Passive, active, and digital filters (3ed., CRC, 2009) - tiera.ru

Single-Amplifier Multiple-Feedback Filters 12-9100908070Q actual(%)Q desired605040302010LF351µA741010 100 1,000 10,000Frequency (Hz)FIGURE 12.3 Effect of finite amplifier gain on the design Q for MFB bandpass realizations using two commercialoperational amplifiers.GainFrequency (Hz) mA741 LF35110 1.3 3 10 5 3.16 3 10 5100 10 4 3.16 3 10 41,000 10 3 3.16 3 10 310,000 10 2 3.16 3 10 2Figure 12.3 shows the rather dramatic fall-off in actual Q as frequency increases (and hence gaindecreases). Thus, for designs of modest Q (note that A 2Q 1), a very high quality amplifier is neededif the center frequency is more than a few kilohertz. For example, the LF351 with a unity gain frequencyof 4 MHz will yield 6% error in Q at a frequency of only 1 kHz.12.5.3 TuningLimited functional tuning of the bandpass section is possible. For example, the midband (peak) gainK o ¼G 1 C 3G 5 (C 3 þ G 4 )(12:20)may be adjusted by means of either G 1 or G 5 .Subsequent adjustment of either Q p or v p is possible via G 2 . In view of the discussion above, it is mostlikely that adjustment of Q p will be desired. However, since the expressions for Q p and v p are so similar, anyadjustment of Q p is likely to require an iterative procedure to ensure that v p does not change undesirably.A more desirable functional tuning result is obtained in circumstances where it is necessary to preservea constant bandwidth, i.e., in a spectrum analyzer. Since