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Old 21st June 2004, 12:38 PM   #1
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Question Low Ron N-channel MOSFET

My JLH isn't even finished yet, but I can't keep thinking about the next project: a preamp .

So, I will have a maximum of 4 audio PCBs to allow for 7.1 input and I was thinking that the easiest way to ensure that no signal would come out of those extra channels when only stereo input was used is to simply cut the power to them. Now, since I need at least 5 different voltages for each board using relays isn't a good idea, so I was thinking MOSFETs. This seems like a good idea, since in the PPA headphone amplifier JFETs are used. Although it isn't used there to turn on/off the power, they say that it attenuates rail modulated crosstalk. Sound fancy . (In case you're interested, here's the link: http://tangentsoft.net/audio/ppa/amp/pguide.html#Q3)

Basicly, I've got 2 problems. The first: can N-type MOSFETs conduct negative voltages, I know you can use low Ron MOSFETs like a relay to conduct, let's say, 12V, but can they also be used to conduct -12V?

2nd problem: there's so much MOSFETs . Can anyone recommend a good N-channel MOSFET with: low Ron (the lower the better), logic level switching, doesn't burn at 2A (or more of course)?

In case my idea is just plain dumb or in case I should use another type of FETs, please tell me, before I route the biggest PCB I've ever routed only to find out later that it won't work .

* Edit: Ok, I guess the 1st question is pretty stupid, because they do it in the PPA headphone amp.
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Old 21st June 2004, 01:08 PM   #2
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I think it's not worth doing. If you are only inputting a stereo signal, the other channels will just be idling so there will be nothing to crosstalk to the live ones, and you simply won't be able to hear anything from the unused ones, if you do you have a very poor design and/or layout. You may suffer pops and clicks as they power up/down as well.

It's also not that easy to turn a MOSFET on in source-follower mode because you need to bias the gate up a good few volts higher than the source -- you need a high transconductance device rather than a low Ron device. It won't matter if Ron is even 10 ohms as it will be negligible in small signal circuits. For the -ve rail you need a P-channel device as MOSFETs have protection diodes so you can't use them in reverse.

Do you really need 7.1? I just picked up off ebay recently a centre rear speaker to finish off my matching setup, and I'm darned if I can find a 6.1 film to test it! So 7.1 I think you are wasting your money and time.
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Old 21st June 2004, 04:22 PM   #3
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Hm, well, the preamp will be like this: 8 stereo inputs + 2 inputs for each 7.1 channel (so: 2x 5 inputs). I don't think there's a reason to worry about pops & clicks on turn on, because the chip that will take care of the volume (WM8816) mutes on power on plus there's an extra hardware mute line that I can use if needed. It's just that I don't want the extra channels to be consuming power while they are disabled.

And maybe I don't need 7.1, but since this is going to be quite a big project anyway, I might as well include options for it. I will leave it out, but when I need it, it can then be added. It isn't any trouble adding it, I've got plenty of I/O ports on the C to take care of everything.

Ok, back on topic now, I remembered that Texas Instruments has some PMOS switches (datasheet here). Now, on page 9 you can see the MOSFET being used like I what I had in mind. Also, they're using P-channel MOSFETs to turn on/off a +ve rail. Wouldn't a "logic level" MOSFET take care of the "source needs to be higher than gate"-problem? Also, I googled a bit and it seems that P-channel MOSFETs are being used to switch positive voltages and N-channel MOSFETs for negative voltages.

So, I don't really understand what you're trying to say. Forgive me for being such a newbie, certainly when it comes to MOSFETs and the likes .

If you've got another idea that can do what I want, please tell me. (No, no relays, those are only an option when I'm on the edge of insanity because I can't find a solution .)

(Hm, I'm probably making such a fool out of myself right now. But eventually, the answer will come .)
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Old 21st June 2004, 09:20 PM   #4
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Thanks for the app note, it has enlightened me. I had not considered using P-ch to switch +ve rails and N-ch to switch -ve rails. It will work and has even given me an idea how to make a soft-start for massive PSU cap banks

You don't need logic-level devices, but it would be better to use them given their lower Vgs requirement if you only intend on switching supplies of 5 V.

Basically you just need to make the gate a few volts (see Vgs threshold parameter and Vgs v Id curve in data sheet) different potential from the source. Be careful not to exceed the Vgs max.

If you are using a non-logic level device (with Vgs max likely to be 20 V or so) then all you need to do to turn on the +ve supply is connect the gate to ground. Put a resistor between gate and source to turn off when no power is present.

IRF540 and IRF9540 should do the job nicely for rail voltages of 9 V or more. You may find the N suffix type is all you can get hold of now as the older non-suffix type is no longer made by International Rectifier.
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Old 21st June 2004, 09:31 PM   #5
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Oh, that should clear things up a lot. Basicly I've been searching for info and suitable MOSFETs for the last 2 hours. However, my basic electronics book is so unclear on MOSFETs that I didn't know what to look for . I did notice that Vishay seems to have a lot of suitable stuff (I think ).

Wohoo, only 2 more exams to go !
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Old 22nd June 2004, 08:01 AM   #6
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Ah, c'mon, I'm sure somebody knows the answer to my question.
To make it a bit clearer, I drew a simple block schematic.

Maximum amperages:
+12V: 2A
-12V: 2A
+5V analog: 0.5A
+5V digital: 0.5A

For the 5V, I really only need something like 0.1A, but it's better to be safe than sorry.

So, the "schematic":
Click the image to open in full size.

Oh, btw, relays really aren't an option, since then I'd need to make yet another power supply, as I don't want them to use the 12v analog supply. Plus, they're big and slugish and make clicking noises and consume way to much current and are expensive and ... (yeah, you get it ).
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Old 22nd June 2004, 10:14 AM   #7
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I've already answered your question! The schematic was on the datasheet page 9 like you said. See my previous post re connecting the gate to ground to turn on...

However, the schematic you have just posted is not the same thing as what you originally wrote about. What you have drawn there is a CMOS switch. CD4016 has 4 of them in a 14-pin DIL package. They will only handle a few mA though.
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Old 22nd June 2004, 11:05 AM   #8
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Lol, sorry, I must have read over it.

I didn't intend to draw a CMOS switch, it's just a symbol I came up with. On the lower half of the drawing you can see that there's a question mark behind it, because I didn't know what to use for it.

Now, one last time .
Positive voltages: P-MOSFET (R from S to G)
Negative voltages: N-MOSFET (R from S to G)
To turn on: positive signal into base of NPN which then connects G to gnd (would this work for both P & N types?)

That should work, correct?
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Old 22nd June 2004, 11:49 AM   #9
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Yup, you got it now

The problem with using a transistor to switch to ground though is that the 'off' state of the upper and lower halves is different, i.e. the +ve half is +ve for off, and the -ve half is +ve for off and you cannot do this with a single control line. This is where a CMOS switch or reed relay would come in useful, but if you are going to use a reed relay, you may as well use it to switch the power in the first place.
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Old 22nd June 2004, 12:21 PM   #10
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Quote:
Originally posted by richie00boy
... i.e. the +ve half is +ve for off, and the -ve half is +ve for off ...
Wouldn't that be the same ? (1 is +ve, 0 is gnd) I guess you mean that the +ve is 1 for off and the -ve is 0 for off. In case you mean that, I'll just use a simple NOT port (TI Little Logic SOT23 or something like that), that should fix it, right? (Oh, and I'm going to need those Little Logic NOT chips anyway, so I might as well order a few more ).
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