JFET Biasing in Cascode: Designing a High Input Impedance Differential Amp

[SadScholar]

Hi, I'm trying to design a simple differential amp with extremely high input impedance. I'm using a couple of JFETs at the input stage to accomplish this. However, because I'm sending the signal from there to a differential amp, I don't want any DC offset, but I'm a little bit confused about how to bias J1, the first JFET. Basically my question is, should the blue wire be there. Even assuming I am dealing with two JFETs on the same silicon chip, and using the same value for both resistors, if I leave out that blue wire, I get a DC offset. But if I leave that blue wire in, I get almost no current through the bottom JFET and resistor, which is confusing to me. Any insight would be appreciated.
Here is the circuit diagram:
http://i516.photobucket.com/albums/u324/chrysolite13/CascodeCircuit.png

 

[berkeman]

Hi, I'm trying to design a simple differential amp with extremely high input impedance. I'm using a couple of JFETs at the input stage to accomplish this. However, because I'm sending the signal from there to a differential amp, I don't want any DC offset, but I'm a little bit confused about how to bias J1, the first JFET. Basically my question is, should the blue wire be there. Even assuming I am dealing with two JFETs on the same silicon chip, and using the same value for both resistors, if I leave out that blue wire, I get a DC offset. But if I leave that blue wire in, I get almost no current through the bottom JFET and resistor, which is confusing to me. Any insight would be appreciated.
Here is the circuit diagram:
http://i516.photobucket.com/albums/u324/chrysolite13/CascodeCircuit.png

Your blue wire would appear to short the input to the output, no?

 

[SadScholar]

Hah, that explains why it removes the offset.

 

[vk6kro]

There should be high value resistors from the gate to the bottom of the bias resistor for both FETS. So cut your blue wire and put a 100 K resistor in there.

But wouldn't you drive the bottom one and bypass the gate on the top one?

This would give more gain.

 

[skeptic2]

The attached is the schematic of TL081 JFET Opamp. Does that help?