JFET differences: GP vs. SW vs. VHF vs. UHF

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scm6079
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JFET differences: GP vs. SW vs. VHF vs. UHF

Post by scm6079 » Sat Aug 09, 2003 3:45 pm

Could someone help explain the difference between GP (general purpose) SW (switch?) VHF and UHF JFETs? <p>I am designing a circuit and using a JFET as a 'relay replacement' for a very small current flow. In picking the correct JFET for the job, I don't know the difference between these different available types. <p>Any help would be *greatly* appreciated.<p>Thanks,
_Scott

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Chris Smith
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Re: JFET differences: GP vs. SW vs. VHF vs. UHF

Post by Chris Smith » Sat Aug 09, 2003 6:09 pm

"Switches" VS standard JFET [or transistors] in general, are what could be considered on or off, with very little progressive ramping while turning on. Square wave VS Sine wave opening.<p> For example a transistor is proportionally on, to the amount of current at its base. More current, more on. Like a adjustable water valve. Switches tend to be like a gate, open, or closed, little or no in between. <p>JFETs are also used over the Transistor because they don’t use current, they merely need a voltage present at the gate, to open that gate. <p>There are JFETs that are proportional to the voltage present but "Switches" tend to be more on/off, and even faster in the case of UHF/VHF as well as possibly lower Capacitance in the gate area which tends to slow things down. <p>This is Some of the info, there is more.<p>Also, from memory?, UHF/VHF uses "Chopper" fets, which are really fast snap action on/off. Designed to be like a clean switch, with out bounce.<p>[ August 09, 2003: Message edited by: Chris Smith ]</p>

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Re: JFET differences: GP vs. SW vs. VHF vs. UHF

Post by haklesup » Sat Aug 09, 2003 10:02 pm

UHF and VHF transistors are designed to operate at higher frequencies with respect to a number of specific specifications. See a data sheet for all those graphs that have frequency on the X axis. Specific methods of construction and design influence the suitability of a device to be in this category.<p>GP switches are selected by the manufacturer based on looser specifications. Note the huge ranges for some of the specs in the data sheets. These devices are used in designs where these particular specs (like gain) are not critical (like buffering an IC output from a relay to boost the current- a switching application). They are desired by designers because of their generally lower cost and greater availability.<p>When comparing data sheets, if all or most of the DC specifications are similar, the devices are likely to operate the same at low frequency or DC.<p>For the purposes of just plain switching off and on (fully saturated) just about any transistor will do the job so long as you do not exceed the breakdown voltages of the junctions or the rated current (Ic or Id). <p>As to the choice of FET (N or P, Enhancement or Depletion), JFET, Bi-polar (NPN or PNP) a lot depends on what type of signal the previous circuit stage produces and what output polarity and type (voltage or current)you want. Other descisions based on power usage, efficiency, leakage and other properties may also influence the decision of one technology over another.

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Re: JFET differences: GP vs. SW vs. VHF vs. UHF

Post by scm6079 » Sat Aug 09, 2003 11:55 pm

Thanks for the great information, I'm understanding that allot more now. <p>In my case, I'm looking to use some external logic (running at 3.5vdc) to cotrol what is currently a very low voltage/current 'physical' switch.<p>I believe that JFET seems the most appropriate, since I don't have any current flow from my logic circuit to the existing circuit I'm controlling.<p>Given my low level of current/voltage it sounds like either GP/VHF or UHF will work for my needs.<p>Since I don't want to influence the 'original' circuit (part of a cell phone) I don't think a typical NPN/PNP transistor will work.<p>Last questions (I promise!)-- should I be very concerned with the voltage that is running through this JFET being too low (it is also logic level, <4vdc for one part, a headset mic pickup for the other) from source to drain? Knowing my circuit, I'd want a gate (off) max voltage of as close to 3.5V as I can get, and what else should I "look out for?" <p>Thanks in advance for all your help!<p>-Scott<p>PS - If you are curious, I'm a software programmer, integrating a bluetooth module into my phone for voice and data (wireless modem) transfer. It all works (yeah!) except I have a big ugly 3PDT switch to control the mess, that really should be a logic controlled flat pushbutton.<p>http://i500.nopdesign.com/hw/
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Re: JFET differences: GP vs. SW vs. VHF vs. UHF

Post by toejam » Mon Aug 11, 2003 9:43 am

check out a 4066 switch on google.

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Re: JFET differences: GP vs. SW vs. VHF vs. UHF

Post by haklesup » Mon Aug 11, 2003 12:13 pm

In that case, don't even mess around with discrete transistors. Since you are basically trying to make a 3pdt switch solid state you should use a switch like the DG411/DG412 or the MAX313/314 (quad SPST) or the MAX4661 (lower on resistance). <p>Similarly numbered Maxim (www.maximic.com) chips have other switch configurations and all operate under a wide range of supply voltages and come in DIP and SOIC and SSOP packages for small mounting. They also have good input protection to hepl prevent failure (which discrete parts are prone to). You could probably get a few free samples from Maxims website.

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