JFET Analysis & Use of LTSpice

Thread Starter

Sh3rlock44

Joined May 7, 2020
7
Good Afternoon,

Hope everyone is keeping safe and well.

I have found myself stumbling across this forum in search of information to help me understand a particular homework assignment. I understand the rules in place and don't want the question answered for me but would like a gentle nudge in the right direction. I am studying Instrumentation Electronics through open uni and am currently on a module dedicated to Diodes, Transistors and Amplifiers. Its been a long time since I completed Basic Engineering Analysis and my Electronics core module so some of the valuable knowledge required is a little rusty or has been replaced with other less useful information (im finding returning to education quite a challenge).

Anyway, I have nearly completed all of the required units to achieve my HND but it seems I have saved the hardest for last. I am on assignment 2 and im struggling to understand or work out the calculations required to answer the following question. Also, my Spice experience isn't the best for circuit analysis. I have tried to contact my lecturer but with whats going on in the world, I'm not sure if he's working at the moment. Anyway, see question below;

q5.JPG


I have built said circuit;

Capture.JPG

Thanks in advance for any help offered. Much appreciated.
 

crutschow

Joined Mar 14, 2008
24,987
Do you understand anything about that circuit, as to what the three resistors and JFET do?
Do you need a tutorial on that?

What are equations 1 or 2 of Lesson 4?
 

Thread Starter

Sh3rlock44

Joined May 7, 2020
7
Hi,

Thank you for the speedy response.

I feel like I have a decent understanding of the circuit and JFET operation but going over the basics again might help me with working out the characteristics.

Equations provided are;

equations.JPG

Sorry if this is pretty basic to some. I've not struggled with any other unit, am just finding this one overwhelming.
 

Thread Starter

Sh3rlock44

Joined May 7, 2020
7
Ok, this is what i've done so far (see attached). Unfortunately this is where my knowledge of spice is holding me back because I am not sure how to utilise it to give me values for Vout and my voltage gain. Take a look at picture 4, am I correct in assuming this is my Vout? Therefore in order to calculate my gain is it just change in vout/change in vin?

Sorry if im sounding abit dull.

page1.JPGpage2.JPGpage3.JPGpage4.JPG
 

Alec_t

Joined Sep 17, 2013
11,309
I suggest you get into the habit of giving your circuit nodes labels, using the label tool. Otherwise we have to guess where nodes 001 and 002 are in your circuit.
Here's an example :-
jfet.png
In this circuit the input voltage is between nodes g and gnd. The output voltage here is developed across R1, i.e between nodes V+ and d. You can plot that by running the sim then clicking and dragging the voltage probe between V+ and d.
The LTspice Help is a good starting point for developing your Spice-fu, and there are many tutorials online.
 

Thread Starter

Sh3rlock44

Joined May 7, 2020
7
Taken that on board and this is what i've come up with, although I am a little confused as to which Vout value I should be using?

jfet1.JPGJFETvout.JPG
 

Alec_t

Joined Sep 17, 2013
11,309
I am a little confused as to which Vout value I should be using?
If you really want the DC operating point of Vout then the DC analysis gives that (2.27V). If you want to work out the AC voltage gain then the Transient analysis enables you to determine the peak-to-peak voltage across Rd. Divide that by the peak-to-peak value of Vin.
 
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