JFET Common Source Aplifier AC load line gain

[blah91]

Hello

I am currently struggling with a JFET common source amplifier and I was wondering how to estimate the gain from the AC load line.
And one more question what is meant by "low frequency and high frequency break points" of the amplifier.
As far as I can understand I am applying ac voltage to the jfet , not ac frequencies.
Any help will be greatly appreciated. Thanks guys.

 

[audioguru]

You forgot to post the load line.
It shows the output voltage and current swings, not the input so it does not show gain. If it shows many curves of input voltage then it is easy to see the gain.
The gm of the Jfet determines the gain.

AC voltage can be at any frequency.
RC networks as input and output coupling capacitors determine the low frequency breakpoint. The datasheet for the Jfet lists its high frequency gm.

 

[blah91]

Thank you , finally manage to find my way through it.
Just one last question , apparently there are some "stray capacitance" in my circuit that can be modeled as capacitance between the drain pin of the transistor and the ground. Any idea how to include this thing in my small signal model or how to measure it/

 

[audioguru]

All circuits have stray capacitance in the wiring. Circuits built on breadboards have much more stray capacitance that is between all the wires that are all over the place and between the many rows of contacts. Therefore a circuit built on a breadboard doesn't work with a frequency above about 1MHz (the middle of the AM radio broadcast band).

Don't forget about the very high capacitance in a Mosfet between its gate and its source because it is made with thousands of little Mosfet cells in parallel.