ENEE 306: Electronics Analysis and Design Laboratory

Transcription

1 ENEE 306: Electronics Analysis and Design Laboratory Neil Goldsman Department of Electrical and Computer Engineering University of Maryland College Park, MD Spring 2005 Instructor: Professor Neil Goldsman Department of Electrical and Computer Engineering University of Maryland College Park, MD 20742

2 Contents 2 Simple Transistor Amplifiers BJT Forward Active Operation, Equivalent Circuit and β Theory DC Levels, Loop Equations and β Common Emitter Amplifier: DC Bias Theory CE Amp Small Signal Voltage Gain at Midband Frequencies Theory: Approximate Analysis Experiment

3 Laboratory 2 Simple Transistor Amplifiers 2.1 BJT Forward Active Operation, Equivalent Circuit and β Theory Figure 2.1: BJT Basic Structure (NPN) 2

4 Large Signal Equivalent Circuit & Forward Active Mode BJT in forward active to be a three-terminal device composed of a diode and a current controlled current source as shown in Fig Forward Active V C >V B >V E (2.1) Figure 2.2: Large Signal Equivalent Circuit BJT circuit symbol on the left of Fig. 2.2 with the equivalent circuit on the right of Fig The BJT then has three terminals, the base, collector and emitter, and thus three terminal currents, I b, I c,andi e, which are defined in the Figure above. In forward active, the collector current is equal to the base current times the current gain β or I c = βi b,whereβ is typically about 200. From Kirchoff s current law we have Substituting I c = βi b we have I e = I c + I b (2.2) I e = I b (1 + β) (2.3) 3

5 Diode Equation I b = I S exp V be (2.4) V t I S is the saturation current which is a parameter like β that depends on the specific BJT construction. V t is the thermal voltage which is equal to KT where K, T, q are q Boltzmann s constant, absolute temperature At room temperature V t =0.026V. 4

6 2.1.2 DC Levels, Loop Equations and β Fig DC voltage levels at the base (V B ), emitter (V E ) and collector (V C ). Applying the following loop equations: where V CC in the figure is 10V. V B = V CC I B R B (2.5) V E = V B V BE V B 0.7 =I E R E (2.6) V CC I B R B 0.7 I E R E = 0 (2.7) Since β 1, I E I C, we can use the following expression for the collector voltage: V C = V CC I C R C V CC I E R C (2.8) Figure 2.3: Circuit for Determining β 5

7 2.2 Common Emitter Amplifier: DC Bias Theory Amplifies small AC signals, but first must set up a DC bias and operating point. The circuit in Fig. 2.4 is a Common Emitter (CE) Amplifier. Figure 2.4: Common Emitter Amplifier First use R 1, R 2, C 1,andC 2 to set up a DC operating point with v in =0. Purpose of C 1 and C 2 is to isolate the DC operating point currents and voltages from the rest of the world, i.e, the signal source and the load. Replace voltage divider with its Thevenin equivalent, and then by directly applying loop equations to the circuit. To see this consider the circuit in Fig Using KVL on the base emitter loop, we obtain V BB = I B R B + I E R E + V BE (2.9) where R B = R 1 R 2 and V BB = V CCR 2 R 1 +R 2. 6

8 Figure 2.5: Determining DC Bias The B-E loop gives one equation and three unknowns. We can easily reduce the number of unknowns by making the very good approximations I C I E,andV BE =0.7V. Using these approximations and recalling that βi B = I C,wecan obtain the following equation for I C in terms of known parameters. I C = V BB 0.7 R B (2.10) β + R E With I C determined, V C and V E are readily obtained by observing that: V C = V CC I C R C (2.11) V E = I E R E (2.12) V B = V E +0.7 (2.13) 7

9 2.3 CE Amp Small Signal Voltage Gain at Midband Frequencies Theory: Approximate Analysis CE configuration is useful for amplifying small signal voltages. Voltage gain is A v = v out v in Figure 2.6: Common Emitter Amplifier = V c V b. General procedure: Find V c and V b from simple applications of Kirchoff s laws, and then find the small or incremental changes in these voltages due to an applied signal at the base. Using KVL directly on the base emitter loop, and recalling that formostbjt si e I c, and thus substituting I e for I c,gives V b = I c R E + V be (2.14) If we make an incremental change in V b by applying a small signal to the base we obtain: V b = I c R E + V be (2.15) V be is almost always very small. Therefore, a zero order approximation can often be made to neglect V be compared with I c R E to 8

10 yield: V b I c R E (2.16) Look at the collector voltage V C. From KVL we have V c = V CC I c R C (2.17) Since V CC represents a DC power supply V CC =0 V c = I c R C (2.18) Taking the ratio v out v in = V c V b for the voltage gain yields: v out v in R C R E (2.19) Experiment Figure 2.7: CE Circuit for small signal gain 9