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....
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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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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?
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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.
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.
ok, with the source resistor cranked up, I see a source voltage of 2.54 (top tip - thanks)
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?!!!!
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?!!!!
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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.
I quickly checked the datasheet (always read them, you see now why!), J110 has a minimum IDSS of 10mA.
You have to repeat the measurement of IDSS...
As soon as you have ID settled at the operating point (let's say it's 5mA), you can calculate the mid point (i.e. at 0V input).
To swing 3Vpp, you should aim to around 2V below rail = 3V. So you will swing from 1.5V to 4.5V.
With 5mA and 2V drop, it's easy, isn't it? 2V/5mA = 400 Ohm.
BUT, i fear with 400 Ohm, you won't get any gain.
Gain of a common source amp is Rd/(1/gm+Rs).
But let's see what you find out regarding IDSS.
As i have to leave now, you'll be on your own. Don't know when i might be back....
Well, earlier when trying to establish the IDS max, I followed this article...
http://www.qrp.pops.net/fetbias.asp which has 100R in the drain, but clearly that knocks things way out of whack, becuase without 100R in the drain (drain to VCC), I'm now measuring 100mA IDS!
http://www.qrp.pops.net/fetbias.asp which has 100R in the drain, but clearly that knocks things way out of whack, becuase without 100R in the drain (drain to VCC), I'm now measuring 100mA IDS!
What??
Are source and gate tied together to the negative voltage / ground, without resistor?
Do you have enough headroom for those measurements? 100mA, that would mean 10V across the source resistor.....
What is your supply voltage?
Did you make some progress?
No I didn't progress becuase the weekend arrived ....& then I got a sudden bout of man flu (serious stuff)
Because I want to square JFETs away in my head (& I'm a long way from that at the moment!), I will revisit today, but that earlier 100ma IDS measurment was with the gate grounded, the source grounded & 5V applied to the drain (no resistors in situ at all as I wanted to establish the max IDS)
In fact just double checked the measurement & it is showing 120mA....therefore I guess we can summise that the JFET 'on' resistance is 41 ohms?
And how did you measure the current? With an Amp Meter? Or do you still have the 100 Ohm resistor between 5V and drain? With 100 Ohms and a 5V supply, you can't be measuring 10V (10V / 100 Ohm = 100mA)
Put the 100 ohm resistor back, increase the voltage to 10-15V, and measure again....
because the 100 ohm drain resistor (as recommended by that article I linked to earlier), seemed to really give me out of whack results....I removed the drain resistor completely...essentially the 120mA reading is with my DVM (ie current setting!) slotted in series between source & ground....no resistors, gate grounded, 5V on the drain.
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