Courier.

. . +--|-[9.1K]-|---+-||--+ | | | | . | | | [18K] | | [1K] . CMOS | | | | | | | . IN | [1k] +-|>|-+--||-----+ . | | | | | . | | | | +-|

"Walter Harley" a écrit dans le message de news: snipped-for-privacy@speakeasy.net...

really

the

a

Then, to have just one trimpot, you can simply buffer one with another:

1M ( IC(Q1) ~ 10uA ) ___ +Vs-----|___|---+-----+------. | | | | .-. | | | |1M | | | | | Q1 | '-' | >| | | 2N5087 |---+ | /| | | | | .-. | | | | === | | | 22M GND | '-' >| | 2N5087 |----+--->50nA /| Q2 | | === GND

but this doubles the trimpot value, so you can do:

1M ___ +Vs-----|___|-+---------+------. | | | | | .-. | | | | | | | | 10M | | '-' | >| | | 2N5087 |----+--->50nA | /| B | Q2 | \\| | 2N5089 |-------+ 0.4V

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Thanks,
Fred.

"Fred Bloggs" a écrit dans le message de news: snipped-for-privacy@nospam.com...

one

reduced

delete

Hmmm, it'll work but in case of RadioShack components I guess I'll keep my fixed 18K-1M current divider and variable current sources: the 10K trimpots will produce non negligible current offsets due to the output opamp offset (potentially several mv). The 1M resistor reduces this current offset to a low enough value that you won't noptice its effect.

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Thanks,
Fred.

You move quickly Walter, thanks for that story. Have fun with your radio station. I spent my summer before starting college designing and building a main-console mixer (this was 1960, so I used tubes!) for a small community FM station, so I have some idea how much fun that can be. In my mixer, all ramp-ups and ramp-downs were by hand, by rotating big Gates-style knobs!

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 Thanks,
    - Win