Common emitter amplifier calculation

Thread Starter

mahela007

Joined Jul 25, 2008
45
After figuring out how to properly connect a transistor, I want to make the necessary calculations to use it as an amplifier.
The datasheet for the transistor shows a very wide rance.. from 130 to 520
http://www.alldatasheet.com/datasheet-pdf/pdf/108447/ETC/2SC828.html

How do I determine which beta value to use, so that I can calculate at what point the transistor will become saturated?
(I plan to calculate the maximum base current required for saturation by first calculating the maximum current that can flow from Collector to Emitter and then dividing that by the beta value)
 

Hi-Z

Joined Jul 31, 2011
158
I think you'll need to give more information regarding how you wish to use the transistor, and a circuit diagram would help. Why are you interested in saturated operation?

It's perfectly normal for a transistor to have a wide variation in current gain, and in practice it would be very unusual for this to present problems.
 

Hi-Z

Joined Jul 31, 2011
158
It occurs to me that you may be talking about a saturated switch, rather than amplifier. If so, yes, use that calculation, but do make sure you have plenty of base current to ensure a nicely saturated transistor. Usually, you'll have more problems if your saturated switch isn't properly saturated than if the base current is overdone (though you might have to watch out for slow switch-off if you're very heavily saturated).
 

Audioguru

Joined Dec 20, 2007
11,249
The Oriental transistor has a wide range of current gain. But an American 2N3904 transistor and many more or a European transistor with an A, B or C suffix have a much narrower range like 100 to 300.

A resistor in series with the emitter and a voltage divider for the base voltage reduce the effects of the range of current gain.
 

Thread Starter

mahela007

Joined Jul 25, 2008
45
It occurs to me that you may be talking about a saturated switch, rather than amplifier.
I want to know the base current level for which my amplifier circuit will saturate. My problem is that I don't know what beta value to use in my calculation because the datasheet gives such a large range of beta values.. Could you please show me how you would calculate it?
 
Last edited:

Audioguru

Joined Dec 20, 2007
11,249
I want to know the base current level for which my amplifier circuit will saturate. My problem is that I don't know what beta value to use in my calculation because the datasheet gives such a large range of beta values.. Could you please show me how you would calculate it?
BETA is used when the transistor is a linear amplifier with plenty of collector to emitter voltage so it is NOT saturated.

The datasheet for most transistors show its saturated collector to emitter voltage when its base current is 1/10th its collector current.
 

Thread Starter

mahela007

Joined Jul 25, 2008
45
Yes.. but to find out the base current that would saturate the transistor, the beta value would have to be known right?
 

debjit625

Joined Apr 17, 2010
790
Yes.. but to find out the base current that would saturate the transistor, the beta value would have to be known right?
Audioguru already said about the 1/10 stuff i.e.. to saturate the transistor you need 1/10 of collector current for base.Actually that gives you the beta value at saturation i.e.. 10 times.For example your transistor's max collector current is 100mA then ,if you bias the transistor using 10mA base current i.e 1/10 th of collector current then your transistor will be saturated.

Many European transistors will have this value of 1/20 of collector current like BC547.

Good Luck
 

Audioguru

Joined Dec 20, 2007
11,249
But that is WRONG because the author does not read datasheets.

The datasheets for nearly all American small transistors say the base current should be 1/10th the collector current to saturate the transistor and the datasheets for nearly all high gain European small transistors say the base current should be 1/20th the collector current even if the Beta is as high as 900.
 
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