《学科竞赛ch》PPT课件.ppt

Ch6 Basic BJT Amplifiers,6.1.1 Graphical Analysis,6.1.2 Small-Signal Model Analysis,6.1.3 Common-Emitter Amplifiers,6.1 AC Analysis of Transistor Circuits,6.1 AC Analysis of Transistor Circuits(CH4),Analog Signals,A signal contains some type of information. There are two kinds of signals: analog and digital. Analog signal, such as temperature, pressure, velocity, can take on any value and may vary continuously with time. The magnitude of digital signal has a finite number of digits at an instant of time, and no longer is a continuous function of time. Electronic circuits that process analog signals are called analog circuit.,6.1.1 Graphical Analysis,1. AC and DC Values,Common-emitter circuit,We add a sinusoidal signal source in series with the dc source. vs produces ac vbe superimposed on VBE, so vBE=VBE+vbe iB =IB+ib iC =IC+ic vCE =VCE+vce,(1) Common-Emitter Circuit,6.1.1 Graphical Analysis,1. AC and DC Values,6.1.1 Graphical Analysis,1. AC and DC Values,Conclusions: 1. vi vBE iB iC vCE |-vo| 2. Phases of vo and vi are opposite_inverter. 3. Getting voltage gain. 4. Determining the max of vo .,# When the circuit is operating， are the real directions of iB and iC changing, and real voltage polarity of vCE is changing？,6.1.1 Graphical Analysis,2. AC Load Line,vce= -ic (Rc /RL),From ac equivalent circuit, we get,Due to vce= vCE - VCEQ ic= iC - ICQ and assume RL = Rc/RL so, vCE - VCEQ= -(iC - ICQ )RL,That is iC = (-1/RL) vCE + (1/RL) VCEQ+ ICQ,The ac and dc load lines are different. The ac load line must pass through the Q-point, and its slope is -1/RL , so we plot the ac line shown in fig.,6.1.1 Graphical Analysis,2. AC Load Line,6.1.1 Graphical Analysis,3. Nonlinear Distortion 非线性失真,There are three regions for i-v characteristic. If Q-point is not suitable, there are saturation or cut-off distortions occurred.,The i-v characteristic of npn,6.1.1 Graphical Analysis,3 .Nonlinear Distortion,Q-point is low, or vs is too large, cut-off distortion occurs.,6.1.1 Graphical Analysis,Q-point is high, or vs is too large, saturation distortion occurs.,Conclusions: Q-point must be suitable. 2. vs is small enough to yield linear relation. 3. Max symmetrical swing can be determined,If we want get the max range of vO, we must: set Q-point at the middle of active region, have suitable ac load line,3. Nonlinear Distortion,6.1.1 Graphical Analysis,The range of vO,3. Nonlinear Distortion,6.1.1 Graphical Analysis,？,1. Answer following questions:,（1）RC increases，how can load line and Q-point change？,（2） RC increases , how can load line and Q-point change ？,（3） VCC decreases, how can load line and Q-point change ？,（4）RL decreases, how can load line and Q-point change ？,Review Questions,6.1.1 Graphical Analysis,2. When VCE of BJT is nearly equal VCC , Which region is it operating in? What may the reasons?,Cut-off region,Answer：,Reasons may:, Rb may open circuit, IB=0, IC=0, VCE= VCC - IC Rc= VCC ., C1 may short circuit VBE=0， IB=0， IC=0， VCE= VCC - IC Rc= VCC .,end,Review Questions,6.1.1 Nonlinear Distortion,？,6.1.2 Small-Signal Model Analysis,Why we consider the small-signal model?,As we know, BJT is a nonlinear device, it is difficult to analyze BJT circuits. The small-signal model is a linear model of nonlinear device BJT, and can simplify the analysis and the design of BJT circuits.,How can we built small-signal model?,When the input ac voltage is very small, we can use the piecewise linear approximation techniques for the i-v characteristics, and take a BJT as a linear device.,6.1.2 Small-Signal Model Analysis,From transfer process of carries we know:,vbe= ibr,1/r is slope of input curve.,So,r is base-emitter input resistor.,1. Hybrid- Equivalent Circuit of BJT,Transconductance传递电导,1. Hybrid- Equivalent Circuit of BJT,For output loop,Incremental values can be represented by ac signal,where, Is ac comment-emitter current gain.,1. Hybrid- Equivalent Circuit of BJT, Hybrid- parameters are the ac parameters. They are related to the Q-point. They are only use in the ac analysis., ib is current-control current source (CCCS). The direction of ib is related to the ib.,2. H-Parameters Equivalent Circuit of BJT,Units of four parameters are different，so called hybrid parameters.,Input resistance, when vce is short for ac signal;,AC current gain, when vce is short for ac signal；,Voltage feedback ratio 反向电压传输比, vbe is open for ac signal;,Output admittance 输出电导, vbe is open for ac signal;,2. H-Parameters Equivalent Circuit of BJT,hre very small, 10-310-4, negligible. hie =rb+ rr . hoe can be calculate ro=1/hoe=VA/ICQ. VA=Vce Ro Very large, 100k, negligible. We get simplified small-signal equivalent circuit.,6.1.2 Small-Signal Model Analysis,3. Circuit Analysis with Small-Signal Model,(1) Find Q-Point Values in DC Equivalent Circuit,3. Circuit Analysis with Small-Signal Model,(2) Small-Signal Equivalent Circuit,The small-signal equivalent circuit,3. Circuit Analysis with Small-Signal Model,(3) Find the voltage gain,Thus,Due to,The small-signal equivalent circuit,(4) Find the input resistance,3. Circuit Analysis with Small-Signal Model,(4) Find the output resistance,Get equivalent circuit of finding output resistance Ro,3. Circuit Analysis with Small-Signal Model,Example 1,Calculate the small-signal voltage gain of the bipolar transistor circuit shown in Fig. Assume =100, VCC=12V, VBE=0.7V, RC=6k, RB=50k, VBB=1.2V. If vi=0.25sint V, find vo .,Sol: (1) Find the Q-point values,3. Circuit Analysis with Small-Signal Model,Example 1,and,(2) Find the small-signal parameters,(3) Find AC parameters,3. Circuit Analysis with Small-Signal Model,Example 1,(3) In small-signal circuit, ac base current is given by,3. Circuit Analysis with Small-Signal Model,Example 2,Find the small-signal voltage gain, input resistor and output resistor in single base resistor biasing CE circuit.,(1) Small-Signal Equivalent Circuit,(2) Voltage gain,3. Circuit Analysis with Small-Signal Model,Example 2,(3) Input resistor,Output resistor,Observing,6.1.3 Circuit Analysis with Small-Signal Model,Example 3,The circuit shown in Fig. Draw its small-signal equivalent circuit.,1. What condition can we get the small-signal equivalent circuit?,end,Review Questions,6.1.2 Small-signal model,？,6.1.3 Common-Emitter Amplifiers,1. Voltage Divider Biasing Circuit,(1) Find Q-point Values,Forming Thevenin equivalent circuit,Q-point,1. Voltage Divider Biasing Circuit,(2) Sketch small-s