JFET based audio source selector - Page 2

Obe1 · 08:34 PM

Quote:

The 8 volt can come from a zener... the current needed is essentially zero and the -0.3 from a germanium diode (as I used) although I can't see why just resistive dividers couldn't be used if the supplies to the opamp are stable. Tie the gates to the -0.3v via say 2.2meg resistors... that put's both FET's off. Then use the switch to overide that by feeding the 8 volts via say 220k

This looks very good and uses less parts than what I was thinking about.

I can't wait to try this out. Hope my local toy store has what I need to test this baby out.

Thanks again Mooly!

Obe1 · 22nd November 2010, 09:14 PM

Hi Mooly,

Please verify the voltage polarities for me. I thought an n-channel JFET would need 0.0 to say plus (+) 0.3 volts to turn on and a minus (-) 6.0 to (-) 8.0 volts to turn off.

wakibaki · 22nd November 2010, 09:35 PM

That's an N-channel jfet? That would make the control voltage polarities in the original diagram correct. If you take the gate of an N-channel jfet more than about 0.5V more positive than the source, then the gate-channel diode will conduct.

w

Obe1 · 22nd November 2010, 10:26 PM

Thanks wakibaki,

I got twisted around when I looked at Mooly's original schematic that shows gate control voltages of +0.3 volts and -8.0 volts.

abraxalito · 22nd November 2010, 11:08 PM

Quote:

Originally Posted by Obe1

Hope my local toy store has what I need to test this baby out.

You might find the JFET a bit tricky to source, its a Sony part. Doug Self's book 'Small signal audio design' has a whole chapter dedicated to signal switching circuits and goes into some depth with JFET switching arrangements. He prefers to use the J112 which is very widely available.

Obe1 · 23rd November 2010, 12:08 AM

Thanks abraxalito,

It's nice to have choices!

I'll also say that the more I try to learn about JFET's the more I get confused. For example, here are specs for several popular n-channel JFET's that might be candidates in this application:

Datasheet for 2N3819: Vgs (off) = minimum not stated, maximum +8.0 Volts

Datasheet for BF245C: Vgs (off) = minimum -0.5 Volts, maximum -8.0 Volts

Datasheet for J112: Vgs (off) = minimum -1.0 Volts, maximum -5.0 Volts

So why do some of these have a positive Vgss (off) value and others are negative?

abraxalito · 23rd November 2010, 12:17 AM

There's a very simple answer to this question

Datasheets sometimes contain errors - you've found one in the Fairchild datasheet for the 2N3819. Onsemi's is just as bad. The correct (and most comprehensive) data is found in Vishay 2N3819

Obe1 · 23rd November 2010, 12:32 AM

Thanks again abraxalito,

From now on I'll check the datasheet's from multiple manufacturers on any given part.

abraxalito · 23rd November 2010, 12:38 AM

Good move. My working assumption is this - the datasheet containing the most details is going to be the source datasheet, thus the least susceptible to errors. The others will be copies.

Calvin · 23rd November 2010, 06:18 AM

Hi,

I wouldn´t recommend JFETs as swiching devices, especially not those named here. The 2N3829 for example features an Rds(on) of 150Ohms, the J112 50Ohms. What You want is a as low and linear-over-signal-voltage as possible device. I´d rather opt for a low Rds(on) MOSFET instead or a dedicated switch or multiplexer like the ADG- and DG-series of AnalogDevices or the 460x-series (and others) of Maxim.

jauu
Calvin

23rd November 2010, 12:17 AM	#17
abraxalito diyAudio Member Join Date: Sep 2007 Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though... Blog Entries: 62	There's a very simple answer to this question Datasheets sometimes contain errors - you've found one in the Fairchild datasheet for the 2N3819. Onsemi's is just as bad. The correct (and most comprehensive) data is found in Vishay 2N3819 __________________ When a measure becomes a target, it ceases to be a good measure. C.A.E. Goodhart

23rd November 2010, 12:38 AM	#19
abraxalito diyAudio Member Join Date: Sep 2007 Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though... Blog Entries: 62	Good move. My working assumption is this - the datasheet containing the most details is going to be the source datasheet, thus the least susceptible to errors. The others will be copies. __________________ When a measure becomes a target, it ceases to be a good measure. C.A.E. Goodhart

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22nd November 2010, 09:14 PM	#12
Obe1 diyAudio Member Join Date: Jan 2010 Location: Stumptown	Hi Mooly, Please verify the voltage polarities for me. I thought an n-channel JFET would need 0.0 to say plus (+) 0.3 volts to turn on and a minus (-) 6.0 to (-) 8.0 volts to turn off.

22nd November 2010, 09:35 PM	#13
wakibaki Banned Join Date: Jan 2008 Blog Entries: 2	That's an N-channel jfet? That would make the control voltage polarities in the original diagram correct. If you take the gate of an N-channel jfet more than about 0.5V more positive than the source, then the gate-channel diode will conduct. w

22nd November 2010, 10:26 PM	#14
Obe1 diyAudio Member Join Date: Jan 2010 Location: Stumptown	Thanks wakibaki, I got twisted around when I looked at Mooly's original schematic that shows gate control voltages of +0.3 volts and -8.0 volts.

23rd November 2010, 12:08 AM	#16
Obe1 diyAudio Member Join Date: Jan 2010 Location: Stumptown	Thanks abraxalito, It's nice to have choices! I'll also say that the more I try to learn about JFET's the more I get confused. For example, here are specs for several popular n-channel JFET's that might be candidates in this application: Datasheet for 2N3819: Vgs (off) = minimum not stated, maximum +8.0 Volts Datasheet for BF245C: Vgs (off) = minimum -0.5 Volts, maximum -8.0 Volts Datasheet for J112: Vgs (off) = minimum -1.0 Volts, maximum -5.0 Volts So why do some of these have a positive Vgss (off) value and others are negative?

23rd November 2010, 12:32 AM	#18
Obe1 diyAudio Member Join Date: Jan 2010 Location: Stumptown	Thanks again abraxalito, From now on I'll check the datasheet's from multiple manufacturers on any given part.

23rd November 2010, 06:18 AM	#20
Calvin diyAudio Member Join Date: Nov 2004 Location: close to Basel	Hi, I wouldn´t recommend JFETs as swiching devices, especially not those named here. The 2N3829 for example features an Rds(on) of 150Ohms, the J112 50Ohms. What You want is a as low and linear-over-signal-voltage as possible device. I´d rather opt for a low Rds(on) MOSFET instead or a dedicated switch or multiplexer like the ADG- and DG-series of AnalogDevices or the 460x-series (and others) of Maxim. jauu Calvin