SELECTING A SMALL SIGNAL BIPOLAR JUNCTION TRANSISTOR (BJT)
If you are not familiar with transistor data sheets, using them can be daunting. Most data sheets are very good theses days, but because they are comprehensive, they present a myriad of data using seemingly arcane acronyms which can be completely overwhelming at first. At the other extreme some data sheets are deficient and do not provide enough data for you to use the transistor. If possible, it is best to avoid such transistors but, with experience, you can normally make informed guesses to arrive at working parameters.
The good news is that for most applications you only need to worry about a few parameters, and once you decode some of the acronyms, using a transistor data sheet is quite straight forward.
There are three broad areas that define a transistor:
(1) Mechanical Characteristics
(2) Absolute Maximum Ratings
(3) Electrical Characteristics
Mechanical Characteristics should not present you with any problems so only Absolute Maximum Ratings and Electrical Characteristics will be further covered.
Absolute Maximum Ratings
The maximum ratings give the values for various areas: temperature, voltage, current etc, that must not be exceeded under any operating modes, or the performance of the transistor will be permanently reduced or the transistor will be destroyed.
When selecting a small signal BJT only these absolute maximum characteristics need to be taken into account for the vast majority of applications:
(1) VCEo
(2) IC
(3) Ptot
VCEo means Voltage Collector Emitter with the third terminal, the base, open circuit.
IC is the maximum permissible current flowing from the collector to the emitter.
Ptot is the total power dissipated by the transistor and can be closely approximated by the product of VCE and IC. Ptot is normally specified at either a case temperature of 25 deg C or with the case in air at an ambient air temperature of 25 deg C. In practice, to check that the transistor is not over dissipating do the finger test. If you just cant quite keep your finger on the case that is around 70 deg C and the transistor will be fine. Note that, once again, there is much more to transistor power dissipation than this, but if you use the finger test the component will be safe.
Electrical Characteristics
The electrical characteristics specifies how the transistor will perform under defined operating conditions. For nearly all applications only the following electrical characteristics need be considered:
(1) hFE
(2) VBE
(3) VCEsat
(4) ft
hFE is the current gain of the BJT at DC. It is IC/IB. hfe is the same thing except at a defined frequency. h stands for hybrid characteristic which is a set of models for transistor analysis, which you do not need to worry about for the majority of applications.
VBE is the Voltage across the base emitter terminals that will arise under defined base and emitter or collector currents. A very good data sheet will give maximum and minimum values for VBE, while a good data sheet will just give maximum VBE.
VCEsat or sometimes just Vsat is the voltage across the collector and emitter when the collector current is so high that it drops the supply voltage across the collector load resistor. Vsat is specified under defined conditions. Vsat for a small signal transistor at an Ic of 1ma would typically be 100mV. Many newbees cannot accept this because the base voltage would be around 600mV so how can the collector volts be almost zero. Hard as it is to believe, it is fact.
ft stands for transition frequency. As frequency increases the transistor hfe falls until it reaches one. This point is the transition frequency (ft). (this is a gross simplification but is quite adequate for most work). In general, the further away from ft that your circuit operates the better. Take the BC546. The average ft is 300MHz, so it would be fine for an audio amplifier where the maximum frequency required would probably be around 10MHz. Note that the audio frequency range is 20Hz to 20KHz so you may think that 20Khz would be the maximum frequency required, but this is not the case because you need the transistor to operate beyond the signal pass band so that frequency stability can be ensured. In general, the transition frequency of the BC546 will be adequate for most general purpose applications.
On a really good transistor data sheet, additional information will be given. This often comes under the heading Typical Performance Characteristics. As you can see above, the BC546 data sheet has a comprehensive collection of data under this heading. This information is invaluable when applying the transistor.
The data sheet might have information describing any specialist areas associated with the transistor and finally the data sheet may give applications information, typically showing a range of circuit applications. Most application information though is normally available from the manufacturer as a separate application report or application note.
You may wonder why there is a family of transistor types on the datasheet: BC546 thru BC550 and you may think that each transistor is specially made to have a lower noise or a higher VCE for example. While that may be the case for certain specialist transistor families, more normally, the transistors from a particular family are all made the same, even on the same semiconductor crystal. But, because of the nature of fabrication, individual transistors will have different characteristics so the manufacturer tests and sorts the transistors into groups and allocates a group a number in the family range.
As a general approach in electronics design it is wise to choose a range of components that will suit most general applications. These include, small signal transistor, medium power transistor, high power transistor, and the same for MOSFETs. You might also choose one type of resistor and a couple of types of capacitor and finally, a couple of inductors, one for switch mode power supplies and one for small signal filtering. Collect data sheets and application data for all of these universal devices so that you can look up any information that you might need when you are designing in any of these components. You will now have reduced the mountain of information available for the millions of devices available, into a manageable size.
By studying the data sheets as you go along, you will be surprised how soon you get the feel for the layout and lingo. Quite soon you will know the important characteristics of your set of selected components and that will be an enormous help in your electronics adventures. You can take a similar approach with integrated circuits. For example, about three opamps will meet most of your needs, and everyone needs to know the 555 timer chip inside out, the same for the LM317/LM337 voltage regulator chips.