When I search for "FET preamp", like here:
https://www.google.com/search?q=fet+preamp&tbm=isch good half of the schematics shows that there is no pull up resistor from gate to VDD, for example like the one in attachment.
When I make these (in breadboard or in simulator), they never work. Which is kinda logical as FETs usually require some minimal gate voltage (around 1V similar to bjts). When I put pull up resistor (e.g. 3M pull up, 3M pull down which puts it to the middle of VDD) it start working.
My question is, what am I missing here? Good half of the schematics shows no pull up. I've tried them all, they never work. Am I missing some crucial bit of information that prevents me from making them work? I just don't understand how half of the schematic can be wrong, so I assume they are correct and I'm doing it wrong. Thanks.
Edit: I'm using BS170 (fet, N-Channel, Enhancement Mode, BS170)
The circuit diagram shows a JFET. Essentially all JFETs are depeletion mode and they thus work OK in the linear region with zero gate votlage.
The BS170 is a enhancement MOSFET and thus need some postive bias to work. If it needs to by a MOSFET, there are also depletion mode MOSFETs (like BSP135 ( Microchip also has a few in To92).
Strictly, in this circuit they work with negative gate-source voltage because source is elevated above ground due to voltage drop across the source resistor. The value of that resistor regulates operating current (which also depends on characteristics of the transistor: Ic vs Vgs; different JFETs will yield different current with the same resistor).
While people do make a habit of calling both "FET", MOSFETs and JFETs are not really interchangeable, if you search for a "FET preamp" you are going to mostly get JFET designs.
You may find this presentation helpful in understanding how a JFET works and the differences from MOSFETs.