Electronics, Power Electronics, Optoelectronics, Microwaves ...

7AmplifiersGordon L. CarpenterCalifornia State UniversityJohn Choma, Jr.University of Southern California7.1 Large Signal Analysis ........................................................... 7 -1* *DC Operating Point Graphical Approach Power Amplifiers7.2 Small Signal Analysis ........................................................... 7 -7*Hybrid-Pi Equivalent Circuit Hybrid-Pi Equivalent Circuit* *of aMonolithic BJT Common Emitter Amplifier Design*Considerations for the Common Emitter Amplifier Common*Base Amplifier Design Considerations for the Common*Base Amplifier Common Collector Amplifier7.1 Large Signal AnalysisGordon L. CarpenterLarge signal amplifiers are usually confined to using bipolar transistors as their solid-state devices because ofthe large linear region of amplification required. One exception to this is the use of VMOS for large poweroutputs due to their ability tohave alarge linear region. There are three basic configurations of amplifiers:common emitter (CE) amplifiers, common base (CB) amplifiers, and common collection (CC) amplifiers. Thebasic configuration of each is shown in Figure 7.1.In an amplifier system, the last stage of avoltage amplifier string has to be considered as alarge signalamplifier, and generally EF amplifiers are used as large signal amplifiers. This then requires that the dc bias ordc operating point (quiescent point) be located near the center of the load line in order to get the maximumoutput voltage swing. Small signal analysis can be used to evaluate the amplifier for voltage gain, current gain,input impedance, and output impedance, all of which are discussed later.DC Operating PointEach transistor connected inaparticular amplifier configuration has aset of characteristic curves, as shown inFigure 7.2.When amplifiers are coupled together with capacitors, the configuration is as shown in Figure 7.3. Theload resistor is really the input impedance ofthe next stage. To be able to evaluate this amplifier, adcequivalent circuit needs to be developed as shown in Figure 7.4. This will result in the following dc biasequation:I CQ ¼V BB V BER B = beta þ R EAssume h FE 44 1where beta ( h FE )isthe current gain of the transistor and V BE is the conducting voltage across the base-emitterjunction. This equation is the same for all amplifier configurations. Looking at Figure7.3, the input circuit canbe reduced to the dc circuit shown in Figure 7.4 using circuit analysis techniques, resulting in the following7 -1