LTSpice: Adding an Ideal BJT NPN Transistor to Your Circuit - Tips and Tricks

[SpartanG345]

I am just wondering, how do we add an ideal BJT npn transistor to an LT Spice circuit?

From the component button i have added an 'npn1'

will the default values be ideal? And what if you want to change the beta for the transistor how do you do this.

I found a file called standard bjt, it seems to have all the model properties of the transistors that appear when your right click the bjt and click "pick new transistor" Unfortunately there is no ideal bjt in that list.

eg 2N2222 is 1st on the list.

do we have to create our own ideal model of a BJT? eg with Va at inf and Beta = 100
Is there one which we can download?

 

[vk6kro]

Yes, you can do it.

With a text editor, copy the parameters for the 2N2222 and paste the new line at the top of the list with a new name. Maybe call it 2N2222-100 if you intend to give it a current gain of 100.

Then modify the parameter BF= 200 to BF= 100 or whatever.

 

[Redbelly98]

I think the OP wants a transistor where beta is a fixed value as long as the transistor is in the linear region, i.e. not saturated. This is not the case in general -- to convince myself of this, I built a simple (2N2222) transistor circuit in LTspice, and graphed Ic/Ib for varying Ib. Definitely not a constant ratio.

I don't know enough about the spice parameters to figure out how to do this.

p.s. SpartanG345, does this have something to do with a school assignment?

 

[vk6kro]

I have done this previously, but I again used a 2N2222 which had had its parameters changed to give a gain of 100.

Using a current generator to give a variable input and using the DC sweep mode it seems to simulate OK. It has a very slight curve to it, but quite acceptable, I think.

Here is a screen grab of the result:
[PLAIN]http://dl.dropbox.com/u/4222062/Spice%20current%20gain%20test.PNG

The vertical scale is the collector current and the horizontal scale is the base current.

 

[Redbelly98]

Not bad, beta varies from about 120 to 105 in that plot. However, I tried the same circuit using the 2N2222 parameters as-is (beta=200), and found more variation in beta:

• With V2 at 10 V, beta changes from 217 to 150 as the base current increases from zero to 1.0 mA
• With V2 at 30 V, beta changes from 259 to 177 as the base current increases from zero to 1.0 mA

I guess it's a matter of what is reasonable and acceptable in practice, vs. an ideal constant beta.

p.s. If you plot ic(q1)/ib(q1) for the circuit, you'll see directly how beta varies. You'll need to start i1 out at a small-but-nonzero value in the simulation, as it doesn't like to do the calculation when the base current is zero