Reading a transistor datasheet ?

Ive got some basic questions about how to interpret data from a transistor data sheet basically when there is no graph to look at to get data from only a select list of x and y data points in a list.

Take for example a 2n2222 npn transistor it says in the data sheet

vbe(sat) at Ic = 150ma,IB = 15ma vce(sat) = 0.4v
vbe(sat) at Ic = 500ma,IB = 50ma vce(sat) = 1.6v

How would u get an idea of the expected vce(sat) if the Ic was 70ma and Ib was 10ma, also for any other values that lye between these points ?


In the datasheet under dc gain(hFe) it says this
IC = 0.1 mA; VCE = 10 V 35 hFe
IC = 1 mA; VCE = 10 V 50 hFE
IC = 10 mA; VCE = 10 V 75 hFe
IC = 150 mA; VCE = 1 V; 50 hFe
IC = 150 mA; VCE = 10 V;100hFe

So how would you get an idea of the dc gain(hFe) if Ic is 50ma and vce is 5v,
also for any other values that lye between these points ?

Transistors have wide ranges for their spec's. one might saturate well with a low base current but another might need the max current shown in the spec's which is 1/10th the collector current even if both transistors have the same part number.

If you want all your circuits to work properly without testing, documenting and sorting each transistor then use the max ratings with a base current that is 1/10th the collector current for the spec'd saturation voltage loss.

Okay so use 10 hFe for the ib when using transistor as switch i understand !

If every transistor even the same part number have a different hFe values then what are the hFe values in a transistor data sheet useful for ?

Does the 1/10th base current rule still stand for a Darlington transistor setup to ensure saturation condition ?

crazyjohn said:

If every transistor even the same part number have a different hFe values then what are the hFe values in a transistor data sheet useful for ?

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Usually a datasheet lists the guaranteed minimum and maximum hFE values and shows graphs of typical transistors.

Does the 1/10th base current rule still stand for a Darlington transistor setup to ensure saturation condition ?

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No.
A darlington has much more current gain than a single transistor. Its datasheet shows that it saturates well with its base current usually 1/250th its collector current.

crazyjohn,
Follow the simple BJT eqautions, transistors have same basic eqations relating all its parameers like Ic, Ib, Vce,Vbe,hfe etc.
First check it with given values in data sheet. then you calculate required value with available values of other parameters.
And well, if you designing something, the dig every thing until you satisfy....Give more time...just dig it more...so that second time you will be free to think & design effectively.

When you see on line places like radio***** selling transistors and saying this transistor has a gain of 300 hFe what is this value useful for operating the transistor as a switch or amplifier ?


If transistor hFe reduces as IC increases what mode of operation would a transistor be in to simulate this behavior ?


Other than changing a ac signal what other uses do transistors have when they are operating within their active region ?



Thanks for replying to my posts!

crazyjohn said:

When you see on line places like radio***** selling transistors and saying this transistor has a gain of 300 hFe what is this value useful for operating the transistor as a switch or amplifier?

Click to expand...

A transistor's hFE is its DC current gain (the increase of collector current when the base current is increased) at a certain collector to emitter voltage and a certain temperature. Its hFE is a range of numbers, 100 to 300 for a 2N3904 at 10mA and it changes when the collector current, collector to emitter voltage and/or the temperature changes.
A transistor's hfe is its AC current gain that changes as above plus reduces as the frequency increases.

When designing a transistor circuit then simply use enough base bias current so that a transistor that has the minimum spec will work properly then all transistors with that part number will work properly in the circuit.
Negative feedback can be used to cancel any small change that remains when different transistors are used (but having the same part number).
Most transistors can be used as a switch or as an amplifier.

If transistor hFe reduces as IC increases what mode of operation would a transistor be in to simulate this behavior?

Click to expand...

When a transistor is used as an amplifier then its collector current increases and decreases with the signal. if there is enough base bias current then the drop in hFE with increasing current doesn't matter. The hFE is current gain and has nothing to do with a transistor amplifier's voltage gain.

Other than changing a ac signal what other uses do transistors have when they are operating within their active region?

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Transistors amplify and control DC voltage and DC current in a voltage regulator.