PSpice Model of diode, BJT, MOS

Discussion in 'Homework Help' started by mau80, Oct 24, 2010.

1. mau80 Thread Starter New Member

Oct 24, 2010
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Hi to all!

I'm trying to solve some didactical circuit which includes ( diodes, BJT, MOS ). For solve this exercise I've to use a simplified characteristic of devices.
For example for:

• diode use piecewise linear model Ideal diode in series with voltage source
• BJT Piecewise model

See attached file att.jpg for characteristic formulas and circuit that Ive to solve

I'd like to be able to solve this problem with PSpice in order to better understand how to solve this kind of exercise and be able to inspect various nodes of circuit.

My problem is that I'm not able to use such model in PSpice, I've tried to search in the forum but I didn't find anything.

I'd like to ask if you can provide to me some information about.

Thank you very much for you help.

Regards
Maurizio

• att.jpg
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Last edited: Oct 24, 2010
2. Jony130 AAC Fanatic!

Feb 17, 2009
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Well for this circuit and DC analysis you can use simply
voltage source instead of Diode. And voltage source + current-controlled current source instead BJT.

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3. Jony130 AAC Fanatic!

Feb 17, 2009
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And of-course we can quickly verify the results, assuming β=∞.
So voltage on emitter Q1 is equal:

Ve1 = (9V - 0.7V) * 40/(80+40) = 2.76666667V

then Ie1 = Ve1/ Re1 = 1.38333334mA

Ie1 = Ic1 = 1.38mA

Ie2 = (Ic1 * Rc1)/Re2 = ( 1.38mA * 2K )/100 = 27.6666668mA

And voltage on Q2 collector

Vc2 = Ie2 * 100 = 2.76666668V

4. mau80 Thread Starter New Member

Oct 24, 2010
21
0
Thank you very much for you help!
I'll analyze your mail in dept this afternoon.
I'd like to ask if you could attach the schematics to the message.

Futhermore I've to analyze circuit like that in attachement for Vi from 0 to Vdd, I'd like to ask if the substitution made in Pspice could be applied also to that circuit.

Thank you again for your great help!

Bye
Maurizio

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5. mau80 Thread Starter New Member

Oct 24, 2010
21
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I've study the circuit that you post. Using your substitution, I can reach the correct results.

Using your circuit the result it's correct, furthermore I've noted that I can apply Kirchhoff KCL e KVL after that substitution, but if I want to analyze a circuit with an input signal varying by time, I've to redraw the circuit substituting the devices with it's corresponding model for different state which device can operate: AD, LIN, AR, SAT in case of BJT. So I'd like to ask if there is a way for have a model of devices working piecewise in PSpice. This for avoid to redraw every time the circuit considering. I've attach an example of this required analysis.

Thank you very much to everybody for your great help!

Maurizio

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6. Jony130 AAC Fanatic!

Feb 17, 2009
4,577
1,281
Well simply use "normal" model in PSpice.

But for this simply circuit yo don't need a PSpice simulation.

For expel for Vin = Vcc = 5V
T1 will be in saturation If : Ib1*β >> Ic2+IR3

Ic1 = β*(5V - 0.75V)/20K = 21.25mA

Ic2 = Ib2 *β

Ib2 = (Vcc - Vbe - Vce_sat)/ ( Rb + (β+1) * Rc ) = ( 5 - 0.75V - 0.2V ) / ( 5K + 101*500) = 4.05V/55.5K = 72.97uA

Ic2 = 7.297mA

IR3 = (5V - 0.2V)/2K = 2.4mA

So
Ib1*β = 21.25mA and Ic2+IR3 ≈ 9.7mA
So for sure for Vi = 5V T1 is in saturation and Vu = 0.2V
And T1 will start to go out from saturation for Vin < then
9.7mA/β = 97uA---> Vin = 97uA*20K + 0.75V = 2.69V.
And Vu = 2.5V for Vin equal
Ic1 = Ic2 + IR3
Ic2 for Vu =2.5V is equal --->
Ic2 =β* (Vcc - Vbe - Vu)/ ( Rb + (β+1) * Rc ) = (5V - 0.75V - 2.5V)/55.5K = 100* 31.53uA = 3.153mA
IR3 = 2.5V/2K = 1.25mA
So Ic1 = 4.403mA we have Vin = 1.6306V
Summarize
If Vin = 0 V then Vu = 5V
Vin = 1.6V then Vu = 2.5V
and for Vin greater than 2.69V output voltage is equal Vu = 0.2V

• att2.PNG
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Last edited: Oct 25, 2010
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7. mau80 Thread Starter New Member

Oct 24, 2010
21
0
I've try to resolve it and I was able to find the solution.
I'd like to thank you very very much for your precious help and your time!

Bye
Maurizio