I have read in the book that beta is the ratio of collector current to the base current in common emitter configuration and alpha is the ratio of collector current to emitter current to common base configuration.
Also, I know that alpha and beta are the current gains in common emitter and common base configurations.
My questions are:
What does both the terms signify actually?
Why are we calculating these current gains?
And Why is there a range of beta for a particular transistor? What does minimum and maximum values of beta mean?
How much does these alpha and beta affects our Early Effect?
Best Answer
𝛼 and β are directly related - 𝛼 = β/(β+1); It doesn't matter what circuit configuration is used (or if any circuit is used at all -- these parameters are properties of the transistor, not the circuit).
However, in a common-emitter circuit, it may be more intuitive to consider β as the non-ideal factor, and in common-base circuits to use 𝛼.
The physical significance is that these parameters relate the effects of (usually considered non-ideal) base currents to collector currents. Beta for a transistor depends on the ratio of emitter doping to base doping, as well as the width of the base diffusion. It is quite constant over a range of current levels, and so is a useful parameter to use for calculations. Physically, however, a (BJT) transistor can be considered to be controlled by the base-emitter voltage; but since the current is exponentially dependent on this value, it is not generally used this way in a circuit (except in a current mirror).
A lot of circuit design involves making approximations or estimates of component performance because component parameters (especially transistors) are not precise, and may vary with temperature or current levels). Good circuits will setup components so that the performance remains acceptable (constant, or 'high enough') as component parameters may vary.
This often takes the form of 'sufficiently large' or 'close enough' calculations. This can for example be seen in opamp circuits where overall gain is defined by resistor ratios (which can be quite precise), where the equation's accuracy is acceptable if the opamp gain is 'high'.