Bipolar transistor project6/24/2023 At frequencies below the smallest of the three, the gain will fall off at a rate of 60 dB/ decade (I DB/octave). the lower cutoff frequency will be the largest of the three. If these are all distinct, and reasonably well removed from each other. an emitter follower and was previously expressed as (stages (see equation 5-64):Figure 10-27 lower cutoff frequency due to tire bypass capacitor, f is the frequency where the repentance of Ct equals the resistance R.We see that there arc three possible break frequencies in the amplifier of Figure 10-27( (1)( C2). Thus, we can define a lower cutoff frequency due to CE, NCE) that affects the amplifier’s low frequency response in the same way as the coupling capacitors C and C If the coupling capacitors arc large enough to have negligible effect, then the gain will fall to 12/2 times its midland value at the frequency where the repentance of Ct: equals the resistance R, looking into node A in Figure 10-27 R is the same as the output resistance of. Recall that ac signal degeneration occurs in the absence of the bypass capacitor because of the ac voltage drop across RF At sufficiently high frequencies the reluctance of the capacitor is negligible so the emitter is effectively at ac ground and there is no signal loss across RI At low frequencies, the reluctance of C£ can become significant, to the extent that the voltage gain is reduced. designed to eliminate signal degeneration, as discussed in Chapter 6. from equation 10-40.įigure 10-27 shows a common-emitter amplifier having an emitter bypass capacitor Cf. Since roll(stage) = (ISO kl) II (1.5 kH)= 1.5 k we have. Assume that the resistance looking into the base of the transistor is 1500 H and that the transistor output resistance at the collector r is 100 kH. The next example illustrates the application of equations 10-40 and 10 41 to determine the lower cutoff frequency of a fixed-bias common-emitter amplifier.Find the lower cutoff frequency of the amplifier shown in Figure 10-26. Similarly r in equation 10-41 is r(stage) and its value depends on particular amplifier characteristics. the bias resistors, and the values of the transistor parameters. Its value depends on the transistor configuration. the resistance seen by the source when looking into the amplifier. the term appearing in equation 10-40 is r (stage). What is the approximate bandwidth of the amplifier? Solution From equation 10-39,In a BJT amplifier. Example The specifications for a certain oscilloscope state that the rise time of the vertical amplifier is 8.75 ms. Equation 10-39 is used when the lower equency is 0 (dc) or very small, so that the bandwidth is essentially the same withe upper cutoff frequency The relationship is exact if the high- frequency response beyond cutoff is the same as that of the single RC low-pass network (Figure 10-25). Low- Frequency Response of BJT Amplifiers We have learned that the lower cutoff frequency of an amplifier is approximately equal to the larger of (CI) and NC), where where B is the bandwidth, in hertz.
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