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

[Terminator3]

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

 

[biff44]

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.

 

[Terminator3]

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?

 

[biff44]

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

 

[Terminator3]

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