* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download lecture 2:bjt small
Dynamic range compression wikipedia , lookup
Stepper motor wikipedia , lookup
Power inverter wikipedia , lookup
Variable-frequency drive wikipedia , lookup
Three-phase electric power wikipedia , lookup
Signal-flow graph wikipedia , lookup
Electrical ballast wikipedia , lookup
History of electric power transmission wikipedia , lookup
Electrical substation wikipedia , lookup
Pulse-width modulation wikipedia , lookup
Ground loop (electricity) wikipedia , lookup
Analog-to-digital converter wikipedia , lookup
Power electronics wikipedia , lookup
Two-port network wikipedia , lookup
Surge protector wikipedia , lookup
Schmitt trigger wikipedia , lookup
Current source wikipedia , lookup
Voltage regulator wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Power MOSFET wikipedia , lookup
Stray voltage wikipedia , lookup
Voltage optimisation wikipedia , lookup
Buck converter wikipedia , lookup
Alternating current wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Mains electricity wikipedia , lookup
Network analysis (electrical circuits) wikipedia , lookup
LECTURE 1: BASIC BJT AMPLIFIER -AC ANALYSIS- 1 BJT LINEAR AMPLIFIER - BJT small signal operation - BJT AC equivalent circuits 2 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 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 Block diagram of a compact disc player system. 5 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 a linear amplifier. 6 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 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. Figure (c) – voltage transfer characteristic graph. To become amplifier, transistor need to be biased with dc voltage @ 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 Figure a Figure b Figure c 9 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 Fig d) Common-emitter cct with a time-varying signal source in series with dc source. Fig e) Common-emitter transistor characteristic, dc load line and sinusoidal variation in base current, collector current and collector-emitter voltage. 11 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. 12 Based on Fig. d & e (time-varying signals linearly related & superimposed on dc values) iB I BQ ib (1) iC I CQ ic (2) vCE VCEQ vce (3) vBE VBEQ vbe (4) If signal source, vs = 0: VBB I BQ RB VBEQ (B - E loop) (5) VCC I CQ RC VCEQ (C - E loop) (6) – For B-E loop, considering time varying signals: VBB vs iB RB vBE ( I BQ ib ) RB (VBEQ vbe ) Rearrange: VBB I BQ RB VBEQ ib RB vbe vs (7) (8) Base on (5), left side of (7) is 0. So: vs ib RB vbe (9) – For C-E loop, considering time varying signals: VCC iC RC vCE ( I CQ ic ) RC (VCEQ vce ) (10) VCC I CQ RC VCEQ ic Rc vce (11) – Base on (6), left side of (11) is 0. So: ic Rc vce 0 (12) Signal source, vs in base cct generate time-varying component in base cct –- iB and vBE. Figure f) 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. 15 From figure above, relation between vBE and iB is: iB v . exp BE 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 16 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 BE i B I BQ 1 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 17 RC ic vO RB vs ib + vce + vbe - AC equivalent circuit of C-E with npn transistor 18 Input loop: vs ib RB vbe I BQ vbe ib VT Output loop: 0.026 V ic RC vce 0 Set all dc current and voltage to zero – voltage become short cct & current become open cct. ic ib 19 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 20 Definition of small signal Small signal : ac input signal voltages and currents are in the order of ±10 percent of Q-point voltages and currents. e.g. If dc current is 10 mA, the ac current (peak-topeak) < 0.1 mA. Figure shows iB vs. vBE with small-time varying signal superimposed at Q-pt. Since sinusoidal signals are small, the slope at Q-pt treated as a constant, has units of conductance. The inverse of this conductance is smallsignal resistance, rπ 22 We can relate small-signal input base current to smallsignal input voltage by: v be i b r Finding rπ from Q-point slope lead to: v be VT VT r ib I BQ I CQ rπ also known as diffusion resistance and is a function of Q-point parameters. VT is known as thermal voltage. 