The operating point of a device, also known as bias point, quiescent point, or Q-point, is the point on the output characteristics that shows the DC collector–emitter voltage ( V ce) and the collector current ( I c) with no input signal applied.Ī bias network is selected to stabilize the operating point of the transistor, by reducing the following effects of device variability, temperature, and voltage changes: By selecting the proper resistor values, stable current levels can be achieved that vary only little over temperature and with transistor properties such as β. The voltage divider configuration achieves the correct voltages by the use of resistors in certain patterns. Much more elaborate biasing arrangements are used in integrated circuits, for example, bandgap voltage references and current mirrors. In circuits made with individual devices (discrete circuits), biasing networks consisting of resistors are commonly employed. The hybrid equivalent of a transis tor alone when connected in CC configuration is shown in Fig.A load line diagram, illustrating an operating point in the transistor's active regionīipolar transistors must be properly biased to operate correctly. Input signal source across its input terminals and load resistance across output terminals may be connected Its V/I characteristics are described by the following equations The hybrid equivalent of the transistor alone when connected in CE configuration is shown in Fig. It is all the reason to call it a hybrid equivalent circuit (b) Hybrid CE Circuit ![]() Incidentally, it may be noted that the ac equivalent circuit contains a Thevenin's circuit in the input and a Norton’s circuit in the output. It may be noted that no external dc biasing resistor or ac voltage sources have been connected to the equivalent circuit as yet. Similarly, ac voltage polarities have been taken by considering upper terminal positive and lower one as negative As per current convention, collector ie is shown flowing inwards though actually this current flows outwards as shown by the arrow inside the ac current source Similarly, current ic in the output terminals must equal the sum of two branch currents These equations are self-evident because applied voltage across input terminals must equal the drop over hib and the generator voltage Base Bias with Collector And Emitter Feedbacks It is a combination of (2) and (3) above. Base Bias with Collector Feedback It is also known as collector-to-base bias or collector feedback bias. Base Bias with Emitter Feedback This circuit achieves good stability of dc operating point against changes in β with the help of emitter resistor which causes degeneration to take place.ģ. Base Bias or Fixed Current Bias It is not a very satisfactory method because bias voltages and currents do not remain constant during transistor operation.Ģ. ![]() Some of the methods used for providing bias for a transistor are :ġ. collector-base junction is always reverse-biased This type of biasing is known as FR biasing emitter-base junction is always forward biased ![]() Biasing- Types of biasing- DC equivalent circuit of BJT- Load Line-DC and AC Load Line Analysis – Hybrid Model of BJT- Hybrid Model Analysis of CE, CB, CC - Calculation of Input Impedance, Output Impedance, Voltage Gain, Current Gain using hybrid model- Approximate Model of BJT- CE, CB and CC Analysis- Small signal equivalent circuit of BJT- Small Signal Analysis of CE, CB and CC.īiasing For proper working of a transistor, it is essential to apply external voltages of correct polarity across its emitter-base and collector-base junctions. UNIT – II TRANSISTOR BIASING CIRCUITS ANS SMALL SIGNAL ANALYSIS OF BJT AMPLIFIERS 9 Hrs.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |