My calculated and simulated values for operating point not matched, why???

Thread Starter

valorous

Joined Oct 28, 2021
18
I have used transistor 2N3904 having beta value (= 300) . My value for op are (5V, 7.5mA) but i m getting around (4.33V, 7.66mA). I am bascially simulating the fixed bias config of transistor by varying the beta value.
1638354595486.png
1638354713217.png
 

MrChips

Joined Oct 2, 2009
27,115
No two transistors are the same. The actual value of beta will be different. Your simulation might not model the part exactly.
 

MrChips

Joined Oct 2, 2009
27,115
My point is, simulation might not model the real part exactly.

Your simulation shows the cursor position. It does not fall exactly on the load line.
 

Thread Starter

valorous

Joined Oct 28, 2021
18
o
My point is, simulation might not model the real part exactly.

Your simulation shows the cursor position. It does not fall exactly on the load line.
ok,but it can be clearly seen that operating voltage is less than calculated(5V).
is this error valid or should i do something?
 

Thread Starter

valorous

Joined Oct 28, 2021
18
How did you calculate the op?
i used the standard formulas...
like first i calculated Ibe by applying kvl in the base-emitter loop then using beta and Ibe, I found Ic. then I apply kvl in collector emitter loop, using the relation between Vce and IcRc and Vcc, i found the Vce.
 

ericgibbs

Joined Jan 29, 2010
16,365
I don't get it, what u mean by the standard.bjt?
and what are these graph saying.....can u please explain me in detail...
hi,
The standard.bjt are the bjt files in your LTS folders under lib/cmp.

The plots are showing there is a spread in the parameters for a particular transistor type.

E
 

Thread Starter

valorous

Joined Oct 28, 2021
18
hi,
The standard.bjt are the bjt files in your LTS folders under lib/cmp.

The plots are showing there is a spread in the parameters for a particular transistor type.

E
what does it mean by spread in parameter?
r my graph were correct or wrong ?
 

Papabravo

Joined Feb 24, 2006
19,266
A couple of points:
  1. The BJT model parameter "BF" is NOT the Beta value, it is the "Beta Factor". It is part of a calculation that will determine the actual performance in the simulation.
  2. It seems suspect that you are using the Letter B for two different purposes, in both the .step and the .measure statements.
I quote from the LTspice Help pages:

.MEASURE -- Evaluate User-Defined Electrical Quantities

.MEAS TRAN res6 PARAM 3*res1/res2

Print the value of 3*res1/res2. This form is useful for printing expressions of other .meas statement results. It's not intended that expressions based on direct simulation data, such as V(3), are present in the expression to be evaluated, but if they are, the data is taken from the last simulated point. The result will be labeled res6.
 

Thread Starter

valorous

Joined Oct 28, 2021
18
A couple of points:
  1. The BJT model parameter "BF" is NOT the Beta value, it is the "Beta Factor". It is part of a calculation that will determine the actual performance in the simulation.
  2. It seems suspect that you are using the Letter B for two different purposes, in both the .step and the .measure statements.
yes, in .step i vary the beta value and in .meas i want to find the value of Ic for different beta values.(spice error log file)
 

Papabravo

Joined Feb 24, 2006
19,266
yes, in .step i vary the beta value and in .meas i want to find the value of Ic for different beta values.(spice error log file)
That is not what your statement is asking the simulator to do. You apparently did not read the LTspice Help file with much diligence. See my edit to post #15 as I was curious enough about your intention. Beta and Ic(q1) are NOT the same thing. The calculated value of Beta at any point is the instantaneous collector current divided by the instantaneous base current.
 
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Thread Starter

valorous

Joined Oct 28, 2021
18
That is not what your statement is asking the simulator to do. You apparently did not read the LTspice Help file with much diligence. See my edit to post #15 as I was curious enough about your intention. Beta and Ic(q1) are NOT the same thing. The calculated value of Beta at any point is the instantaneous collector current divided by the instantaneous base current.
1638361427299.png
What are these values then??
i was assuming it the value of Ic at operating point for the corresponding value of beta.
 
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