Beta

[Electronman]

Somebody told me that we are not able to use even 2 similar transistors for some works BECAUSE their Beta Is not completely similar (I.e the company is not able to produce 2 transistors with the same Beta even if they have the same mark).
Now that I am thinking about it A question arrases,
Here's the question:
Beta is Ic/Ib that means beta is the ratio between collector current and the base current, we identify those currents by the biasing resistors at the collector and the base so why the beta depends on the transistor itself?
For instance if 2 similar marked transistors have a beta rated at 400 by company (no matter if one has a beta more than that value) why we are not able to get the same results if we bias the collector and base resistors so that both the similar transistors give us the same response?
Make sense?

 

[MikeMl]

No. The biasing network and emitter resistor are usually designed to desensitize the ciruit against unit-to-unit variations of beta if different transistors are put into the circuit.

The beta is measure of current gain of the transistor; not the circuit.

 

[Electronman]

So you are telling that betas of 2 transistors are not so important when they are used as a differential amplifier for instance?
But somebody told me that we must consider the transistors when try to make a differential amplifier out of transistors? Did he refer to another parameter when he told so?

 

[audioguru]

A modern differential amplifier is inside an IC where the transistors and their temperatures are almost identical. If you try to make a differential amplifier with two separate transistors then you must fiddle around with trying to match the transistors and with different resistor values. Even the different temperatures of the transistors will mess up the differential amplifier.

 

[Electronman]

A modern differential amplifier is inside an IC where the transistors and their temperatures are almost identical. If you try to make a differential amplifier with two separate transistors then you must fiddle around with trying to match the transistors and with different resistor values. Even the different temperatures of the transistors will mess up the differential amplifier.

I do not want to design anything, just want to know why the beta's of the transistors must be completely similar when We design some circuits?
I am not talking about the beta which we create by biasing resistors, I am talking about beta which is given by the datasheet.
Makes snse?

 

[Nigel Goodwin]

I do not want to design anything, just want to know why the beta's of the transistors must be completely similar when We design some circuits?
I am not talking about the beta which we create by biasing resistors, I am talking about beta which is given by the datasheet.
Makes snse?

Your main mistake is in assuming a transistor with a specified beta of 400, no manufacturer specifies like that, and none will. They specify the beta as a range - say 150 to 500 - and you should assume the lowest when doing your calculations, but design so the beta being higher makes no difference.

Certain circuits require transistors to be closely matched, such as the input stages of IC's, this is accomplished by making them identical on the same slice of silicon. At one time you could even buy double transistors, with two on the same slice of silicon, this ensures close matching of the pair. They are quite hard to come by these days though.