Single Jfet amp How to calc gain?

Zero Cool · 20th April 2010, 03:11 PM

I am looking to calculate the gain of this single Jfet amp stage. I found this previous thread:
Have to calculate gain for typical small JFET preamplifier

and using the simplified version of John Curls calculations: Vg=R(1)/{R(2)+1/Gm} or R(1)/R(2) approximately

R1/R2 = gain

I find that we have about 6K/12K = gain of 2 or 6db Correct? seems to make sense.

Zc

Zero Cool · 20th April 2010, 08:15 PM

I am hoping someone can confirm my math here?

richie00boy · 20th April 2010, 08:34 PM

Yes, 6k in the source and 12k in the drain would yield you gain of 2 when output is taken from the drain and loaded with a reasonably high impedance (10k would be fine).

Zero Cool · 20th April 2010, 09:27 PM

Thank you!

richie00boy · 20th April 2010, 09:49 PM

Remember that the output will be inverted with respect to the input, so you may want to include another stage or simply live with it.

Zero Cool · 21st April 2010, 03:48 AM

This is actually part of a GK 1001RB bass amp. This Jfet is in there for flavor only from what i gather. there is an op-amp before and an op-amp after so this Jfet really doesn't DO anything other then overload and change the sound. But once i started looking at the circuit i got curious as to how much gain it was providing. I didn't figure much if any at all.

h_a · 21st April 2010, 04:58 AM

That's a simple common source stage, for the basic euquations look in wikipedia:

Common source - Wikipedia, the free encyclopedia

R1/R2 = gain is a bit too simplified as the jfet itself does not matter at all and of course there are high gain (transconductance) and low gain parts

Also, 6k in the source are a little...uncommon. You will be driving any small signal jfet near pinch-off voltage limiting signal swing. Better try smaller source resistor and smaller load resistor.

Have fun, Hannes

EDIT: just thought over your last post: sure, that's ok if you want to add sound to the circuit

20th April 2010, 03:11 PM	#1
Zero Cool Audio Junkie diyAudio Member Join Date: Sep 2004 Location: MN	Single Jfet amp How to calc gain? I am looking to calculate the gain of this single Jfet amp stage. I found this previous thread: Have to calculate gain for typical small JFET preamplifier and using the simplified version of John Curls calculations: Vg=R(1)/{R(2)+1/Gm} or R(1)/R(2) approximately R1/R2 = gain I find that we have about 6K/12K = gain of 2 or 6db Correct? seems to make sense. Zc

20th April 2010, 08:34 PM	#3
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	Yes, 6k in the source and 12k in the drain would yield you gain of 2 when output is taken from the drain and loaded with a reasonably high impedance (10k would be fine). __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

20th April 2010, 09:49 PM	#5
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	Remember that the output will be inverted with respect to the input, so you may want to include another stage or simply live with it. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

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20th April 2010, 08:15 PM	#2
Zero Cool Audio Junkie diyAudio Member Join Date: Sep 2004 Location: MN	I am hoping someone can confirm my math here?

20th April 2010, 09:27 PM	#4
Zero Cool Audio Junkie diyAudio Member Join Date: Sep 2004 Location: MN	Thank you!

21st April 2010, 03:48 AM	#6
Zero Cool Audio Junkie diyAudio Member Join Date: Sep 2004 Location: MN	This is actually part of a GK 1001RB bass amp. This Jfet is in there for flavor only from what i gather. there is an op-amp before and an op-amp after so this Jfet really doesn't DO anything other then overload and change the sound. But once i started looking at the circuit i got curious as to how much gain it was providing. I didn't figure much if any at all.

21st April 2010, 04:58 AM	#7
h_a diyAudio Member Join Date: Sep 2006 Location: Graz, Austria	That's a simple common source stage, for the basic euquations look in wikipedia: Common source - Wikipedia, the free encyclopedia R1/R2 = gain is a bit too simplified as the jfet itself does not matter at all and of course there are high gain (transconductance) and low gain parts Also, 6k in the source are a little...uncommon. You will be driving any small signal jfet near pinch-off voltage limiting signal swing. Better try smaller source resistor and smaller load resistor. Have fun, Hannes EDIT: just thought over your last post: sure, that's ok if you want to add sound to the circuit

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