![]() The degree of success achieved in stabilizing l C in the face of variations in I CO is expressed in terms of Stability Factor of Transistor S and it is defined as the rate of change of collector current w.r.t. Heat sink and air blast may be used to remove heat from the transistor. Air-conditioning or thermostat chambers (wherever possible) may be used for maintaining the ambient or case temperature constant. Such devices produce compensating voltages and currents so as to make operating point stable.Īdditional means are adopted to maintain the ambient and junction temperatures constant. Stabilization technique makes use of a resistive biasing circuit that permits such a variation of base current (biasing current) I B as to maintain collector current l C almost constant inspite of variations in reverse saturation current I CO, base-emitter voltage V BE and β.Ĭompensation techniques make use of temperature sensitive devices such as diodes, transistors, thermistors etc. Operating point Stability Factor of Transistor may be achieved by adopting either stabilization techniques or compensation techniques. Means of Achieving Stability For Operating Point: This will achieve thermal stability resulting in bias stability. However, if by some modification, I B is made to fall with increase in temperature automatically, then decrease in the term βI B can be made to neutralize the increase in the term (1 + β)I CO, thereby keeping l C almost constant. The self destruction of an unstabilized transistor is called the thermal runaway. Being a cumulative process, it can lead to thermal runaway resulting in burn out of the transistor. This leads to increased power dissipation with further increase in temperature. (iii) Thermal Runaway: The collector current I C, being equal to β I B + (1 + β)I CO, increases with the increase in temperature. Such problems do not arise in case of vacuum tube circuits because it is possible to manufacture vacuum tubes with identical characteristics. Thus when a transistor is replaced by another transistor of the same type, the operating point may shift. Such little variations result in a large change in the transistor parameters such as β, V BE etc. For an instance, it has not been possible to control the base width and it may vary, although slightly, from one transistor to another one of the same type. For an example BC147 is a silicon transistor with β varying from 100 to 600 (for one transistor β may be 100 and for the other it may be 600). (ii) Individual Variations: The values of β and V BE are not exactly the same for any two transistors even of the same type. Though with the change in temperature collector-emitter voltage V CE also changes but the change is very small and collector current l C is not affected. Base-emitter voltage, V BE, which decreases by 2.5 mV per ☌.Īny or all of the above factors can cause the bias point to shift from the values originally fixed by the circuit because of a change in temperature.Transistor current gain, β, which increases with the increase in temperature. ![]() Reverse saturation current (leakage current), I CO, which doubles for every 10☌ rise in temperature.(i) Temperature Dependence of Collector Current I C: The instability of collector current I C, being equal to βI B + (1 + β)I CO, due to variations in temperature, is caused because of the following three main factors. The stabilization of operating point is essential because of (i) temperature dependence of collector current I C (ii) individual variations and (iii) thermal runaway. The maintenance of the operating point stable (independent of temperature variations or variations in transistor parameters) is known as stabilization.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |