Help Troubleshoot 2 FET amp.

Thanks for taking another look.

I don't see where I posted new voltage readings after I fixed the layout. But, you are correct that my J271 source is lower than the gate. J271 G=5.2v, Source=5.1v, D=0v J301 G=4.2v, Source=6.8, D=12. To be more precise, there is 10Meg meter loading, calculating that out, J271 G=5.4v, Source=5.1v, D=0v J301 G=4.6v, Source=6.8v, D=12v.

I don't have enough knowledge to know that J271 Gate must be higher than the source. It is conducting and has 0.7mA of current flow.

I hesitate to rework it, (yet), this was a production unit and did work as drawn. I think the original design had a 100 ohm instead of 220 ohm.

Here's a write up about the design and a pissing contest from this author with another designer.

Specifically the first paragraph. It has this line, "Since the gate voltages are independently set, it's possible to arrive at the correct bias voltage to make the P and N FET's operate at a point where the voltage drop across each is at the desired 50% or so of Vdd.? I certainly can't make that happen!

Thanks, Mikek

I just notice you said "The gate of J 271 must be positive with respect to the source" It is! But not on the J301. ?

Because one is a P channel and the other an N channel. But the P channel one is "upside down" which can confuse the issue.

I need context on that statement, do you still think the voltages are improper? It seems to be working, as I said it does have -6db gain. But when I hook up my long wire antenna, I get a 1 to to 2 units increase in the AM band over the long wire alone and a beacon at 326kHz has a 3.5 S unit increase over the long wire. The gain is from the from the active impedance conversion. Mikek

l one is "upside down" which can confuse the issue.

The gate voltage must be positive with respect to the source. It's possible that it is working upside down, that is using the drain as source and vice versa, if so it is not in the proper operating range. Do you have harmonic s in the output ? (I mean that aren't in the input)

If not then the P channel is probably acting as a resistor and might as wel l be one. Maybe the original designer intended that, but the reason is beyo nd me. People have done stranger things. If that was the idea then someone around here with a better grasp of this please explain why anyone would do that.

That Dallas Lankford article shows two gate bias adjust pots, in your first post your schematic shows a pot only for the J310 with the J271 at fixed 6v bias (1meg to 0 and 1meg to +12). I think if you could tweak the J271 gate bias with a second pot you'd find it possible to get where you want to be.

piglet

d it possible to get where you want to be. "

Yup, but then wouldn't the adjustment where I have it automatically have th e setpoint where you want it ? The resistors going to ground and + 12 would be equal. It would operate pretty much like the bias adjustment in an audi o amp except that if the pot (rheostat actually) opens up it decreases the current and doesn't destroy things.

Not really because the J271 and J310 are not really matched complements, the J310 datasheet Idss is higher and Vgs(off) lower than the J271 so getting equal Vds headroom on both devices may be impossible without truly independent control of gate bias. J-fet parameters have really huge production spreads.

piglet

I may have that article, but can you post a link?

Actually it's 1Meg to +12 and 820K to 0v.

I'm pretty sure the voltages are exactly where they should be.

As it is now, if instead of the 200 ohm resistor I had 2-100 ohm resistors in series, the middle point would be at 5.95 volts. Wait, OK, I modified the circuit, it now has 2 series 100 ohm resistors instead of the 200 ohm. With B+ at 12.0v the center point of the 2-100 ohm resistors is 6.0V J310 G=4.36v D=12.0v, S=6.90v, J271 G=5.18,V D=0V, S= 5.14V.

Caveat, I'm using a 10Meg DMM. It does load the Gate bias.

Doing the math to correct for the loading, I get, J310- G=4.73v, S=6.90v, D=12.0v, J271- G=5.39V, S=5.14V, D=0V.

Voltage Drain to Source of the J301 is 5.1v Voltage Drain to Source of the J271 is -5.14v

I don't know how separate bias adjustments could make anything better.

I'm open to disagreement, but as I've said the amp works, I can't find any harmonics, I have other feedback that the voltages are correct. I tend to agree. Mikek

You are correct. Those voltages are just dandy!

Didn't you just implement the bias adjustment by changing 1meg to 820k?

piglet

Nope. Oh, I see the confusion, caused by me. There seem to be several iterations of the schematic. I unknowingly posted the one that does have 1 Meg and 1 Meg in my OP. I'm working from one that uses 820k. Note the schematic in your last post does suggest you can replace the

1Meg pot with 820k with almost as good IP2. I have no way to measure IP2 or IP3 so... Thanks, Mikek

All of the iterations make it hard for me to follow this thread. Lankford drops the J271 pot from the 3/11/2014 iteration of his schamatic. [1]

Note.

1. formatting link

Thank you,

--
Don Kuenz, KB7RPU

/lankford/J310%20-%20J271%20FET%20Follower.pdf

I'm confused by that reference. The last papers I have by Lankford are 1-9-2013 and it is on a phaser. The other 5-06-12,it is an active array which does have the 2 FET amp with an 820k resistor and a 12V regulator. I searched the

Sorry clicked to soon.

I'm confused by that reference. The last papers I have by Lankford are 1-9-2013 and it is on a phaser. The other 5-06-12, it is an active array which does have the 2 FET amp with an 820k resistor and a 12V regulator. I searched the okdxf site by adding Lankford, it said I wasn't authorized. Once he pulled his stuff off Yahoo, I haven't seen anything from him.

Mikek

There are opamps that would work too.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Are you ready for more confusion? Although the heading of the pdf at the link says "3/11/2014" the schematic embedded within it says "1/23/2011." google doesn't want to be my friend today. It's complaining about unusual traffic from my network. Be that as it may, when duckduckgo.com is used to search for lankford site:okdxf.eu the pdf shown above appears on the second page of /my/ results. YMMV.

Thank you,

--
Don Kuenz, KB7RPU

Don, Thanks for that search term "lankford site:okdxf.eu" There are a few pages I find with that, that I don't have. I used Google and it worked just fine. I built a couple a Dallas's version of the Norton noiseless feedback amplifiers from this article,

he sent me around 1995. I also built the Slotka/Lankford phasor unit and antenna system at the same time, had a lot of fune with it. Reliving old times! :-) Mikek

John, Would they have the high intercept points? IP2 ~ 88dbm, IP3 ~ 41dbm Mikek

What's the frequency?

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com