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AC ANALYSIS BJT 1 Content BJT SMALL SIGNAL - BJT small signal operation - BJT AC equivalent circuits 2 Objectives Understand the concept of an analog signal and the principle of linear amplifier. Investigate the process a single-transistor circuit can amplify a small, time-varying input signal. 3 Introduction Signals contain some type of information. The electrical signals in form of time-varying current and voltage are analog signal. Electronic circuit that process analog signal – analog circuit, example linear amplifier. Linear amplifier – magnify an input signal and produce an output signal whose magnitude is larger and directly proportional to input signal 4 Example of linear amplifier Block diagram of a compact disc player system. 5 Cont.. From figure, a dc voltage source connected to amplifier. The amplifier contain transistors that must be forward biased so that they can act as amplifying devices. We want the o/p signal to be linearly proportional to input signal o/p of speaker is an exact reproduction of signal from compact disc. So, we need linear amplifier. 6 Cont.. 2 type of amplifier analysis: dc analysis due to applied dc voltage source. ac analysis due to time-varying signal source. dc analysis is performed by ac source set to zero ~ large signal analysis. ac analysis is performed by dc source set to zero ~ small signal analysis. 7 Bipolar linear amplifier Transistor -- heart of an amplifier. Bipolar transistors is used in linear amplifier cct because of their high gain. Figure (a) – cct where input signal vI contain dc and ac signal. Figure (b) – VBB is dc voltage to bias transistor at Q-point and vs is ac signal that is to be amplified. DC biased transistor @ Q-point transistor biased in forward-active region. A time-varying (sinusoidal) signal is superimposed on dc input voltage, VBB, o/p voltage change along the curve producing a time-varying o/p voltage. 8 9 Cont bipolar linear amplifier If o/p voltage directly proportional to and larger than i/p voltage linear amplifier cct. If transistor is not biased (in cutoff or saturation), o/p voltage doesn’t change with a change in i/p cct is not an amplifier. Summary of notation Variable iB, vBE IB, VBE ib, vbe Ib, Vbe Meaning Total instantaneous values DC values Instantaneous ac values Phasor values 10 Graphical analysis & ac equivalent circuit The graph shows collector current, iC vs c-e voltage, vCE for different values of iB. Q-point is chosen where distance between iB curves are even to get linear amplification. Line between VCC / RC and VCC –- dc load line. Signal source, vs produce ac base current superimposed on quiescent base current. ac collector current produce a time-varying voltage across RC, that induces an ac c-e voltage, vCE. vCE or vO will be larger than i/p to produce signal amplification. 11 Graphical analysis cont.. Common-emitter cct with a time-varying signal source in series with dc source. Common-emitter transistor characteristic, dc load line and sinusoidal variation in base current, collector current and collector-emitter voltage. 12 Graphical analysis cont.. Signal source, vs in base cct generate time-varying component in base cct –- iB and vBE. Figure shows the exponential relationship between iB and vBE. If magnitude of time-varying signal superimposed on dc quiescent pt is small => develop a linear r/ship between ac vBE and ac iB. This r/ship corresponds to the slope of curve at Q-point. Slope at Q-point is inversely proportional to a small-signal parameter, rΠ. 13 Cont.. From figure above, relation between vBE and iB is: v BE iB . exp VT IS If vBE is composed of dc term with sinusoidal component superimposed, vBE=VBEQ + vbe, then V BEQ v be iB . exp VT IS The term [IS/β].exp (VBEQ / VT) is quiescent base current, we can write v BE i B I BQ . exp VT 14 Cont.. The base current eq. is not linear and can’t be written as ac current superimposed on dc quiescent value. If vbe << VT, we can expand the exponential term in a Taylor series, keeping only linear term which lead to small signal approximation. v i B I BQ 1 BE VT I BQ I BQ .v BE I BQ i b VT Where ib is the time-varying base current I BQ i b VT v be 15 AC equivalent circuit RC i c vO RB i vs b + vbe - + vce - AC equivalent circuit of C-E with npn transistor 16 Equations Input loop: vs ib RB vbe I BQ vbe ib VT Output loop: ic RC vce 0 Set all dc current and voltage to zero – voltage become short cct & current become open cct. 0.026 V ic ib 17 Rules for ac analysis Replacing all capacitors by short circuits Replacing all inductors by open circuits Replacing dc voltage sources by ground connections Replacing dc current sources by open circuits 18