#1 almost never does, but we have developed techniques so that these should be in the ballpark and, in many circuits, useable with little or no modification.There are three answers:
1) by calculation
2) by simulation
3) by real circuit measurements
Which one gives the "correct" answer?
Sorry I have a question . Can you give an example about calculation techniques ? So it means that Multisim is wrong medel right?#1 almost never does, but we have developed techniques so that these should be in the ballpark and, in many circuits, useable with little or no modification.
#2 should be the one to go with since the simulations can take into account far more variations then prototypes can. But you have to have good enough models (which usually do exist) for this to be the case.
#3 produces results which are, pretty much by definition, correct for that particular set of components operating under those particular conditions. This, of course, is even assuming that prototyping the circuit is even an option.
@ericgibbs Does LTspice model use typical values ?hi Hero,
There is a spread in the parameters for most semiconductor devices, this is mainly due to manufacturing.
I use the typical values from the device's datasheet, other users choose the lowest parameter listings.
E
A transistor is a highly non-linear device and so analyzing and designing circuits that use them would be, in general, difficult to do even if we knew exactly how every transistor behaved. So we develop analysis techniques that rely on approximations that get us close to how the actual circuit works -- treating the voltage drop across a forward-biased diode (or the base-emitter junction of a bipolor junction transistor) as a constant voltage regardless of current is just one example. We take this a step further and develop techniques, such as small-signal analysis, that let us treat the transistor as if it behaves like a linear element provided certain conditions are met. We then design circuits that try to ensure that those conditions are met.Sorry I have a question . Can you give an example about calculation techniques ? So it means that Multisim is wrong medel right?
The big difference is that your model of the small-signal response did not take into account the finite output resistance of the transistor.In my main question I used my calculation and voltage gain is 713 but in LTspice it's 535 . My calculation is correct right ? But there is some percentage error in my calculation because In LTspice they use many parameter to calculate right?
hi hero,Does LTspice model use typical values ?
The generic "beta" of a transistor is a pretty non-specific ratio of collector current to base current. It tends to vary over a wide range of values, both for a given transistor as operating condition changes, and from one transistor to another.Now I'm confused between BF and beta, Hfe . Is it the same thing? In LTspice BF is 200 but Multisim is 153 first I think that BF is beta , Hfe and beta = Ic/Ib . Both of Multisim and LTspice , Ic and Ib are close therefore beta also are close but BF is different.
Does that mean beta and BF are different?
You can pretty much expect any generic model to depend on the "typical" device parameters. You can then go in and modify specific parameters within the model that you want based on your needs.@ericgibbs Does LTspice model use typical values ?
Yes, this is true - and this difference (and the associated tolerances) no longer matters at all when the transistor is operated in a circuit with negative feedback (which is practically always the case).The small-signal and large-signal values are often close enough that the overall uncertainty in what a particular transistor's beta is makes the distinction irrelevant, but the small-signal beta tends to drop as the signal frequency increases, so for high frequency applications, the distinction because very relevant.
There's a limit to what negative feedback can do when the h_FE is 200 and the signal is at such high frequency that h_fe has dropped to 2.Yes, this is true - and this difference (and the associated tolerances) no longer matters at all when the transistor is operated in a circuit with negative feedback (which is practically always the case).
Are beta and BF same thing?The generic "beta" of a transistor is a pretty non-specific ratio of collector current to base current. It tends to vary over a wide range of values, both for a given transistor as operating condition changes, and from one transistor to another.
h_FE is the large-signal (or DC) beta, which is the ratio of the total collector current to the total base current when the transistor is operated in the Forward current amplification mode in the common Emitter configuration (hence the F and the E in the subscript).
h_fe (note the lower case instead of the upper case) is the small-signal beta, which is the ratio of the change in collector current to the change in base current.
The small-signal and large-signal values are often close enough that the overall uncertainty in what a particular transistor's beta is makes the distinction irrelevant, but the small-signal beta tends to drop as the signal frequency increases, so for high frequency applications, the distinction becomes very relevant.
As an aside, is there a reason that you are using the ancient 2n2222? If you use a more modern transistor, even the still-old 2n3904, you can expect to find better models (and better performance).
Before you ask, don't ask what the best transistor to use is. Not only is the only truly defensible answer, "it depends,", but it is likely to start a religious war.
Oh yes, of course. These feedback effects are relevant for a sufficiently large loop gain only.There's a limit to what negative feedback can do when the h_FE is 200 and the signal is at such high frequency that h_fe has dropped to 2.
Sorry I have a question. What is BF ?To be able to answer this question I extract 2N2222 BJT's model from Multisim.
And if we do this, we see that Early's voltage (VAF) has a very low-value VAF = 10V. Thus, the transistor output resistance (ro) will be extremely low.
ro ≈ (11.3V + 10V)/3.73mA ≈ 5.7kΩ
Therefore the voltage gain will be around
Av ≈ (Rc||RL||ro)/re ≈ (5kΩ||5.7kΩ||100kΩ)/6.9Ω ≈ 2.6kΩ/6.9Ω ≈ 376V/V
So BF is max value of beta right?BF is an Ideal maximum beta ( β, Hfe).
And Beta is not constant.
See yourself:
Multisim 2N2222
View attachment 279293
And LTspcie model
View attachment 279294
by Jeff Child
by Aaron Carman
by Aaron Carman