Next Report - Application circuits for BJTs

ericgibbs

Joined Jan 29, 2010
18,766
Ah ok I got the K meaning... Don't need the link explaining!
Thanks anyway!

What am I doing wrong in my simulation?
So, if I consider 0.6V for Vbe and 4.5V for the Level 1 state limit I have:

5 = 2.2Ic + 4.5 <=> Ic = 227uA

Ib = Ic/β <=> Ib = 227/256.7 <=> Ib = 884nA

Vx = RbIb + Vbe <=> Vx = 100k* 884nA + 0.6 <=> Vx = 688mV
I see that you have added text to an earlier post.

Why are you now stepping Vcc when you are trying to find the Vc for 4.5V.???
Ie 4v< level1 < 5V

Also whats happened to the 100K.??

EDIT:

Vx = RbIb + Vbe <=> Vx = 100k* 884nA + 0.6 <=> Vx = 688mV
With no Rb this becomes.
Vx= 0.600000884V
 
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Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,755
Sorry for the wrong Screen.

Let me try again! ;)

The current question I'm working on is to calculate the max Vx so that Vc is still considered at Level 1, knowing that 4V <= Level 1 <= 5V.

So, I'm going to consider your suggestions of Vbe = 0.6V and 4.5V for the limit to Level 1.

Using equations of "output net", "input net" and Ic = β*Ib:

input net: Vx = Rb*Ib + Vbe
output net: Vcc = Rc*Ic + Vce

5 = 2K2Ic + 4.5 <=> Ic = 227.3μA

Ib = 227.3/256.7 <=> Ib = 885.5ηA

Vx = Rb*Ib + Vbe <=> Vx = 100K*885.5ηA + 0.6 <=> Vx =688.6mV

Now, I just don't know how to setup the circuit...

Should I place a voltage source with a constant value of 688.6mV and Rb = 100KΩ to the base? Or just a current source of 885.5ηA to the base???

What kind of simulation should I setup? Trans, DC sweep or what?
If I choose a trans simulation, I get nothing at the plot window!

Is the attached circuit, the one I should run?
 

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ericgibbs

Joined Jan 29, 2010
18,766
Should I place a voltage source with a constant value of 688.6mV and Rb = 100KΩ to the base?
Or just a current source of 885.5ηA to the base???
What kind of simulation should I setup? Trans, DC sweep or what?
If I choose a trans simulation, I get nothing at the plot window!
You can do either for the Base drive, also use a .tran as its a DC condition

The Rb 100K, condition would be more usual
 

ericgibbs

Joined Jan 29, 2010
18,766
The current question I'm working on is to calculate the max Vx so that Vc is still considered at Level 1, knowing that 4V <= Level 1 <= 5V.
I thought we had done some this time ago.?
You have calculated a value for Vx that gives a Vc within those limits.

If you want to see Vx on the Xaxis, why dont you use a PWL sweep for Vx as we did some time ago.?

EDIT:
Like this.
 

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Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,755
Morning...

EricGibbs, please explain me what you have done with that voltage source Vmid, how do you set it up and what is it's function in the simulation?
 

ericgibbs

Joined Jan 29, 2010
18,766
Morning...

EricGibbs, please explain me what you have done with that voltage source Vmid, how do you set it up and what is it's function in the simulation?
Morning Psy.

The level1 in the question is between 4V and 5V, so I chose 4.5V as being the mid voltage point.

The V source on the left hand side of the circuit is set for 4.5V, it is only used as a reference plot to show the mid voltage.
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,755
Morning Psy.

The level1 in the question is between 4V and 5V, so I chose 4.5V as being the mid voltage point.

The V source on the left hand side of the circuit is set for 4.5V, it is only used as a reference plot to show the mid voltage.
But you just added a label to the negative side of the voltage source and LTSpice is ok with that to run the simulation? Or what kind of component is that text attached to the voltage source?
 

ericgibbs

Joined Jan 29, 2010
18,766
But you just added a label to the negative side of the voltage source and LTSpice is ok with that to run the simulation? Or what kind of component is that text attached to the voltage source?
The Label Vmid is on the Positive connection of the V source, the negative side is 0V

LTSpice will plot Vmid without any problem.
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,755
Hi again...

I was trying to figure out why LTSpice is not even close to my calculations for that question about the maximum value we could have at Vin to VCol is still considered to be at Level 1.

My teacher told me today to use the values he has suggested in calculations so that we could compare them between our colleagues in case of doubts.

So I have considered the 4V at VCol (instead of 4.5V as you suggested) and 0.7V for Vbe (instead of 0.6V).

I did the follwoing:

Vx = RbIb + Vbe
Vcc = RcIc + Vce
Ic = βIb

5 = 2K2*Ic + 4
Ic = 455μA

Ib = 455μA/256.7 = 1.77μA

Vx = 100K*1.77μA + 0.7 = 877mV

So this was supposed to mean that when I set up Vin with 877mV VCol should be somewhere close to 4V, right? But LTSpice gives me something like 3.5V.

Ib = ~2.65μA also looks far from calculations for 877mV in Vin.
Ic = ~684μA is also far from my calculations...
 

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ericgibbs

Joined Jan 29, 2010
18,766
Vx = 100K*1.77μA + 0.7 = 877mV

So this was supposed to mean that when I set up Vin with 877mV VCol should be somewhere close to 4V, right? But LTSpice gives me something like 3.5V.
If you consider the low value of Ib, its not realistic to assume a Vbe 0f 0.7V + 0.177V = .877V.

Your low value of Vc is due to the Vbe being too high.

EDIT:

Look at this plot, it also shows the Vbe... its ~ 0.6V
 

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Jony130

Joined Feb 17, 2009
5,487
The answer is simple in LTspice Vbe is not equal to 0.7V.

Vbe = Vt*ln(Ic/Ise) and Ib = Ic/beta
So for Ic = 455μA; Beta 260; and Is = 29.13E-15 we have
Vbe = 610mV and Vx = 0.785V
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,755
The answer is simple in LTspice Vbe is not equal to 0.7V.

Vbe = Vt*ln(Ic/Ise) and Ib = Ic/beta
So for Ic = 455μA; Beta 260; and Is = 29.13E-15 we have
Vbe = 610mV and Vx = 0.785V
Never saw that formula before... Our teachers always said to us to consider Vbe = 0.7V.

But I appreciate that info. Thanks

hi Jony,
Thanks for re-stating what I have been telling Psy for days.:) phew!

Eric
I have not your level of knowledge about this stuff or about LTSpice.

I know you have been trying to tell me how things go as close to reality as possible, but I also need to follow teachers instructions because if I follow all your suggestions, if my teacher asks me why have I not considered the values they have been teaching us, I might not be able to answer them! :confused:

Anyway, I'm going to put a note in the report saying that there are slight differences due to Vbe = 0.7V being too high for such low Ic.
 

ericgibbs

Joined Jan 29, 2010
18,766
Anyway, I'm going to put a note in the report saying that there are slight differences due to Vbe = 0.7V being too high for such low Ic.
OK, that would help to explain to your tutor why your answers are different to what LTSpice shows.

E
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,755
Now I need your knowledge about LTSpice to simulate the attached circuit.

I need to run 4 different combinations at point A and B to simulate the 4 possible logic values of 0;0, 0;1, 1;0, 1;1, where 0 can be any value between 0V and 1V and 1 can be any value between 4V and 5V.

Can I use 2 Voltage sources and set them up somehow to run those 4 combos to try to simulate the whole circuit and check VCol behavior as a NAND port?
 

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