|Radio and Electronics (DED Philippinen, 66 p.)|
|11. ACTIVE COMPONENTS -2- / TRANSISTORS|
|11.2. CHARACTERISTICS OF TRANSISTORS|
|11.2.1 HANDLING OF CHARACTERISTICS OF TRANSISTORS|
Even if you find in a given four-quadrant-characteristics a so called mutual-characteristics this is very often not fitting, because this characteristics is changing when collecter-emitter-voltage is changing. For this reason it is necessary to know how this characteristics can be found for each voltage you want it.
A) draw a vertical line into the 1. quadrant at the collector-emitter-voltage for which you want the mutual-characteristics.
B) draw horizontal lines through the points at which this vertical line is crossed over by the lines of the output-characteristics.
C) Find out the values of the base current represented by the crossed over lines in the output-characteristics and mark the horizontal lines at those values of the base-current in the 2. quadrant.
D) Connect all the points which you have found to get a new graph - this is the mutual characteristics for the desired collector-emitter-voltage.
If the transistor is connected in a circuit the mutual characteristics changes according to the parameters given in this very circuit. The graph, showing this characteristics, is called the DYNAMIC MUTUAL CHARACTERISTICS and it shows actually the ability of amplifications of this very circuit.
To construct it, we enter the loadline of the collector-resistor into the output quadrant and find the values of the collector-current for each given base-current. The relation Ic/Ib is represented then in the 2. quadrant as shown in fig. 133.
For the given circuit of a transistor with the given characteristics, an Rc of 300 Ohms and a supply voltage of 9 Volts, we find that the amplification is rather constant as long as the base-current does not exceed 3 mA. Above this value the amplification is decreased down to 0.
Mind too, that the output voltage Vce is high at low base-currents (low-input-signal) and vice versa.
Each transistor has a maximum power rating. Which means: this is the power which can be dissipated by it, without danger of destruction.
Therefore it is extremly important to make sure that the transistor is always operated on the safe side - which means in our field of characteristics: on the safe side of the maximum power line. For this reason it is necessary to be able to construct the maximum-power-line.
A) find in the data sheets the maximum permissible power for the transistor.
B) Calculate for various collector-emitter-voltages the permissible collector-current for this power. (The power dissipated for the input-values is so minute, that it can be neglected).
Ic = Ptot / Vce
C) Construct the Power-line by inserting the values of Ic for the different collector-emitter voltages.
The maximum powerline represents the field of application in a safe-area and a not allowed area.