Two radio stations, WHEW and WHAT, broadcast in the same listening area: WHEW broadcasts at 100 MHz and WHAT at 98 MHz. A single-stage tuned amplifier, such as that shown in Fig. 12.35, can be used as a tuner to filter out one of the stations. However, single-stage tuned amplifiers have poor

Question

Two radio stations, WHEW and WHAT, broadcast in the same listening area: WHEW broadcasts at 100 MHz and WHAT at 98 MHz. A single-stage tuned amplifier, such as that shown in Fig. 12.35, can be used as a tuner to filter out one of the stations. However, single-stage tuned amplifiers have poor selectivity due to their wide bandwidths. To reduce the bandwidth (increase the quality factor) of single-stage tuned amplifiers, designers employ a technique called synchronous tuning. In this process, identical tuned amplifiers are cascaded. To demonstrate this phenomenon, let us generate a Bode plot for the amplifier shown in Fig. 12.35 when it is tuned to WHEW (100 MHz), using one, two, three, and four stages of amplification.

Accepted Answer

Using the circuit for a single-stage amplifier shown in Fig. 12.35, we can cascade the stages to form a four-stage synchronously tuned amplifier. If we now plot the frequency response over the range from 90 MHz to 110 MHz, we obtain the Bode plot shown in Fig. 12.36.
From the Bode plot in Fig. 12.36 we see that increasing the number of stages does indeed decrease the bandwidth without altering the center frequency. As a result, the quality factor and selectivity increase. Accordingly, as we add stages, the gain at 98 MHz (WHAT’s frequency) decreases, and that station is “tuned out.”

Question 12.15: Two radio stations, WHEW and WHAT, broadcast in the same lis...

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