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Re: Phaser boards


 
6/10/1998 10:37 PM
hys chip
email
Re: Phaser boards
Wow I have drew a blank on the 2n5492 JFET, except for American Electronic Resources Inc, and they have a texas location R.G.. The only problem is the min. order. They have a website  
http://www.aeri.com/index2.html  
Let me know if the link will not work. chip
 
6/11/1998 6:55 AM
R.G.
.... uh... (blush)... that's because I got in a hurry and typed it in wrong...  
 
That was supposed to be 2N5952...  
 
Let me know if you find a cheap place. I'll probably buy 100 and see how hard selecting sets of 8 is.
 
 
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6/11/1998 5:16 PM
hys chip
email

The same place has the 2n5952 I so far have not found them in any of my low volume freindy sorces, I will keep looking. do you have the specs on the 2n5952 ? or a cross ref.# on it.worst case we could get 4 or 5 people to chip in and get em from American Electronic resources???? anyway have a great day all chip...
 
6/12/1998 9:04 AM
moocow
From the Fairchild Semiconductor site:  
 
2N5952  
 
Vgs(off): 1.3-3.5 V  
Rds(ohms): <300  
 
 
The 2N5484 also has a maximum Vgs(off) of 3 volts. Allied has them for 32 cents each.  
 
Any FET will work in this circuit, however the .05 uF capacitor feeding the + input of the phase shifter stage may need to changed. This capacitor, along with the Rds of the FET, controls the frequency range over which the phase shifter works best. If the Rds of the FET is not similar to that of the 2N5952, some tweaking will be necessary.
 
6/12/1998 10:26 AM
R.G.
Almost.  
 
The Rds spec is Rds(on), the lowest expected value of Rds. Actual Rds in a phaser is diddled between Rds(off), which is substantially infinite compared to the rest of the circuit, and some value small compared to the resistor leg of the circuit, which is usually 10K to 100K, so the lowest Rds in phasers is usually 1K to 10K.  
 
The range over which the phaser works best is determined by the fixed resistor and the cap, not the fet resistance, assuming that the fet can get a big enough Rds, which they all can.  
 
So yes, almost any JFET with low enought Vgs(off) can be made to work, but the value of the cap doesn't really interact with the FET selection. It interacts with the fixed resistor side of the phase network.
 
6/15/1998 10:31 AM
moocow
For once, my statements were not based on theory, but upon my own measurements !  
 
During my phase shifter experiments, I tried every FET I could find, from small signal to power MOSFETs. I built a few circuits with the 2N3819, and then tried the MPF102, only because it is the cheapest FET that Digi-Key sells. At first, it didn’t shift properly. But after tweaking, I found it required a capacitor half the size as that needed for the 2N3819 circuit. Figuring that using smaller caps would be cheaper, I switched to the MPF102.  
 
At the time, I did not use a resistor in parallel with the drain and source of the FET. This probably made my circuit more dependent upon the type of FET, since the 'fixed resistor' was missing !  
 
Eventually, I added the parallel resistor, but I chose 47k instead of 27k. Anything smaller introduced its own shift, thereby reducing the max. to min. shift per stage.  
 
I match FETs based on phase shift vs. Vgs. I adjust Vgs until the phase shift is 90 degrees. FETs with that require the same Vgs for a 90 degree phase shift get grouped together. This matches them on not on the minimum Rds, but at some midpoint value.  
 
There are people out there who may not have access to a wide variety of parts, or who have a whole box full of some other kind of FET that they don’t know what to do with. I just wanted them to know they can use whatever is available to them, and what to expect when a substitution is made.
 
6/12/1998 10:59 AM
R.G.
Took a look at the Fairchild site.  
 
Looks like the 2N5484 and 2N5953 would work as well. The device needs a Vgs(off) of something like 3V max because it has to be less than the 9V splitter used in the P90. The source is tied to a 1N5232B zener, so you have to be able to drag the gate low enough to (almost) turn it off, so 3V is a reasonable value to pick.  
 
Devices with Vgs(off) of greater than about 4.5V can not be guaranteed to work; devices with ranges like 1-9V would have to be even more heavily selected.  
 
JFETs suitable for this application need to be matched for Vgs(th) and Vgs(off).
 
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