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Old 17th August 2002, 07:03 PM   #1
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Default Discrete op amp circuit

i was going thru diyopamp by Nelson Pass and got hook on the idea of building a discrete op amp with jfets input(2sk389)
However i have problem sourcing the output stage of the circuit
the circuit calls for a 2vp3310A and 2vn3310A. Can anyone sugest an alternative to this transistors. Also i was wondering about using jfets(sk170) for the ccs. For 4 ma the resistor used is 75 ohm, so for 10 ma i should use 30 ohm resistor rite? Thanks for the help
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Old 17th August 2002, 07:16 PM   #2
JoeBob is offline JoeBob  Canada
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Digikey caries both of those MOSFETs just search for VN3310A and VP3310A (it's not listed with the 2 in font). Hope that helps.
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Old 17th August 2002, 10:20 PM   #3
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Quote:
Also i was wondering about using jfets(sk170) for the ccs. For 4 ma the resistor used is 75 ohm, so for 10 ma i should use 30 ohm resistor rite?
Not if 10 mA exceeds Idss, the saturated drain current.

What "color" are your fets?

Jocko
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Old 17th August 2002, 11:05 PM   #4
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Default What "color" are your fets

Black with gray legs........why?
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Old 18th August 2002, 12:02 PM   #5
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Why using fets?
I think you will get yourself a better one with normal transistor,s because off the noise .
GRTZ Don
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Old 18th August 2002, 04:15 PM   #6
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the color will be blue so using the 30 ohm sets the current a 10mA rite?

what will be better than fets thaN? bipolar?jfets?
anything other than VN3310A will be better.
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Old 18th August 2002, 07:32 PM   #7
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A normal fast transistor i think will give a better totall result.
The time of the fets which where much faster than bipolair is over.
A freq respons from normal transistor,s will even be higher than the fets and what i mentioned the fet with his input resist is much more sensitive for noise and other hf distortion.
You can see it back in several power output stages .
In the 80,s the power fet was simply better than a bipolair transistor but the time changed with the well known fast transistor,s see the sheets off toschiba nec sanken etc.
Much easier to handle too
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Old 18th August 2002, 07:36 PM   #8
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Quote:
Originally posted by NickC
the color will be blue so using the 30 ohm sets the current a 10mA rite?

what will be better than fets thaN? bipolar?jfets?
anything other than VN3310A will be better.

Sorry i don,t know this type off operational amp.
The ones you see the most in europe is the ne5532.
A change to an OPA from Burr Brown will give most off the time a more satisfactioner vieuw.
BUT sometimes teh ne5532 will do his work very well and when you used a lott off them parallel with a restistor in the output it will give a much better result than the most expensive op amp for less money
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Old 18th August 2002, 08:11 PM   #9
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Default Black with gray legs........why?

How did I know he was going to say that......

Hey Hairy......show us your **TS.

Anyway.....blues......I'll measure a few and let you know.

Ah.......what does speed have to do with constant-current sources anyway? I thought constant current and high impedance where the prime parameters.

Jocko
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Old 18th August 2002, 08:31 PM   #10
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Default Just as I suspected....

The answer is a resounding NO!

1.) I measured about a dozen at random from my bag o' hundreds, and the Idss is around 7.5 - 8.0 mA (roughly). No way you can get 10 mA from one unless you have violets.

2.) Two in parallel, with a 10 ohm resistor gives a tad less than 10 mA.

I would only use a JFET for a ccs if the current is in the range of a few mA. Go to a bipolar here is my suggestion since you do not wish to use a MOSFET.

Jocko
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