Exploring the world of 3DQ or the mystery of zero bias devices

If you say zero bias in the wrong places they will start collecting fire wood and a stake. Let's don't go into this with a closed mind. After all the manufacturer named these devices. Even if the device is not zero bias the term is being used to define a new class of device. Now that I've set the ground rules here we go.
Hold on it will be a quick run threw without a lot of discussion. I will try to let the datasheets tell the story.

UHF ,high gain, low noise, and dual gate. It's a cascode amplifier. Look at the drain current? Plus or minus 30ma? What's up with that? Can the drain conduct in either directions? That's what it says. Let's turn the page and see what else we see.

Look at the center graph at the top of the page. It conducts at zero volts bias and can respond to positive or negative gate voltage. Ummm! It's and enhancement depletion device! As I said above it's a new family of devices. Let's look at a 'normal' MOSFET for a moment and compare.

We don't have to look at much more than the symbol. The 2N7002 has a protection diode built in to keep the drain current from reversing. If you use a MOSFET in an inductive load it needs a protection diode to keep the inductive kick back from destroying the device.

The depletion MOSFET conducts with no bias and can be biased into higher or lower conduction. This is accomplished by doing an extra step is the doping that enhances the channel. While this is not new and exciting technology it seems when it was new it failed to be exciting enough to become common place to us hobbyist. Now for the good news. These devices are still being made. A quick check on Ebay for 3DQ finds some at $5.50 each. Seems they are obsolete.  But a the depletion MOSFET is alive and well.

This circuit is built using a DN3545 MOSFET. The schematic is for a 'normal' enhancement MOSFET. It would require biasing to conduct. I made the circuit with a Depletion N channel 3545. I did use a 1 meg gate resistor but no forward bias resistor. My plan is to build a ham receiver 80 and 40 meter. I'm sending it a 4 Mhz signal in the test. So it requires no bias and works at 4 Mhz. I think the term zero bias would apply. Now that we know what to look for we can find 3DQ equivalents a lot cheaper. I think I paid 10 cents each for the last ones I bought.
Just look for Depletion in the name.