Dear friends. I have made AC analysis for a simple 2N5486 based common source amplifier. The circuit is below:



My LTspice AC analysis of the circuit delivered the following result:



I was surprised to see magnitude plot having the magnitude increasing with frequency increase. I do not understand what to make out of it!
Next, I replaced the 2N5485 with the 2N5485, and got the plot below:



This time a got a plot shape that I expected to see.
Besides the above-mentioned transistors, I also tried simulating various JFETs in same circuit. The models for is transistor were provided with my LTspice software. I found that some of the transistors have plot shapes like the 2N5486 but the others have plots similar to 2N5485.
Please explain what is causing this strange 2N5486 plot. Why the plot for the 2N5486 is different from the plot for 2N5485?
What does it tell about this transistor’s properties?
What does this strange plot shape tells about the application of 2N5486 type transistor vs. the application of type 2N5485 transistor?
What will happen at high frequencies in areal circuit having the 2N5486?
Thanks.

 

hi bis,
Please post your LTS asc file.
E

 

Dear friends. I have made AC analysis for a simple 2N5486 based common source amplifier. The circuit is below:

View attachment 355217

My LTspice AC analysis of the circuit delivered the following result:


View attachment 355218
I was surprised to see magnitude plot having the magnitude increasing with frequency increase. I do not understand what to make out of it!
Next, I replaced the 2N5485 with the 2N5485, and got the plot below:

View attachment 355219

This time a got a plot shape that I expected to see.
Besides the above-mentioned transistors, I also tried simulating various JFETs in same circuit. The models for is transistor were provided with my LTspice software. I found that some of the transistors have plot shapes like the 2N5486 but the others have plots similar to 2N5485.
Please explain what is causing this strange 2N5486 plot. Why the plot for the 2N5486 is different from the plot for 2N5485?
What does it tell about this transistor’s properties?
What does this strange plot shape tells about the application of 2N5486 type transistor vs. the application of type 2N5485 transistor?
What will happen at high frequencies in areal circuit having the 2N5486?
Thanks.

Per ericgibbs request I attached my .asc files.

 

hi bis,
The two jFET devices have different Von's
E
Note the Source and Drain voltages.

model 2N5485 NJF(Is=.25p Alpha=1e-4 Vk=80 Vto=-2.0 Vtotc=-3m Beta=3.5m Lambda=10m Betatce=-.5 Rd=10 Rs=10 Cgs=4p Cgd=4p Kf=3e-17 mfg=Siliconix)
.model 2N5486 NJF(Is=.25p Alpha=1e-4 Vk=80 Vto=-4.0 Vtotc=-3m Beta=4.0m Lambda=10m Betatce=-.5 Rd=10 Rs=10 Cgs=4p Cgd=4p Kf=3e-17 mfg=Siliconix)

 

Thanks Eric, I see the difference in Vto values. However, how does it affect the magnitude plot? It is my understanding that the "Vto" is the pinch-off voltage for the JFETs. Both circuits are not operating around this point. I have simulated both circuits with signal voltage of 0.001V and it did not make any difference in Magnitude response. It must be something else.

 

Hi bis,
As you can see on the ‘bad plot’ right side jFet the Source to Drain voltage is only 0.5V, that’s why the Bode plot is incorrect.
Reduce the Drain resistor to say 270R and rerun the analysis.

The other point for the AC value when running an Analysis is to set the AC to 1V, this gives a direct plot of the actual dB range.

To display the actual DC voltage on any point of the circuit, place the cursor on the test point and right click the mouse, then select the '88' option on the pop-up menu.

E

 

Thanks a lot Eric.
It is my conclusion that that my quiescent operating point for the 2N5486 in my schematic ended up well inside the triode region of the JFET. That is why my magnitude plot got so weird.
Is it correct?

 

hi bis,
With only 0.4v across the jFet Drain to Source, there was insufficient voltage to enable the jFet to operate correctly. By reducing the Drain resistor to 270R this increased the Drain to Source voltage to approx 2.7v, so the jFet could work correctly.

E

 

Thanks a lot Eric