How are transistors selected for various practical circuits?

Transistor mystery

How are transistors selected for various practical circuits.
Say i want to design a amplifier how do i begin.
How to choose bias and various r and c values ?

What can be a check for transistor working in active mode and how can the gain be calculated ?

Fuh !! lots of question in on stroke !!!! but can' t help it ,i am dying to understand the working of practical transistor circuits.

Thanks !!

Re: Transistor mystery

Please check any basic electronics books.
Milman Halkias is good.
In our forum : Teach yourself electronics and electrical is also good as a beginer guide.

Re: Transistor mystery

As far as the transistor selection you need the Ft value to be greater than the gain x bandwidth of the circuit. Then price and availability further constrict your selection. Then there are specialized needs that dictate JFET or MOSFET as the active device even though they cost more than BJT.

Then you select the bias point. The transistor data sheet will tell you what you gain and lose for various values of current and voltage.

Then you select the bias circuit. The temperature range and variation of transistor DC parameters between production lots will determine the details of the part values.

Then you select the remaining parts for the frequency response and gain you need.

Since you seem to be aiming to a hobby circuit and probably do not have a lot of expensive, high performance test equipment, I would suggest the 2N2222 and its variations as a good first try for your circuit. It is not a microwave transistor and so if your circuit self oscillates, you can catch it on an ordinary oscilloscope. The Ft is 750 MHz or so and the performance will be adequate for most amplifiers you make.

Re: Transistor mystery

hi guy,
i used to deal with β of the transistor as a coinstant, but when i downloaded a data sheet for the 2N3904 ifound that :

there is alot of values β , and all of them depends mainly on Ic, i tried to draw these values to make a charachteristic curves but i shoched when i found it produce nonlinear amplification.

is there anyone can explain this to me???????????????????????? [/size

Re: Transistor mystery

Yes, the BJT is a very nonlinear device. That is why it is for small signals in the common emitter configuration. Adding feedback in the form of an unbypassed emitter to ground resistor helps a lot. If you want further linearity, use feedback around two stages. The JFET is more linear but costly. If you want larger signals to be linear you will have to use valves/tubes with a 300 V power supply so the signal is a small fraction of the 300 V.

Transistor mystery

https://www.allaboutcircuits.com/vol_3/chpt_4/1.html
this is a good book for BLT

Added after 6 seconds:

https://www.allaboutcircuits.com/vol_3/chpt_4/1.html
this is a good book for BTL

Re: Transistor mystery

Hi,
You can also download some SPICE simulator and simulate the general purpose transistor models in the program that you can buy locally, it will help you learn and test the theory of biasing after some self reading on biasing. Also, it will give you a good estimation of biasing resistor values not to mention save you a lot of time messing around building circuits that don't work. Good luck!

You can find some freeware SPICE on the forum, just go to the download sofware section.

Re: Transistor mystery

if u want to strengthen ur knowledge in bjts
u have to read millman and halkias
its THE best resource available...
u might find certain things too hi funda at first but keep reading and re reading ..its totally worth it

Re: Transistor mystery

As the questions you posted are quite fundamental, I would suggest you begin with some popular analog IC design books, such as Ken Martin's Analog Integrated Circuit Design or Razavi's book. Typically you need to understand why the transistor needs to work in different regions, what is the principle behind the amplifier design, the purpose of circuit biasing, etc. It is not practical for anyone to address these issues in this single email.

Re: Transistor mystery

select transistor based on the gain u want. hfe is the most important thing in the selection of transistors

Re: Transistor mystery

transistors are mainly used for two purposes
1. switching
2. amplification


in either case the main parameters that govern the selection of transistor is collector current, power disipatted, collecter to emitter volatage, collector to base voltage, gain factor beta etc.

Transistor mystery

Start reading Electronics Principle by A. P. Malvino. There is lot of discussion about this issue using simple maths. Try his site http://www.malvino.com/

Re: Transistor mystery

try this site to know about transistor from scratch

http://www.101science.com/transistor.htm#HISTORY

Re: Transistor mystery

dear flatulent

"Adding feedback in the form of an unbypassed emitter to ground resistor helps a lot. If you want further linearity, use feedback around two stages. "


asper your say,
emitter resistance are used to increase amt of linearity in transistor. i want from ur side.
actually it is used to make negative feedback to achieve stable gain.pls throw light in this regard

Re: Transistor mystery

Electo_tada said:

How are transistors selected for various practical circuits.
Say i want to design a amplifier how do i begin.
How to choose bias and various r and c values ?

What can be a check for transistor working in active mode and how can the gain be calculated ?

Fuh !! lots of question in on stroke !!!! but can' t help it ,i am dying to understand the working of practical transistor circuits.

Thanks !!

Click to expand...

To be honest this question is better answered by the guys who produce the transistor or the active device.

Your part starts in reading of datasheets. Keep in mind that never is produced a transistor identical to the next transistor. But datasheets are 95% (sometimes more) approximate of what they say. Yes you need to read the datasheets!

You want to calculate gains,etc. etc.? There quite several kinds of transistors and more will be.

When you mention practical transistors you mean most popular ones. I do not think forum space allows explanations space of several chapters.

Read! Read! a good advice.

Re: Transistor mystery

read the book which Sedra or Spencer "introduce to Electronics"

Transistor mystery

You´d better read Sedra (if you want to LEARN, with a scientific approach -- but ignore the semiconductor theory, it´s not necessary) or Malvino (if you want to fix some "easy" electronic devices in your house, it´s a book with practical approach).

I think you should take a look at Opamps before Transistors. Besides being easier to learn and giving you some insight on amplification, maybe it´s an opamp what you need, not a transistor.

Regards
Vilela