ampage
Tube Amps / Music Electronics		 For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  

 

Re: FET Electrical Performance


 :
10/27/1999 2:19 PM
R.G.
 Re: FET Electrical Performance
Supply voltage and Gm do interact in that you need a higher voltage for high Vgs/low Gm devices to get a higher current level. JFETs are usually square-law devices, so the incremental Gm increases with increasing drain current.  
 
However, the Vgs of the JFET has and the transconductance have an even bigger effect. Low Vgs JFETs tend to be high-Gm devices as well. A 2N5384 or 2N5385 should be a big improvement. 2N5457's have Vgsoff of about 4-7 volts usually.  
  
10/27/1999 5:35 PM
Scott
RG  
 
Did you mean 2N5484 or 2N5485? I couldn’t find 2N5385 in the Mouser, Allied, or Newark catalogs. I think I have a 2N5484 around that I could test for distortion. I will also test at higher voltage as Jack suggested.  
  
11/4/1999 9:50 PM
Scott
I tried your suggestion of higher supply voltage and found the following. The voltage source was two batteries, about 18.6V.  
 
1. The higher voltage required a different ratio of drain to source resistors to get correct drain voltage. At 9V, I used 2.2K and 470, and for 18V, 10K and 1K.  
 
2. This means that signifigantly higher gain is available at the higher supply voltage.  
 
3. With the 18.6V supply, 10K drain resistor, 1K source resistor, no bypass cap and Vout = 1 Vrms, gain was 10 and distortion was 0.2% THD. Much better than the 9V supply performance.  
 
This may all be obvious to some, but as I mentioned above my main experience is with tubes, so this was an interesting exercise to me.
 
11/4/1999 11:33 PM
Jack
quote:
"With the 18.6V supply, 10K drain resistor, 1K source resistor, no bypass cap and Vout = 1 Vrms, gain was 10 and distortion was 0.2% THD. Much better than the 9V supply performance."
 
 
Bingo! Removing the bypass cap also contributed to the lower THD. If you can crank the supply up even higher it will improve some more. Three batteries at about 28v might be even better if you need the headroom.  
 
regards, Jack
  

<<First Page<PrevPage 2 of 2