Why use different transistor numbers for switches and amplifiers

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danny davis

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At work on the circuit boards these use these types of transistors , they use transistors are a SWITCH

2N2484
2N3700
2N3440
2N6287
2N4093
2N5566
2N5116
2N3251
2N2222
2N2904

My question is why do they use different type transistors , when the transistor is used as a SWITCH?

What difference does it make when using a different number type transistor

Why don't they just use 2N2904 through out the whole circuit and use it as a switch , why use all these different numbers of transistors? what for?
 

Some of these transistors are NPN, some PNP type, they have different voltage and current rating and probably different switching speed.
 

have you seen the datasheets of these transistors? i assume as said by FVM there would be difference in their power ratings as well as switching speed that is why the designer would probably has used different ones...
 

they have different voltage and current rating and probably different switching speed.
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Why would u want to use different voltage and current rating? for what reason? why not stick to 2N2904 for the whole circuit board , plus it will be cheaper?

How can a transistor have different switching speeds? what determines the switching speed internally inside the transistor? is it the gain? HFE?

If you simply compare their absolute maximum ratings you will get the answer.
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Why would the maximum rating be the answer? cause each transistor number can happen more or less POWER and Stress?
 

danny davis said:
How can a transistor have different switching speeds?
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I think the fall time (Tf) and rise time (Tr) will be dependent on the junction capacitance of the transistor which will end up in affecting the switching speed of it.
 

can i measure the junction capacitance using a lcr meter/?
 

the various transistor parameters that affect it's use as a switch
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What various parameters affect the transistor as a switch?
 

what i think you should really do is to consult a electronic related book, perhaps 'Microelectronic Circuits by Sedra Smith' will help you clear your confusion.
 

I have electronic books but it doesn't say much about the parameters and the inside of the transistor which causes it to switch

The Switching Parameters I mean , plus Comparing the switching parameter to other transistor numbers , power ratings

The Switching Response time of a transistor
 

I also want to know that. thanks for sharing this post.
 

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