Hi, please see attached photo of page 69 of art of electronics text. What does mean to "Choose Rc so that the voltage drop across it is less than the drop across R for the highest normal load current"? Thank you so much!

Please see the image in post #3 -moderator

 

Suppose you pick a Zener diode that wants 20 mA of current to be well past the knee in the reversed bias condition. You will choose the bias resistor R to provide that 20 mA of Zener current. Whatever the voltage drop across R, you need to pick Rc so that whatever load you connect to the output of the transistor, the voltage drop across Rc will be less than the voltage drop across R.

 

Isn't voltage drop across Rc always less than that across R due to zener holding the other side of R and Rc to fixed value? So for example if Zener is 7V and Vin is 10V, Vdiff across R is 3V and across Rc is maybe 2.7V and so on for different values of Vin..



(sorry picture in 90* angle.., see reattach).

 

Isn't voltage drop across Rc always less than that across R due to zener holding the other side of R and Rc to fixed value? So for example if Zener is 7V and Vin is 10V, Vdiff across R is 3V and across Rc is maybe 2.7V and so on for different values of Vin..



(sorry picture in 90* angle.., see reattach).

Rc is there to limit the current if the transistor switches a low impedance load.

 

what I mean is: isn't voltage at collector always >= voltage at base (from figure in photo). And therefore shouldn't Vdiff across Rc is always < Vdiff across R no matter what value Rc you pick?

 

what I mean is: isn't voltage at collector always >= voltage at base (from figure in photo). And therefore shouldn't Vdiff across Rc is always < Vdiff across R no matter what value Rc you pick?

If we had a load resistor the question and the answer would be easier to see. What we know of an emitter follower is that the total current through Rc, Vce, and Rl (the load resistor). determines the voltage drops across Rc, the transistor (Vce), and the load. The transistor is not likely to be in cutoff or saturation if the zener is sufficiently reverse biased to be conducting its zener current. thus it must be in the linear region and that will tell you what you need to know.

 

oh ok, so I guess the author is telling us to operate the transistor in linear region - so as to regulate. And to do that vdiff over Rc has to be < vdiff over R. If Vdiff over Rc is greater than R then the desired load current is only possible when transistor is in saturation - which means no room to regulate.. How very cryptic of the author. Like reading poetry.. Or a riddle..

 

Hi, please see attached photo of page 69 of art of electronics text. What does mean to "Choose Rc so that the voltage drop across it is less than the drop across R for the highest normal load current"? Thank you so much!

Please see the image in post #3 -moderator

Build it! Adjust the components and see what happens.

 

isn't voltage at collector

voltage vs. voltage drop.

 

oh ok, so I guess the author is telling us to operate the transistor in linear region - so as to regulate. And to do that vdiff over Rc has to be < vdiff over R. If Vdiff over Rc is greater than R then the desired load current is only possible when transistor is in saturation - which means no room to regulate.. How very cryptic of the author. Like reading poetry.. Or a riddle..

It is straightforward to me. "The collector resistor Rc can be added to protect the Transistor from momentary short circuits..."

Try this: short Vo to ground and compute how much current is delivered thru the short. Now set Rc to zero and recompute, and try to find a transistor that can safely handle that current.

The point of Rc is to starve out the transistor when excessive current is drawn. Otherwise during normal operation it has little effect beyond sharing the power burned off with the transistor.