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5.2: Frequency Response

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  • In Chapter Four, a number of equations were presented for the various amplifier topologies. These enabled you to find the circuit gain, among other things. These equations are, of course, only valid in the midband region of the amplifier. They say nothing of the amplifier response at the frequency extremes. Chapter One showed that all amplifiers eventually roll off their gain as the input frequency increases. Some amplifiers exhibit a rolloff as the input frequency is decreased as well. Op amp circuits are no exception. There are two things we can say about the average op amp circuit's frequency response: (1) if there are no coupling or other lead network capacitors, the circuit gain will be flat from midband down to DC; and (2) there will eventually be a well-controlled high-frequency rolloff that is usually very easy to find. Item one should come as no great surprise, but you may well wonder about the second. For general-purpose op amps, the high frequency response may be determined with a parameter called the gain-bandwidth product, often abbreviated GBW.