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Old 12th February 2011, 09:19 PM   #1
MondyT is offline MondyT  United Kingdom
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Default JFET experts needed!

Can any FET experts help me here?
I am trying to knock up a very simple FET buffer that incorporates a click-less mute switch on the output. This circuit is to be used for a general purpose unbalanced audio input box for a PA system, where the input can be silenced if necessary without any “pop” or clicks. The input impedance needs to be 1 Meg Ohm and the output impedance as low as possible, the sound quality needs to be as high as possible, and the power source needs to be a single 9V battery with as long a life as possible (conflicting requirements probably)
So far, I have the circuit below which functions and sounds good before the switching section (at C2 negative), but after the switch the sound looses a tiny bit of clarity and “life” if you know what I mean. The muting function works quite well, and this is achieved by disconnecting the D4 from Ground.
Can any of you guys and girls suggest what I can do or change to improve this circuit as to be honest I have no idea if the component values I have in the switching circuit are correct for good sound quality?! The FETs used are the only ones I have available unfortunately and so I would like to use these if at all possible. I know that my aims can be easily achieved using op-amps and CMOS switches etc, but I like the simplicity, low power requirement, and “sound” of the FET buffer and so I would like to stick with it. The circuit needs to drive a 20Kohm mixer input impedance.


Click the image to open in full size.

Hope you can help me and thanks

Ray
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Old 13th February 2011, 10:09 AM   #2
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The key to make a JFET switch work is to have the gate track the voltage of the source in the on-state. The gate drive circuits needs some changes. Here's what you need: http://www.seekic.com/uploadfile/ic-...3021645243.gif

Also you might want to consider paralleling some 2N5460 to lower the effective on-resistance. No matching needed.

Samuel
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Old 13th February 2011, 10:47 AM   #3
SY is offline SY  United States
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Why are you running the signal through the FET? You can accomplish the same goal by having the FET in shunt, shorting the output to ground until enough voltage builds up on the FET gate to cut it off.
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Old 13th February 2011, 01:41 PM   #4
MondyT is offline MondyT  United Kingdom
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Sam

Thanks for your time and help, much appreciated!

I have modified the switch circuit to your suggestion and added another FET in parallel and this seems to work very well in simulation with over half the distortion I was getting before! As soon as I get the time I will bread board it and see if my sound has improved and see if the mute is nice and smooth. I'll let you know as soon as can

Sy

Thanks for your reply and time also! To be honest my friend, I chose the series circuit as there are examples on the net that I can try, although none have worked as well as Sam's suggestion. Shunt seems to make good sense but my electronics knowledge is not up to figuring out the circuit, although I have learned a great deal from you guys already!

Do you guys think the component values I have around the FETs in the circuit below are OK for a stable and reliable circuit?

Click the image to open in full size.

Cheers
Ray
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Old 13th February 2011, 03:38 PM   #5
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Missed something in my first post. I think you should move C2 between the drain of J2/J3 and R9 (I presume R8 just models the mixer--otherwise you could skip it). Otherwise sufficiently negative voltage swing will turn on D4. R2 is not needed then, and R10/R12 could be made about 10x lower which should bring HF distortion down a bit. Also between C7 and V+ there should be a resistor to define the time constant.

I don't think a shunt switch will work particularly well here. To make it effective we a need significant source impedance, which is no what we want here.

Samuel
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Old 13th February 2011, 05:43 PM   #6
MondyT is offline MondyT  United Kingdom
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Thanks Sam!

I altered the circuit to below as I think you suggested, and although it improved the on performance a little (a little less distortion) when on, the muting action does not seem to be as good or working. When muted it seems to cut positive swings completely but lets about 12mV through on negative swings. If the input is increased to about 1.0vrms then the positive peaks get let through by about 450mV rms. The resistor for the RC time constant seems to have no effect when set to about 5mSecs with 1K (I think!)

Click the image to open in full size.

