could i ask this way every one?
BJT small analog signal amplification application, setup as per picture.
let's just say the definition of saturation is when Ic no longer changes with Vb.
hard on, small steady collector current, not enough Vce, collector voltage close to base voltage, changed beta, and what not...
1) what mechanism triggers saturation, Vcb, Vce, Ib, Ic, and how?
2) what happens to Ic when a BJT goes into saturation mode from forward active mode? for example, if a BJT is biased correctly with its Q point at 1/2 the supply voltage, it is likely to have a relatively large Ic, because that's what happens when it is amplifyitng. now, say, if the input signal is getting too large, sending the BJT into saturation, (don't know how yet at this point, the actual mechanism is still unclear to me, hope answers to question 1) will clear this one up), will Ic suddenly drop to a rather small constant value, because that's what a BJT does when saturated?
thank you for all the replies guys!
(1)Saturation is not "triggered".
If you increase the value of Ib progressively,the curve of Ib versus Ic will slowly begin to level off,until the same percentage change in Ib which earlier caused a large change in Ic produces a very much smaller change in Ic.
Eventually a point is reached where the increase is so small,you may regard the device as saturated.
Your "symptoms" are in error-the value of Ic is at its maximum at saturation.
Look at your circuit,& imagine the transistor as a variable resistor,which I will call Rq1
Initially,the variable makes up a largish proportion of the series resistance RL+Rq1, & Ie = V/(RL+Rq1).
As you reduce the resistance of Rq1,this proportion becomes progressively less,until,its contribution is so minimal, that we can
say,RL+VRq1 is approximately = RLso Ie approx =V/RL.
Further reduction of Rq1 won't make much difference to Ie.
Saturation in this sense,is affected by the circuit constants,as well as device characteristics.
(2) If the transistor is biased as per your example& is driven by an AC signal,
If the drive signal's amplitude is great enough,it will drive the transistor into saturation on the positive 1/2 cycle,and
to cut off on the negative 1/2 cycle,(assuming an NPN transistor),so the result is a distorted ("clipped") signal at the output .
The classic method of design of amplifiers is to draw a "loadline" on the characteristic curves,with one extremity being the current which would flow through the load with a short circuit in place of the device,& the other extremity being zero current (which would flow with an open circuit in place of the device).
If you have modelling software,you should be able to model the various conditions,or get some real transistors & play around with them!