Funny little circuit

[M. Hamed]

I'm probably on my fifth or sixth reread of the AoE FET chapter. At the end of the chapter, there is "Bad Circuit Ideas" section which I love because it gets you to think!

Now on page 173 figure 3.78D there is a JFET inverter with a PFET on the high side and NFET on the low side, and Vin goes from zero to +VDD. I reasoned that obviously it won't work because the FETs get forward biased.

However I thought what would happen if the FETs are reversed and the NFET is up and the PFET is down. It took me so long to understand the circuit even after simulating it, I had to think hard why it's behaving the way LTSpice says it should behave.

I think anyone who can try to predict the behavior of this circuit without simulation is someone who understands FETs so well.

But again, it could be that I'm just too much of a newbie

 

[Helmut Wabnig]

On Fri, 27 Feb 2015 20:53:19 -0800 (PST), "M. Hamed"
<mhdpublic@gmail.com> wrote:

I'm probably on my fifth or sixth reread of the AoE FET chapter. At the end of the chapter, there is "Bad Circuit Ideas" section which I love because it gets you to think!

Now on page 173 figure 3.78D there is a JFET inverter with a PFET on the high side and NFET on the low side, and Vin goes from zero to +VDD. I reasoned that obviously it won't work because the FETs get forward biased.

However I thought what would happen if the FETs are reversed and the NFET is up and the PFET is down. It took me so long to understand the circuit even after simulating it, I had to think hard why it's behaving the way LTSpice says it should behave.

I think anyone who can try to predict the behavior of this circuit without simulation is someone who understands FETs so well.

But again, it could be that I'm just too much of a newbie

Yeah, Newbies talk and forget to link to the source.
Upload a copy of the circuit to any image hoster
and place a link here.

Damned newbies!

w.

 

[M. Hamed]

On Saturday, February 28, 2015 at 12:55:00 AM UTC-7, Helmut Wabnig wrote:

Yeah, Newbies talk and forget to link to the source.
Upload a copy of the circuit to any image hoster
and place a link here.

Damned newbies!

w.

Newbies need to see the circuit, experts can imagine

Here it is:

https://www.dropbox.com/s/fi2pd3sl3amz88p/JFET_inverter.PNG?dl=0

 

Bout the only thing you could use that for is an ersatz inverting Schmitt trigger like device. Also, you will need more input voltagge than the supply, and therefore more than the output. If you want to do that, use a transformer to get the pinchoff voltage of the FETs.

Know what ? That might just make a dandy oscillator. In fact the Wavetek, like 103, 111 and such, their circuit had someting looked like that. I know right away this thign has to be in onbe state or another or it will destroy itself.

The Wavetek impressed me alot with the precision of its technology, FROM THE 1970s ! The thing runs on a triangle wave which is developed by an electronically controlled current source into a precision capacitor. That feeds something that detectcts the slope and goes into one state or the other and generates a square wave. That square wave is what triggers the triangle wave generator and that completes the positive feedback loop. Look it up, the manual is on BAMA.

Took me a little while just to understand how that Wavetek circuit works. It does have a pretty good theory of operation text in the manual but you know how it is, when you can envision it yourself is when you really know how it works.

And the sine convertor is a blast. It is the bomb. I literally variably compresses that triangle to make a sine. Down to 0.5 % distortion.

Mu buiddy has a 103 with pretty much the same circuit. His had a compression at the top of the sine and it was pointy at the bottom. There was a DC offset in the oscillator. I temporarily fixed it by inserting a capacitor in there to remove the DC. There are DC adjustment pots but they didn't have enough range. I wanted it off the bench, he wanted it off the bench so that worked for now. Later he decided to fix it right and now it is all in piecesparts. Whatever, mine still works.

Have fun with this, tha tis the best way. Keep this thread bumped up and I will come up with a few resources forya. Links etc.