[SOLVED] Dual Gate MOSFET Biasing to act as an AGC for RF generator.

[Especialista-B4A]

Hello all!
I'm now trying to build a RF generator to be used as a VFO later.

The IC I'm using to generate RF has a variable output level, accordingly to the frequency being generated.

The main idea is to use a dual-gate MOSFET (like BF982, BF961, BF962, BF963 or BF964) to control the final output stage, keeping its voltage level constant.

So, while the RF is generated and put in one of the MOSFET gates, I would use the another gate as an output sensor (tied at drain), performing the AGC of the output.

So, the questions is: How to make properly biasing of the MOSFET to act as described?

Can someone, please, guide me through the math involved in doing this? (Formulas, schematics, articles, anything that could enlight me?)

Just reading the MOSFET documentation wasn't enough.

Thanks in advance!
View attachment BF982 - BF961-962-963-964 - N–Channel Dual Gate MOS-Fieldeffect Tetrode.pdf

 

[chuckey]

Look at fig 8. with 0V bias on G1, changing the bias on G2 from +4V to -1V reduces the gain from +6 to -38 dB. Both gates are limited to +8V wrt source. So feed G2 via a 100K to a potential divider set to some thing up to +4V. That limits the max volts on G2. From the free end of the 100K connect to the collector of a hi speed transistor, emitter to earth, connect the base to the RF output, and put a decoupling capacitor on the collector. The RF output will switch the transistor on, it will draw current, its collector volts will fall, so will the G2 of the FET, so its gain falls and the output level. Some fiddling around with the DC bias on the transistor will be required in case of your RF not being high enough in level and not changing the transistors collector current.
Frank
Frank