Impedance matching on RF stages of vlf RX

[neazoi]

Hello I have designed this VLF RX for whistlers reception. I have joined together two projects (two fets) from different circuits.
The output volume of the first fet is ok for my ssd recorder.
when I connect the second stage fet, the output volume goes down very much. Being a twin notch filter, the second stage, it works ok, but it attenuated much the total volume as well. I would expect the total volume to be at least equal to that of the first stage alone, so that is why I believe there is an impedance mismatching somewhere.
I believe this is an impedance matching problem, either on the input of the first stage or in its output.

I have no idea what is the output impedance of the first stage or the input impedance of the second stage, and I do not know how to correct this.
Any suggestions???

 

[Ow@i$]

My suggestion would be to draw this stage in ADS (advance design system) , check S22 of first stage over your frequency range, take impedance value at center of your frequency band, note it down..
Now draw the second stage and check S11 now, note center value this would be the input impedance of second stage, write it down.
Now through "Smith Chart Matching" utility of ADS, you can have your matching network, which actually transforms your output impedance of first stage to the input stage of the second stage..
But brother you need to first study about matching basics..

I hope someone has a better solution

 

[neazoi]

My suggestion would be to draw this stage in ADS (advance design system) , check S22 of first stage over your frequency range, take impedance value at center of your frequency band, note it down..
Now draw the second stage and check S11 now, note center value this would be the input impedance of second stage, write it down.
Now through "Smith Chart Matching" utility of ADS, you can have your matching network, which actually transforms your output impedance of first stage to the input stage of the second stage..
But brother you need to first study about matching basics..

I hope someone has a better solution

Yes of course, but this requires ADS (very expensive) and knowledge of how to use it. Very accurate matching is not needed here. If only I could define the components (probably resistors) that affect impedance, I could replace them with potentiometers and find the best matching values.
any suggestions?

 

[Ow@i$]

Hmmm ....then hope someone will have us both in finding solution
Don't you think if you replace your 100pf capacitors at first stage output with variable capacitor, and note your output by varying capacitance would do some good !!!!

 

[neazoi]

Hmmm ....then hope someone will have us both in finding solution
Don't you think if you replace your 100pf capacitors at first stage output with variable capacitor, and note your output by varying capacitance would do some good !!!!

no, the first stage is self biased, whereas the second stage is voltage divider biased. I think I will have to alter the bias resistors accordingly in the second stage, but I am not sure if this will help.

I believe I have to reduce this 1mohm resistor at the second stage G2. This will increase the voltage on G2 and increase the gain of this stage.

 

[neazoi]

I have found the problem. it was the drain and source resistors of the second stage. maybe the transistor was starving. when I changed them to similar values like the first stage resistors, it worked loudly again.
Changing the gate resistor 1M, was not good.

BTW, how should I measure the current flowing through each fet? Is it enough to measure the voltage drop on the source resistor? what about the drain one?