How do you select the transistors (or mosfets, rectifier diodes, etc) to use?

[Bricolo]

Maybe this is something teached to EE, but since I'm not one (or not yet), I don't know things like this.

I'm asking this question to the people who design theyr own circuits.
When I see a schematic, I'm always wondering how have the parts been choosed. When I try to do it myself, and look for a BJT on manufacturer's websites, I see hundreads of transistors in dozens of categories, and I'm incapable of choosing with the small knowledge I have.

Is there a consencus about classical transistors to use, or at least to start with? I'd like to know it



Alex

 

[peranders]

First you must know three things:

Current, voltage and power

Then:

current gain and fT (speed)

Then:

capacitances, linearity (if those data are available)


Last: Can I buy them easily? Second source?

Since you are in Europe, BC8xx and BC5xx for small signal are classical

BD139, 140 for drivers

Many more types to choose from when it comes to output transistors.

Note this are examples only!

 

[Bricolo]

Current, voltage and power requirements are given by the schematic, that's OK


but how do you know the current gain you want?
and for fT and capacitance, is there a rule?

 

[Bricolo]

Since you are in Europe, BC8xx and BC5xx for small signal are classical

BD139, 140 for drivers

Many more types to choose from when it comes to output transistors.

Note this are examples only!

That's the kind of answear I was expecting

 

[peranders]

Current gain for small signal transistors is usually not a problem but sometime it's nicer to have 400-600 current gain instead of 100-300. This will become clearer if you start to calculate every single stage in an amp.

fT is a little bit harder to determine what you need at least. Normally driver and output transistors are most important and also hardest to find speedy parts.

 

[Bricolo]

but should I look for the highest possible Ft, and the smallest capacitance?

 

[peranders]

It's often good but in those cases some other parameters gets worse....

If you have chosen a 100 MHz and the bandwidth gets too poor you may have to change the transistor but sometimes you can change the design and get a faster amp.

You see, it's many considerations. You must know what you are after, then you can start the hunt.

 

[Bricolo]

OK

I was not looking for power transistors. In fact, I was more asking what transistors were popular for low level applications (I/V, discrete regulators)

 

[peranders]

The transistors you have to look for are those in TO92 or SOT23 case and they should be "small signal transistors". Look for fT of 100-300 MHz and current gain of around 300 or more.