I have also attached the files just in case you have LTspice

Can you help?

Cheers
RayAttachment 209627
Attached Files
File Type: zip JFet_Buffer.zip (2.9 KB, 33 views)
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Old 13th February 2011, 06:23 PM   #7
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I think it has to do with the offset at J1 source. Let's simplify and temporarily replace R7 with a current source of desired value--at least 1 mA I'd say. Then adjust R4/R5 such that the source of J1 sits at about 4.5 V. How does off-state headroom look now? The actual optimum operating point will be somewhat different from 4.5 V, but also dependent on the actual transistor parameters, mostly Vp.

Samuel
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Old 13th February 2011, 10:01 PM   #8
MondyT is offline MondyT  United Kingdom
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The value of R7 (6.1K) was found using the actual J201 I have in the circuit. I just scoped up the output of the buffer (minus the switch circuit) with a 1 kHz input and adjusted a 10K pot in place of R7 to enable the largest signal input without any sign of clipping. This finally measured the 6.1K used in the sim. The J201 model in LTspice is probably not the same. I checked the J201 model and found the Idss is 594.1uA and Vgs-off is -600.4mV incidentally

With the circuit as my last post, the waveform at the output with the circuit muted and a 0.25Vrms input is as below

Click the image to open in full size.

and with a 1.0Vrms input it looks like this

Click the image to open in full size.

I swapped R7 out for a DC current source of 1.0mA and adjusted R4 to 2.47MEG Ohms to set the quiescent voltage at J1 source of 4.5V and the resulting muted output changed to the below with a 0.25Vrms input

Click the image to open in full size.

and with a 1.0Vrms input it looks like this

Click the image to open in full size.

Does this shed any light on the issue?

Once again thanks for all your time and help Sam, very much appreciated!!

Cheers
Ray
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Old 14th February 2011, 08:26 AM   #9
Elvee is offline Elvee  Belgium
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Here is a possibility (the 2N4438 is identical to the J201).
There is some distortion in the ON state, but it is caused by the follower, not by the switch: the upstream signal is shown for reference and has similar distortion (a shade higher in fact).
Attached Images
File Type: gif FETswitchON.GIF (43.5 KB, 51 views)
File Type: gif FETswitchONref.GIF (42.2 KB, 56 views)
File Type: gif FETswitchOFF.GIF (55.8 KB, 63 views)
Attached Files
File Type: txt FETsw.asc.txt (2.5 KB, 12 views)
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Old 14th February 2011, 09:10 AM   #10
MondyT is offline MondyT  United Kingdom
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Hi Elvee!

Thanks for your help my friend!

The J201 model I am using is as follows:

.MODEL J201 NJF(Beta=1.621m Betatce=-500m Rd=1 Rs=1 Lambda=2.236m Vto=-600m Vtotc=-2.5m Is=114.5f Isr=1.091p N=1 Nr=2 Xti=3 Alpha=506.8u Vk=251.7 Cgd=4.667p M=227.1m Pb=500m Fc=500m Cgs=2.992p Kf=0.6042f Af=1)

The input condition of 1Vrms is probably the maximum the unit will ever see and the majority of the time the input signal will be less than 0.5Vpk and therefore I think the distortion figures will be acceptable, as this unit will not be used for recording applications or similar. Using a J201, the sound from the buffer only is very good and I was pleased with the results. When the required muting action circuit was added, the sound suffered a little in clarity and seem to lose it’s “edge” (no doubt caused by the additional distortion that my circuit errors had introduced) and hence why I came to you guys for help.

Sam's suggested circuit changes seem to be working well in simulation and when the mute is off, the output THD is around 0.5% or so with 1Vrms input and around 0.1% for 0.5Vpk input, both figures are a big improvement. It is just a case of getting the circuit to mute quietly and reliably which is causing a few headaches right now! Unfortunately, I am now a little out of my depth with FET circuit theory and so I am at your mercy!!

Cheers
Ray
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