Biasing Stability Factor Different Methods for Transistor Biasing, Study Guides, Projects, Research of Art

Download Biasing Stability Factor Different Methods for Transistor Biasing and more Study Guides, Projects, Research Art in PDF only on Docsity! UNIT – II TRANSISTOR BIASING CIRCUITS ANS SMALL SIGNAL ANALYSIS OF BJT AMPLIFIERS 9 Hrs. 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. Biasing For proper working of a transistor, it is essential to apply external voltages of correct polarity across its emitter-base and collector-base junctions.  emitter-base junction is always forward biased  collector-base junction is always reverse-biased This type of biasing is known as FR biasing Stability Factor Different Methods for Transistor Biasing Some of the methods used for providing bias for a transistor are : 1. 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. 2. 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. 3. Base Bias with Collector Feedback It is also known as collector-to-base bias or collector feedback bias. It provides better bias stability. 4. Base Bias with Collector And Emitter Feedbacks It is a combination of (2) and (3) above. 5. Emitter Bias with Two Supplies This circuit uses both a positive and a negative supply voltage. Here, base is at approximately 0 volt i.e. V B ≅ 0. 6. Voltage Divider Bias It is most widely used in linear discrete circuits because it provides good bias stability. It is also called universal bias circuit or base bias with one supply. Base Bias with Emitter Feedback Emitter Bias with Two Supplies This circuit gives a reasonably stable Q-point and is widely used whenever two supplies (positive and negative) are available. Its popularity is due to the fact that IC is essentially independent of β. DC Equivalent Circuit (a) CB Circuit  In an ideal transistor, α = 1 which means that IC = IE.  The emitter diode acts like any forward-biased ideal diode.  However, due to transistor action, collector diode acts as a current source.  The purpose of drawing dc equivalent circuit is to view an ideal transistor as nothing more than a rectifier diode in emitter and a current source in collector.  In the dc equivalent circuit of Fig.(b), current arrow always points in the direction of conventional current.  As per the polarities of transistor terminals shown in Fig.(a), emitter current flow from E to B and collector current from B to C.  The dc equivalent circuit shown in Fig. for an NPN transistor is exactly similar except that direction of current flow is opposite. (b) CE Circuit  Fig. shows the dc equivalent circuit of an NPN transistor when connectedin the CE configuration.  As per the polarities of transistor terminals shown in Fig.(a), base current flow from B to E and collector current from C to E.  In an ideal CE transistor, leakage current is ignored and a.c beta is considered as equal to dc beta. DC Load Line  DC load line of a transistor is a straight line jointing cut-off and saturation points.  For the CE circuit, the load line is shown in figure and A is the cut-off point and B is the saturation point. ie = hfb ie + hob veb  These equations are self-evident because applied voltage across input terminals must equal the drop over hib and the generator voltage  Similarly, current ic in the output terminals must equal the sum of two branch currents  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, ac voltage polarities have been taken by considering upper terminal positive and lower one as negative  It may be noted that no external dc biasing resistor or ac voltage sources have been connected to the equivalent circuit as yet.  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 is all the reason to call it a hybrid equivalent circuit (b) Hybrid CE Circuit  The hybrid equivalent of the transistor alone when connected in CE configuration is shown in Fig.  Its V/I characteristics are described by the following equations  Input signal source across its input terminals and load resistance across output terminals may be connected (c) Hybrid CC Circuit  The hybrid equivalent of a transis tor alone when connected in CC configuration is shown in Fig.  Its V/I characteristics are described by the following equations  Input signal source across its input terminals BC and load resistance across output terminals EC may be connected in order to get a CC amplifier Transistor Amplifier Formulae using h-parameters  As shown in Fig., if we add a signal source across input terminals 1-1 of a transistor and a load resistor across its output terminals 2–2, we get a small-signal, low- frequency hybrid model of a transistor amplifier.  It is valid for all the three configurations and holds good for all types of load whether a resistance of an impedance. We will now find expressions for its gains and impedances Before undertaking the above derivations, let us consider different components in the hybrid model of Fig.  The input resistance looks like a resistance (hi) in series with a voltage generator (hr ν2) o This generator represents the voltage feed-back from the output to the input circuit. It is known as voltage-controlled generator because its value is determined by v2 (as hr is a dimensionless constant)  The output circuit also has two components: (i) h0 component which represents the conductance as seen from output terminals and (ii) the current-controlled generator (hf i1) which simulates the transistor’s ability to amplify. The parameter hf is a dimensionless constant The above model can be described mathematically by using the following two equations Current Gain Taking RS into Account  The source current is not the transistor input current because i1 partly flows along RS and partly along rin.  To illustrate this point, consider the Norton's equivalent of the source (Fig.). Typical Values of Transistor h-Parameters Common Emitter h-parameter Analysis The h-parameter equivalent of the CE circuit of Fig.(a) is shown in Fig.(b). In Fig.(a), no emitter resistor has been connected However, Fig. below shows the CE circuit with an emitter resistor RE. 1. Input Impedance  When looking into the base-emitter terminals of the transistor, hie is in series with hre  For a CE circuit, hre is very small so that hre v0 is negligible as compared to the drop over hie  Hence, rin= hie  Ignoring hrevo, 2. Output Impedance Looking back into the collector and emitter terminals of the transistor in Fig. (b), ro ≅ 1/hoe. 3. Voltage Gain 4. Current Gain 5. Power Gain Common Collector h-parameter Analysis The CC transistor circuit and its h-parameter equivalent are shown in Fig. One can make quick approximations of CC gains and impedance if one remembers that hre = 1 i.e. all of ν o is fed back to the input (Art. 59.23). 1. Input Impedance vin = ib hic + hrc ν o = ibhic + ν o = ib hic + ie RL = ibhic + hfe ib RL = ib (hic + hfe RL)