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Section D6: BJT Amplifier Applications

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Section D6: BJT Amplifier Applications Section D6: BJT Amplifier Applications In the last several sections, we have extensively studied each of the four basic BJT amplifier configurations and developed analytic expressions for basic amplifier properties. In this section, we are going to compile all this information (not the equations again, don‘t worry!) and discuss uses for each of the configurations based on their characteristics. To start with, let‘s see what characteristics an ideal amplifier would have. The two-port network representation of Figure 5.2 is given to the right, with the amplifier inside the black box. Each configuration we discussed has its own expression for the characteristics of input and output resistance (Rin and Rout), but in every case, Rin is considered to be parallel to vin and Rout is in series with the load RL. So… an ideal amplifier would have Infinite input resistance. This would mean that the entire signal applied to the input would make it to the amplifier (refer back to the voltage divider discussion of section D2 where now Rckt=∞ and vin=vsource). Zero output resistance. The higher the output resistance, the larger the voltage drop across Rout for a given current. This means that less current can be drawn from the amplifier without a significant drop in the output voltage. Since Rout is in series with the load, a zero output resistance (short circuit), would ensure that the load can draw maximum current, for maximum power to the load. Note that, although ideal properties are defined above in terms of resistances, the actual properties of interest are impedances. In subsequent discussions, we may be switching back and forth between the terminology œ don‘t get distressed! When we leave the world of purely resistive behaviors (and we will), the complex quantities that result from inductances and capacitances will be handled exactly the same way, albeit with a little more interesting math! Finally, an ideal amplifier would have infinite (very, very large) gain properties. Well, big surprise… there is no such thing as an ideal amplifier! However, let‘s take a few minutes and compare what we‘ve actually got. Note that Table 5.3 of your text compiles the actual equations for all four configurations. As a lead in to a discussion of potential applications, the table below offers a generalized comparison of some basic characteristics of the four configurations (along with the ideal for reference). Please note that the high, medium and low designations are meant to illustrate the relative behaviors between the different amplifiers œ actual numeric values will depend on circuit components used. Amplifier Configuration Zin Zout Av Ai Ideal ∞ 0 ∞ ∞ Common Emitter (CE) Medium High High High Emitter Resistor (ER) High High Medium High Common Collector (CC)/ Emitter Follower (EF) High Low Low High Common Base (CB) Low High Low* Low *Note: If RB is bypassed, the gain of the CB is much higher. Transistor Amplifier Applications Looking at the table above, we may make some broad observations as to amplifier applications: The common emitter amplifier possesses the advantage of high voltage and current gains. The relatively high input impedance of the CE amplifier is what we‘re looking for, but the high output impedance may create problems when a load is applied directly to the amplifier. The difficulties associated with the high Zout may be overcome by using the CE amp as an intermediary stage in a multistage system, where its high Zout may be matched by the Zin of the following stage. The CE is most often used for voltage amplification. The emitter resistor amplifier is the same as the CE with the emitter resistor not bypassed. It possesses essentially the same properties as the CE, with a somewhat higher input impedance and lower voltage gain. The tradeoff in voltage gain is made up for in the increased stability and lower noise of this configuration due to the negative feedback created by having RE in the circuit. The ER is commonly used for voltage amplification, particularly as a first or intermediary stage of a multistage amplifier. The common collector, or emitter follower, has the benefits of a high input impedance, low output impedance and high current gain. It has a very low voltage gain (≈ 1) and is obviously not to be used for voltage amplification. However, its low output impedance makes it ideal as a voltage buffer between a CE or ER stage and a load. Using the CC (EF) stage, the high output impedance of the CE or ER stage may be connected to a low impedance load without too much loss. The CC (EF) amplifier is also used as a power amplifier and in impedance matching applications. It is normally found as the final (output) stage of a multistage system since it lowers the impedance while providing the necessary power to drive the load. The common base amplifier has the undesirable properties for signal amplification of low input impedance and high output impedance. If the source, or a previous stage, that is driving the CB amplifier has a low impedance and the load is drawing little current, a bypass capacitor may be placed between the base and ground and this configuration may be used as a voltage amplifier. The current gain of less than one also allows a CB stage to be used as a current buffer. In this application, the CB stage accepts a current signal at a low impedance level and delivers an almost equal current at the output at a high impedance level. We‘re not going to be talking about frequency response until next semester, but each of the four configurations above possess unique response characteristics that will be also be a determining factor in appropriate applications (don‘t worry now, just a preview of coming attractions!). Phase Splitter An extremely useful example of a simple multifunctional amplifier may be found in the phase splitter shown to the right (Figure 5.13 in your text). The phase splitter is a single stage amplifier that is a common emitter (with emitter resistor) and a common collector at the same time. The output for the common emitter is taken at the collector (v02), while the output for the common collector is taken at the emitter (v01). Recall from our previous discussions that the output of a common emitter amplifier is 180o out of phase with the input and that the output of a common collector is in phase with the input. Using the notation in the circuit above, the voltage gain expressions for the CE and CC are modified to − R || R A = L2 C common emitter with emitter resistor v r + (R || R ) e E L1 . RE || RL2 AV = common collector re + (RE || RL2 ) If circuit components are chosen such that RC=RE=RL1=RL2, and assuming that RE||RL >> re, the gains will each be of magnitude one. This indicates that the two output signals are equal in amplitude to the input, but are 180o out of phase with each other, as illustrated in the figure above for a sinusoidal input signal. Pretty slick, huh? Just by sticking an extra wire, you can get two distinct outputs. .
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