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 Post subject: FET question
PostPosted: Aug Mon 12, 2019 12:06 pm 
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I have a circuit in a Sylvania AM/FM radio that uses a FET as the FM mixer.

All the service manual lists for it is FET N channel with a manufacturers part number of 13-22692-1.

Will pretty much any N channel FET work?

I don't need to replace it as it is good, but I do want to go ahead and update the schematic with a FET that will work so I know what to use should it fail.


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 Post subject: Re: FET question
PostPosted: Aug Mon 12, 2019 12:59 pm 
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A J310 would work well - we used them as a do-all FM RF-IF Amp/Mixer JFET. Anything lower, we went with the MPF102. Both can be had for a buck-twenty or less.each.

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 Post subject: Re: FET question
PostPosted: Aug Mon 12, 2019 1:14 pm 
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Thanks.

I'll put both numbers on the schematic.


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 Post subject: Re: FET question
PostPosted: Aug Mon 12, 2019 3:30 pm 
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How do you know the part is a junction FET? MOSFETs were very popular for a time. Some had dual gates.

In theory, the symbol used on the schematic should show what sort you have, but this does not work 100%.

Gotta use the right flavor for replacement.

Ted


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 Post subject: Re: FET question
PostPosted: Aug Mon 12, 2019 4:47 pm 
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Here's the part of the circuit showing the FET.

Attachment:
Circuit.png
Circuit.png [ 40.14 KiB | Viewed 352 times ]


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 Post subject: Re: FET question
PostPosted: Aug Mon 12, 2019 6:01 pm 
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^ Yep, that's the symbol for a junction FET.
John

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 Post subject: Re: FET question
PostPosted: Aug Mon 12, 2019 6:11 pm 
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Since the schematic labels it as a J310, maybe that's a good choice.

At Siliconix, we sold millions of J310, but sometimes customers wanted them selected for pinch-off voltage and/or gm.

Rich


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 Post subject: Re: FET question
PostPosted: Aug Mon 12, 2019 6:39 pm 
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I put that label there on the schematic after it being mentioned that the J310 would work as a sub for the unknown JFET.


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 Post subject: Re: FET question
PostPosted: Aug Mon 12, 2019 9:37 pm 
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It looked professional and original to me!

Rich


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 Post subject: Re: FET question
PostPosted: Aug Mon 12, 2019 9:52 pm 
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Thanks.

I use Microsoft paint to make and modify schematics.


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 Post subject: Re: FET question
PostPosted: Aug Tue 13, 2019 12:16 am 
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Another way to distinguish between JFET and MOSFET is by the relationship between gate voltage and source voltage.

The JFET is always a “depletion mode” device. It’s natural state is “on” (conducting). The gate must be biased negative in relationship to the source in order to turn it off. This is analogous to how vacuum tubes/valves require negative bias on the control grid relative to the cathode. An N channel JFET is extremely similar to a vacuum tube. In more than one way: When the control grid of a vacuum tube becomes positive with respect to the cathode, it conducts just like a diode. The same is true of the gate of a JFET. If it becomes positive with respect to the source, it conducts the same as a normal silicon diode.

JFET: Vacuum tube:
    Source Cathode
    Gate Control grid
    Drain Plate

In contrast, most (but not all) MOSFETs are “enhancement mode” devices. Their natural state is “off” (not conducting). The gate must be positive with respect to the source to get a MOSFET to conduct. Because the gate is insulated, there is never any gate current, regardless of the positive/negative state of the MOSFET gate in relationship to the source.

The device in the schematic has its gate at 0V and its source at 1V. This confirms the device is a JFET.

-EB


Last edited by electricboyo on Aug Thu 22, 2019 4:05 pm, edited 2 times in total.

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 Post subject: Re: FET question
PostPosted: Aug Tue 13, 2019 12:31 am 
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Thanks for that bit of information.

I learned something.


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 Post subject: Re: FET question
PostPosted: Aug Tue 13, 2019 1:34 am 
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The schematic does show a junction FET. N-channel. But we do know that sometimes what the draftsman drew may not be what the engineer meant, nor what Production built.

Well, they do make depletion mode MOSFETs, but they are far less common. Very useful sometimes. The distributors have them in stock.

An enhancement mode junction FET won't work so well, as the gate starts conducting to the source. Depletion mode devices are further enhanced right into forward bias on the gate, though this is not a usual way to run them.

Ted


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 Post subject: Re: FET question
PostPosted: Aug Tue 13, 2019 1:35 am 
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Sorry, MOSFETs can be depletion mode:

https://www.infineon.com/dgdl/Infineon- ... 3d1a197d94

Very popular in "start-up" circuits for small power supplies.

Rich

Image


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 Post subject: Re: FET question
PostPosted: Aug Tue 13, 2019 4:30 am 
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Quote:
Well, they do make depletion mode MOSFETs, but they are far less common.
Quote:
Sorry, MOSFETs can be depletion mode:
Come to think of it, I used a FET made by a company named Supertex in an ultra-low-power DC power supply that I designed.
Supertex refers to these devices as "N channel depletion mode vertical DMOS FET"

So you are entirely correct: There are such things as "depletion mode MOSFETs"

I edited my earlier post to include this information. Thanks for the comments!

-EB


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