I have a common source n channel JFET circuit--
with R1/R2 bias divider R1 = 420K, R2 = 180K
RD = 2.7K Rs = 2.7K RL = 4K
cap Cs in || with Rs.
Vp = -4 early voltage = 125 Idss = 12mA
How can I find small signal voltage gain @ mid freq?
I try to compute gm-- but need Vgs to do so. Do I assume Vs = 0 since Cs short @ mid freq and then use dc value of Vg (6V) to say Vgs = 6v?
I'm thinking there has to be additional info to figure this out...
Any clues appreciated...
Bo
I only dimly remember how to do this, so take this all with a grain of salt:
No, you can't assume that Vgs = 6V, because at DC Vs does not equal zero. Your source current will be in the neighborhood of 6V/2.7k-ohm, because while Vgs will be greater than zero, it won't be that much greater compared to 6V. If your data sheet gives you a means of finding gm from the source current, you're done.
The academic way to do this is to note that the source current is equal to some constant times (VT + Vgs)^2. Data sheets often give VT (IIRC it'll be called 'threshold' or 'pinch-off'), and they'll give the source current at Vgs = 0. This is enough to calculate the source current at any value of Vgs; you can solve the quadratic equation involving your source resistor to find the _exact_ Vgs, then you can differentiate the Is as a function of Vgs at that Vgs to find gm.
The academic way to do this leaves out a host of real-world issues, and is a PITA besides. Spice will tell you the gain of your circuit pretty quickly, and you can choke your gm value out of it without too much trouble, _and_ you can do things like Monte Carlo simulations to take into account the wide variation in JFET parameters while you're at it.
-- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
"Bo" schrieb im Newsbeitrag news:90312$480c9108$41a26d4d$ snipped-for-privacy@API-DIGITAL.COM...
Hello Bo,
It's easy to calculate the gain.
G = gm*RD||RL||rd
Just forget the output resistance rd, because it's much higher than RD||RL.
Calculate the DC-current Id. Go to the datasheet and look for gm(Id).
G = gm*RD||RL
That's all
Helmut
Did you try google?
and many more...
Thanks to all for your replies...
I finally found something via google that helped me. Ended up calculating Vgs from a quadratic equation...
I could not look up in datasheet because P/N is unknown...
Thanks again,
Bo
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