Transistor Vce Saturation voltage

Hi All,

Anybody can tell me how to calculate the Vce(sat),Vbe in all modes i.e. common collector, common emitter, common base?
Which mode is better and why?
What are the factors on which Rb & Rc values depends?
Which transistor is preferable NPN & PNP and why?

hi
Vce(sat) for common emitter is equal to Vcc and i think it is the same for common base, but for common collector it might be different .
the best mode is common emitter , because it more stable , have high gain , and it has easy calculations .of course there is different applications for each.
the values of Rb and Rc depends on the Low cutoff frequency in AC operations of the transistor , so they used to control the low cutoff frequency of the transistor of course Re come in to use too.
the preferable transistor is NPN, that you will understand why when you study the AC operation of the transistor .
if that helped you ,press the hand botton please.

Thanks for the reply.
I want to know the voltage drop across Vce(sat), as i know that there is 0.7v drop across Vbe if i am using Si transistor.
In Rb and Rc values i think we have to set Rb > Rc if i am correct then why is it so?

Saturation Vce is a value in the datasheet of a transistor, and it does not change. Its the voltage across C and E when the given (maximum, saturation) current is flowing through C an E (and Vbe sat. is applied on base). If you don't know it, and you have no other information about the specific transistor, you can only measure it. Ideally Vce is zero in saturation. NPN are most used for various reasons

Hi,
Check this similar thread:
https://www.edaboard.com/threads/191860/#post803161

The Vce(sat) value varies with each transistor. It is stated in the datasheet and should be checked from there. eg. BC547 at Ic=10mA, Ib=0.5mA has Vce 0.09V to 0.25V, but at Ic=100mA, Ib=5mA has Vce 0.2V to 0.6V. On the other hand TIP3055 has Vce varying from 1.1V to 3V depending on Ib and Ic.

Hope this helps.
Tahmid.

very much satified with your answer Tahmid. But you didn't tell me that how i select the value of Rb and Rc.

Hi,
Okay, first between NPN and PNP, it depends on your use. NPN is mostly used as it is easier and safer to switch with Vb=+ve rather than Vb=0 or -ve, however if the application asks for it, you'll need to use PNP.
Common base/emitter/collector will depend on the application and your choice as well. To understand this better, read on this:
Common collector - Wikipedia, the free encyclopedia
Common base - Wikipedia, the free encyclopedia
Common emitter - Wikipedia, the free encyclopedia

Okay now Rb and Rc. Rc is only needed for common base or common emitter. There you have to decide the amount of current you want flowing through the collector. Using that and V=IR, select R. Rb is selected depending on Rc. In the sense that, as you know, each transistor has hfe (gain). So, you know Rc and current flowing through collector. It's better to use example. eg. A current of 200mA flows through collector, and the transistor has hfe 300. Therefore minimum Ib = 200/300 = 0.67mA. For safe base turnon reasons you select 3mA, checking the datasheet that this current is within limits. So, you know V=V applied to base - 0.7v, I=0.003A. Assuming you drive the base with 12v. Therefore, R=V/I = 11.3v/0.003A = 3767ohms = roughly 3.8kohms

Hope this helps.
Tahmid.

i have simple equestion
sometimes i read in webpages tell preventing transistor from entering saturation, why it should be preventing transistor from entering saturation ? what is the problem with that? and they advice to use Schottky diode to prevent it from enter the saturation.
i am confused on this point

I understand your points, i have to measure it practically to see the exact values of voltage and current.
Thank's Tahmid for your help.

Transistors are slow to come out of saturation due to charge storage. That is why it is often desirable to prevent it. A Schottky diode prevents saturation by reducing the base drive as the transistor approaches saturation because the forward voltage drop of a Schottky is less than the base-emitter voltage.

Keith

i understand if more current injected into the base that leads to put transistor in deep saturation . so if that happens, that means more current injected into collector and may lead transistor to be heated and will breakdown the transistor
is that right or not?

No. Breakdown is a term normally used with excessive voltage. Provided you keep the base current, the collector current and power within the device limits it will be fine. Even if you put more base current in than required it cannot increase the collector current beyond a point determined by the load and supply voltage. It is quite common to run transistors in saturation where speed isn't important.

Keith