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Old 27th April 2012, 05:00 PM   #21
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Quote:
Originally Posted by Tekko View Post
Why is DC coupling a requirement ?
I really can't say (I'd like to but I can't)... can't we just take it as read that I cant/won't use an opamp (or have the input DC decoupled with a DC blocking cap).

Ok, re pinch off...I thought it would be more dramatic, but with a VGS of -1.9V, the voltage drop across the drain's 100r resistor sees a chunky drop, but there's still a bit of current flowing (with VCC of 5V I'm seeing about 100mv across the 100R resistor) ...it's not until VGS of about -2.5V that there's no IDS current flowing at all (no voltage drop across the drain resistor) .....I'm assuming that cutoff voltage should be deemed -2.5V here?

Next...biasing calculations....

Last edited by peskywinnets; 27th April 2012 at 05:09 PM.
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Old 27th April 2012, 05:05 PM   #22
zinsula is offline zinsula  Switzerland
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Best way to learn is to use such simple devices as a J-Fet. And it's way more interesting what can be achieved with only one active part.
I cannot understand, thread starter clearly said he wants to use a JFet, and so be it.
He will learn while trying, what's possible and what's not.
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Old 27th April 2012, 05:09 PM   #23
zinsula is offline zinsula  Switzerland
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Quote:
Originally Posted by peskywinnets View Post
[...]So long as the source is biased higher than the largest voltage input swing, then I should be ok?
Yes.

Now measure pinch off voltage. Put a 1 megohm or so resistor between source and ground, and ground the gate.
What voltage do you read at the source?

Oh, and if you did not already:
Read this and this article from Erno Borbely. And save the articles on your HDD. Who knows when they disappear....
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Last edited by zinsula; 27th April 2012 at 05:12 PM. Reason: gate, not base.....pffff
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Old 27th April 2012, 05:11 PM   #24
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I did measure the pinch off voltage...(albeit with a grounded gate...and variable resistor in the source)....pinch off seems to be about -2.5V
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Old 27th April 2012, 05:14 PM   #25
zinsula is offline zinsula  Switzerland
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Don't estimate pinch off. Just turn up your source resistor (how high can you go?) and read the voltage at the source.
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Old 27th April 2012, 05:16 PM   #26
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Originally Posted by zinsula View Post
Don't estimate pinch off. Just turn up your source resistor (how high can you go?) and read the voltage at the source.

I didn't estimate it...cranking the variable 60k of resistance (which was all I had immediately to hand) in the source .........at a source voltage of 2.5V there was no current flowing (no drop across the 100R resistor) ... therefore pinch off equals -2.5V

So if I've an IDS max of 4.9mA...should I be shooting for 50% of that for biasing purposes? (which would then mean I'd need to find the gate source voltage that yields an IDS current of about 2.5mA (or 250mV drop across the 100R drain resistor)

Edit: for a 250mV drop across the 100R drain resistor sees a 2.1V voltage on the source - so is this my biasing point?

Last edited by peskywinnets; 27th April 2012 at 05:22 PM.
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Old 27th April 2012, 05:27 PM   #27
zinsula is offline zinsula  Switzerland
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Look, you do not have to read the current flowing. Just crank the source resistor up to max (the voltage will not go higher than pinch off) and read G-S voltage. Much easier. And less error prone, in case your voltmeter isn't that sensitive to measure such low voltage drops.

OK, let's say -2.5V is ok. Now, you can bias the JFet with your pot at say V GS = -0.75 V, i believe this will give around 2.5mA at operating Point. That means approx. 300 Ohm source resistor.
Check if this is correct please.
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Old 27th April 2012, 05:33 PM   #28
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ok, with the source resistor cranked up, I see a source voltage of 2.54 (top tip - thanks)

Quote:
Now, you can bias the JFet with your pot at say V GS = -0.75 V, i believe this will give around 2.5mA at operating Point. That means approx. 300 Ohm source resistor.
Where did you get the GS 0.75V = 2.5mA ID figure from?

300 ohms in the source sees a 1.92V drop across the resistance ...which is an ID of 6.4mA - this is puzzling as that's higher than my earlier measured IDS max of 4.98mA?!!!!

Last edited by peskywinnets; 27th April 2012 at 05:38 PM.
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Old 27th April 2012, 05:38 PM   #29
zinsula is offline zinsula  Switzerland
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Quote:
Originally Posted by peskywinnets View Post
[....]Where did you get the GS 0.75V = 2.5mA ID figure from?
Quick calculation....
IDSS=5mA
Vp=2.5V (V pinch off)

Desired V GS = 0.75V (so you can swing between VGS of 0.25 and 1.25, Vpp=1V)

ID at VGS = IDSS * (1- VGS/Vp)

Its also written in the documents i linked before.
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Old 27th April 2012, 05:41 PM   #30
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Quote:
Originally Posted by zinsula View Post
Quick calculation....
IDSS=5mA
Vp=2.5V (V pinch off)

Desired V GS = 0.75V (so you can swing between VGS of 0.25 and 1.25, Vpp=1V)

ID at VGS = IDSS * (1- VGS/Vp)

Its also written in the documents i linked before.
I see the methodology now - thanks.....but how to decide on the drain resistance?

I've not had time to read those docs yet...just wanted to take some early readings....I'll have a read now.
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