Next report - Jfets and MosFets

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,446
Hi...

I'm back to these jobs...

I was asked to do a simulation of a BF245C, when Vgg going up from 0V up to 6V and Vdd of 10V, plotting the Id(Vgs) curve.

I would like to know what is the difference between the 2 simulations attached and why aren't they the same!
 

Attachments

Last edited:

tshuck

Joined Oct 18, 2012
3,527
One is a DC sweep simulation, the other (first one) is a transient simulation. It also appears the .step isn't referencing a particular parameter in the transient analysis, however, I don't have LTSpice in front of me, so I can't say definitively.

The DC sweep would make more sense for your simulation, since you are interested in sweeping the DC value at the gate.
 

crutschow

Joined Mar 14, 2008
23,141
You don't have the .param source specified (use Vgg in place of value) in the first sim.
 
Last edited:

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,446
OH yes... The print screen got messed... I'm going to replace the screen... Just a sec!


Edit;

Ok, files replaced.

What I want to know is why can't I get a similar plot with either the simulations!
 
Last edited:

crutschow

Joined Mar 14, 2008
23,141
You circuit works for me (I had to change the JFET model since I don't have a copy of yours). But it gives a series of stepped currents, not a smooth curve.
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,446
You circuit works for me (I had to change the JFET model since I don't have a copy of yours). But it gives a series of stepped currents, not a smooth curve.
How come, mate?

Edit;
By the way, I got my fjt file from here!
 
Last edited:

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,446
You circuit works for me (I had to change the JFET model since I don't have a copy of yours). But it gives a series of stepped currents, not a smooth curve.
How come, mate?

I was trying to get the same result with both simulations!
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,446
Ok, I need help to a small detail.

What can I say about the gate and drain current when we change the temperature to 100ºC in a circuit like the one attached???

I check the values for -4.62V(V inch-off). The Id dropped from 100μA to ~70μA.
 

Attachments

Last edited:

ericgibbs

Joined Jan 29, 2010
8,581
hi,
Rds is Resistance between the Drain and Source of the FET,

Rds is a function of the Drain current and the Drain voltage, so the Rds changes as the Vgs changes.

E
 

Attachments

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,446
And doesn' it depends on what zone the FET is? Can I always say the Rds "on" = Vds/Ids????

Can I do the same to a MOSFET?
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,446
Hi again guys...

I need help to answers a few questions, if you can help!

In a circuit like the one attached, why is it used an alternated voltage source and why not a DC fixed voltage source?

Why the diode D1 is needed?

Could we use a triangular or a pulse wave form in input????

I need help to answer these 3 questions!
 

Attachments

#12

Joined Nov 30, 2010
18,076
In a circuit like the one attached, why is it used an alternated voltage source and why not a DC fixed voltage source?

You will have to ask the person that designed that circuit.

Why the diode D1 is needed?

To avoid sending the wrong polarity to the transistor.

Could we use a triangular or a pulse wave form in input????

Yes.
..........
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,446
Could you be a little more specific????

About the first question, the teacher made us that question, so we are the ones who has to answer, not the teacher.

About the last question, I ask you to justify why is "yes". Can you help?

I have this report hung by these questions.

Thanks
Psy
 

#12

Joined Nov 30, 2010
18,076
Well, the function of the circuit is to allow a jfet with a variable Vgs to conduct current (up to 163.7 ma peak) from a 12 volt RMS sine wave to ground. With no part number for the jfet, I can't say whether the jfet is the limiting factor. Some of them can't conduct 163.7 ma.

Conduction can only happen in one polarity because the jfet is only one polarity. The diode is added to refuse any of the wrong polarity voltage from the transformer to arrive at the jfet. As you change the Vgs with Rpot, a different amount of current (maximum) will flow when the applied voltage is the correct polarity. You can drive the drain with any wave shape and any polarity you want, as long as it does not exceed the rated voltage of the jfet. You can drive the gate with any wave shape you want, as long as it's negative with respect to ground and does not exceed the voltage ratings of the jfet. At any given Vgs, the jfet will allow up to a certain current limit that is directly related to the Vgs.

These are simply the rules about how a jfet works. This circuit is a variable current limit drain for a positive voltage. I still have no idea why anybody would want to use this circuit.
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,446
I'm sorry... I'm using BF245C...

The circuits we are testing are only academic examples to understand the basics of FETs and MOSFESts. So, probably no usage to them! Just to observe currents and voltages behaviors...
 

ericgibbs

Joined Jan 29, 2010
8,581
hi Psy,
Reply to your PM.

This a LTSpice sim shows some curves, but what information you can read into the sim is limited.:rolleyes:

Which parameters are you required to measure.?

E
 

Attachments

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,446
Hi Eric...

At this point I was looking for an explanation for "why are we using an AC voltage source? Why can't we use a DC voltage source and if could we use a pulse or a triangle wave form in the circuit!"

But I think I already answered those question. Now I have 2 last questions but are related with the measurements we have made in laboratory last week and that we are not being able to match them with our calculations.

We are using the Id = IDss(1-(Vgs/Vp))^2 but we can't get even close results...
But don't worry... I think a friend of mine can repeat all lab procedures and later today he will make all measurements again so that we can check if everything is ok.
 
Top