JFET signal shaper for simple frequency counter

[Tukan]

Hello,

I am posting two pictures.



On Picture 1. is the JFET driving PNP BJT through voltage drop on R1 - this is common circuit how I found on internet by many similar variants.

On Picture 2. is the same JFET, but driving NPN BJT through voltage drop on R2 - I would rather use this variant because of presence NPN tranzistor, which is much more simple to obtain for high frequencies (I want measure up to 220 MHz) - but I am confused about using it because sources I read were describing only just driving BJT through R1 - on Drain side.

Can somebody give me opinion about Picture 2. , whether this circuit is usable please?

Thanks in advance.

 

[vfone]

With the given components values R1 have higher gain and lower output impedance.

 

[Tukan]

Hello. Component values are not important for this moment. I just only would like to know if I can use the Picture 2. circuit - if I can use R2 (the Source side) for BJT NPN biasing (driving). Because I have only seen the Drain side as output in all other schematics.
Many thanks.

 

[vfone]

You can use R2, but have to add to the schematic an emitter resistor to the NPN transistor (few hundreds of ohms) decoupled to the ground with a capacitor.

 

[Tukan]

Thank you for suggestion. Can you explain me please the advantage of adding this R and C ?

 

[vfone]

To get better bias point for the NPN, in terms of gain, linearity, thermal, etc.

 

[Tukan]

ok, thank you.

 

[Tukan]

Hello, I am here again with one more question.

Can somebody explain me the advantage of coupling transistor with gate through capacitor on the picture below, instead of tying them directly with wire? (the mentioned capacitor is pointed by arrow)

 

[Tukan]

some opinion to picture above please

 

[ragav777]

Thank you..