The main contents of the chapter consist of the following: Characteristic parameters, the basic structure, configurations, common-emitter amplifier, emitter directly connects to ground, emitter connects to ground by resistor RE
COMSATS Institute of Information Technology Virtual campus Islamabad Dr. Nasim Zafar Electronics 1 EEE 231 Fall Semester – 2012 Basic SingleStage BJT Amplifiers Lecture No. 25 Ø Contents: Nasim Zafar Lecture No. 25 Reference: SingleStage BJT Amplifier Chapter5.7 Microelectronic Circuits Adel S. Sedra and Kenneth C. Smith. Nasim Zafar Introduction Ø Ø Ø The largesignal operation of the BJT amplifier, discussed in lecture 20 (Section 5.3), identifies the region over which a properly biased transistor can be operated as a linear amplifier for small signals Methods for dc biasing the BJT were studied in lecture 22 (Section 5.5), and a detailed study of the smallsignal amplifier operation was also presented (Section 5.6). We are now ready to consider practical transistor amplifiers, and we will do so in this lecture for circuits suitable for Nasim Zafar discretecircuit fabrication. Introduction (contd.) Ø There are basically three configurations for implementing singlestage BJT amplifiers: Ø v The commonemitter v The commonbase and v The commoncollector configurations All three will be discussed in this lecture, using the same basic structure, with the same biasing arrangements Nasim Zafar Introduction (contd.) Ø Ø Ø The basic circuit that we shall use, to implement the various configurations of BJT amplifiers, is shown in slide 8, Ref. SedraSmith (Figure 5.59). Among the various biasing schemes possible for discrete BJT amplifiers, we have selected for simplicity and effectiveness, the one employing constantcurrent biasing (Section 5.5) Slide 8 indicates the dc currents in all branches and the dc voltages at all nodes. Nasim Zafar Introduction (contd.) Ø Ø Ø We would want to select a large value for RB in order to keep the input resistance at the base large (slide 8). However, we also want to limit the dc voltage drop across RB and the variability of this dc voltage, resulting from the variation in β values The dc voltage VB determines the allowable signal swing at the collector Nasim Zafar The Basic Structure Basic structure of the circuit used to realize singlestage, discretecircuit BJT amplifier configurations Nasim Zafar Characterizing BJT Amplifiers Ø Ø Ø To study the BJT amplifier circuits, it is important to know how to characterize the performance of amplifiers as circuit building blocks. During the introduction to this subject, the initial material was limited to unilateral amplifiers. A number of the amplifier circuits however, are not unilateral; that is, they have internal feedback that may cause their input resistance to depend on the load resistance. Similarly, internal feedback may cause the output resistance to depend on the value of the resistance of the signal source feeding the Nasim Zafar Characterizing BJT Amplifiers v For nonunilateral amplifiers, we present here a general set of parameters and equivalent circuits that we will employ in characterizing and comparing transistor amplifiers. Nasim Zafar 10 Summary of CE Amplifier with RE Ø Ø Ø Ø The input resistance Rin is increased by the factor (1+gmRe) The voltage gain from base to collector is reduced by the factor (1+gmRe) For the same nonlinear distortion, the input signal vi can be increased by the factor (1+gmRe) The overall voltage gain is less dependent on the value of β Nasim Zafar 29 Summary of CE Amplifier with RE Ø Ø Ø The reduction in gain is the price for obtaining the other performance improvements Resistor RE introduces the negative feedback into the amplifier The high frequency response is significantly improved Nasim Zafar 30 The CommonBase (CB) Amplifier Nasim Zafar 31 The CommonBase Amplifier Nasim Zafar 32 Characteristics of CB Amplifier • • Input resistance Voltage gain Rin Av re g m ( RC // RL ) ( RC // RL ) Rsig re Gv • Overall voltage gain Rout • Output resistance • Shortcircuit current gain RC Ais Nasim Zafar 33 Summary of the CB Amplifier Ø Very low input resistance Ø High output resistance Ø Shortcircuit current gain is nearly unity Ø High voltage gain Ø Noninverting amplifier Ø Excellent highfrequency performance Nasim Zafar 34 The CommonCollector (CC) Amplifier or EmitterFollower Nasim Zafar 35 The CommonCollector Amplifier or EmitterFollower Nasim Zafar 36 The CommonCollector Amplifier or EmitterFollower Nasim Zafar 37 Characteristics of CC Amplifier Ø Ø Input resistance Rib Voltage gain Av (1 (1 (1 Gv Ø Overall voltage gain Rout Ø Output resistance • Shortcircuit current gain )(re ro // RL ) )(ro // RL ) )(re ro // RL ) RB // Rib (1 )(ro // RL ) RB // Rib Rsig (1 )(re ro // RL ) re Ais Nasim Zafar RB // Rsig (1 ) 38 Summary for CC Amplifier or EmitterFollower Ø High input resistance Ø Low output resistance Ø Voltage gain is smaller than but very close to unity Ø Large current gain Ø The last or output stage of cascade amplifier Ø Frequency response is excellent well Nasim Zafar 39 Summary and Comparisons Ø Ø Ø Ø The CE configuration is the best suited for realizing the amplifier gain. Including RE provides performance improvements at the expense of gain reduction The CB configuration only has the typical application in amplifier. Much superior highfrequency response The emitter follower can be used as a voltage buffer and exists in output stage of a multistage amplifier. Nasim Zafar 40 Example: 5.41 Ø • • Consider the circuit of Fig. 5.59 for the case VCC = VEE =10 V, I = 1 mA, RB=100 kΩ, RC=8 kΩ, and β =100 Find all dc currents and voltages. What are the allowable signal swings at the collector in both directions? How do these values change as β is changed to 50? To 200? Evaluate the values of the BJT smallsignal parameters at the bias point (with β = 100). The Early voltage VA = 100 V Nasim Zafar 41 Example: 5.41 Nasim Zafar 42 Example: 5.41 Nasim Zafar 43 ...Basic SingleStage BJT Amplifiers Lecture No. 25 Ø Contents: Nasim Zafar Lecture No. 25 Reference: SingleStage BJT Amplifier Chapter5.7 Microelectronic Circuits Adel S. Sedra and Kenneth C. Smith. Nasim Zafar. .. Frequency response is rather poor Nasim Zafar 25 The CommonEmitter Amplifier with a Resistance in the Emitter Nasim Zafar 26 The CommonEmitter Amplifier with a Resistance in the Emitter Nasim Zafar 27 Characteristics of the CE Amplifier with a ... emitter Nasim Zafar 20 The CommonEmitter (CE) Amplifier Nasim Zafar 21 CommonEmitter Amplifier Equivalent circuit obtained by replacing the transistor with its hybridpi model Nasim Zafar 22