Why without input turned off my multiplication FET oscillates? (using Class-C bias)

Hello!

After tuning my FET multiplier i found out some disappointing fact: When gate is fed with something about 1GHz, there is good strong output of multiplier. But when gate is not fed with low-freq signal multiplication stage still(!) provides some output. What can i do about that? I am using huge Rs to obtain good pinch-off voltage on gate, can it still oscillate in Class-C? Is it normal, need adjustment?

Thanks

If the gate is truly pinched-off when there is no input, then it can not oscillate. So I would guess that it is NOT pinched-off as biased.

In any case, you have an unstable circuit, and you need to investigate it to change the input or output matching network to make it unconditionally stable.

Can i bias this transistor near pinch-off without using negative power source?
For example, power source of 3.3v, some resistance on gate to ground for maintaining Vg=0v.
From my experiments:
Adding resistor Rs=50 Ohm Rs=100 Ohm - oscillates as hell.
Rs=300 Ohm very weak oscillation, good big harmonics after feeding input with low-frequency signal.
Rs=500 Ohm can't see any oscillation using diode detector, some poor harmonics after feeding input signal.

I guess best result is with load line I=U/R=3.3/300= 11 [mA]. But Rd=0 Ohm, and all resistance put to Rs to obtain Vgs nearer to pinch-off.

Maybe someone can suggest how i can bias this transistor near pinch-off? Using resistance is preferred. Would be happy to hear about some current production easy buying simple negative power-source IC to get about -1v.

Temporary device is NE3210S01, i use it as it is pretty good characteristics and price for learning purposes.

New problem: i tried to use 1.5v battery to ensure pinch-off "-" to gate, "+" to gnd (source grounded). Vd=50 Ohm (similar to **broken link removed** ). In some cases i get very strong "noise"? oscillation. How it can be?

there are ic's that can generate a negative voltage from a positive supply to bias up a fet's gate, such as ZNBG3000

Thanks!
I still have some strange problem. I would be happy if somebody can help me with this graph:
View attachment 89209
i am using potentiometer with 1.5 battery.
Rd=50 Ohm
Rs=0 Ohm
Rg=100 Ohm -> and connected to potentiometer (voltage divider for 1.5v battery).
Turning potentiometer from -1.6v to 0v i get oscillation at -1.09v. Is it normal and it is just oscillation because of wrong matching in class-B, and can be fixed. Or it is wrong? Wanted to add: power supply is 3.3v, potentiometer connected to 1.5 bias battery and then to gate through additional resistor