23 Now, we consider the output terminal characteristic of BJT. Assume o/p collector current is independent of collectoremitter voltage collector-current is a function of baseemitter voltage, so the equation: i C i C v BE .v BE Q pt From eq 5.2 in Chapter 5 Neaman, iC v BE I S exp VT 24 After substitution and rearrange the above, we obtain: I CQ v BE iC 1 . I S exp v BE Q pt VT VT Q pt VT The term ICQ / VT is a conductance. Since this term relates current in collector to a voltage in B-E circuit, it is called transconductance and is written: gm I CQ VT Transconductance also a function of Q-pt parameters and directly proportional to dc bias current. 25 Using these new parameters develop a simplified small-signal hybrid-π equivalent cct for npn BJT. Phasor components given in parentheses. This circuit can be inserted into ac equivalent circuit shown previously. 26 gm=ICQ/VT r=VT/ICQ Transconductance parameter 27 We can relate small-signal collector current to smallsignal base current for o/p of equivalent cct. i C ic .i b i B Q pt Where i C i B Q pt β is called ac common-emitter current gain. Thus: i c i b 28 ib (Ib ) Current gain parameter 29 Combine cct. BJT equivalent cct to ac equivalent Small-signal hybrid-π model 30 Voltage gain, Av = ratio of o/p voltage to i/p voltage. Small-signal B-E voltage is called the control voltage, Vbe or V. The dependent current source is gmV flows through RC produce –ve C-E voltage at the output. Vo Vce g mVbe RC 31 From the input portion of the circuit, using voltage divider: r Vs Vbe r RB The small-signal voltage gain is: r Vo Av g m RC Vs r RB 32 Given : = 100, VCC = 12V VBE = 0.7V, RC = 6k, VT=0.026V, RB = 50k and VBB = 1.2V Calculate the small-signal voltage gain. 33 1. 2. 3. 4. 5. 6. I BQ VBB VBE ( on) RB 1.2 0.7 10 A 50 I CQ I BQ 100(10A) 1 mA VCEQ VCC I CQ RC 12 (1)(6) 6V VT (100)(0.026) r 2.6 k I CQ 1 I CQ 1 gm 38.5 mA / V VT 0.026 r Vo 11.4 Av g m RC Vs r RB 34 Given VCC=5V, VBB=2V, RB=650kΩ, RC=15kΩ, β=100 and VBE(on)=0.7V. Determine: a) Q-points, b) gm and r c) voltage gain. 35 Early Voltage (VA) 36 Figure above show current-voltage characteristic for constant values of B-E voltage. The curves are linear with respect to C-E voltage in forward-active mode. The slope is due to base-width modulation effect Early Effect. When the curves extrapolated at zero current, they meet a point on –ve voltage axis, vce = -VA. VA --- Early voltage with typical value in range of 50 < VA < 300V. 37 Early Effect => collector current, iC is dependent to collector-emitter voltage, vCE (refer Chapter 5Neaman): v BE v CE . 1 i C I S exp VA VT The output resistance, rO: v CE rO i C Q pt Substitute and rearrange both equation, v BE 1 I S exp rO VT 1 . V A Q pt I CQ VA 38 Hence, small-signal transistor output resistance, rO become: VA rO I CQ rO is equivalent to Norton resistance rO is parallel with dependent current sources. 39 Transconductance parameter ro=VA/ICQ Current gain parameter 40 From Neaman textbook, Ac equivalent circuit – pg 386 Transconductance and current gain – pg 386 & 387 Small-signal hybrid-π equivalent circuit – pg 387 Do example 6.3 41 Include 2 additional resistance, rb and rμ. rb series resistance of semiconductor material. Since rb << rμ., rb is neglected (short cct) at low freq. rμ reverse-biased diffusion resistance of B-C junction. Typically in megaohms and neglected (open cct). Normally, in hybrid-π model, we neglect both rb and rμ. 42