Determine op-amp bandwidth requirements

By Ron Mancini -- EDN, 4/13/2000

In my last column, "Op-amp bandwidth and accuracy," I discussed how to select the correct-bandwidth op amp to meet a frequency-performance specification. Now, you learn how to determine the amplifier's bandwidth requirement when marketing doesn't give you a bandwidth specification. The circuit designer's best case occurs when a systems engineer specifies the amplifier bandwidth, and this situation is the case with most data-transmission interfaces, such as digital subscriber lines (DSLs). DSL has detailed specifications for bandwidth, gain, and distortion; thus, the circuit designer can concentrate on designing circuits.

Many times, however, the circuit designer must amplify a poorly defined signal, and the biggest challenge is defining amplifier bandwidth. The simplest method of determining the required amplifier bandwidth is to test the signal. The fastest test method is to solder the new op amp into a circuit and observe the performance. This procedure quickly weeds out inadequate op amps. Even if the performance is good, this process doesn't prove that the new op amp works all the time, because this test method doesn't account for tolerances. This method never tells you whether the op amp has any margin.

Another test method places a tunable filter in the signal path. You decrease the filter bandwidth until the observed performance reaches a lower limit, and you record the bandwidth at that point. The selected op amp must have a greater bandwidth (at the desired gain) than the bandwidth recorded at the lower performance limit. This method enables the designer to determine the amount of margin in the design.

Sometimes you can't use the test method because a tunable filter, a test system, or facsimile signal is unavailable. The last resort is mathematics. The first step is to break the signal into a Fourier series. You accomplish this step by breaking the signal into recognizable components; then you use a Fourier series to represent each component. The temptation is to make the amplifier bandwidth higher than the highest Fourier frequency to preclude signal degradation, but doing so leads to excessive bandwidth. Some signal degradation is allowable in almost every case. The designer should make an educated guess about the required amplifier bandwidth and then discard signal frequencies exceeding this bandwidth. Analyze the reconstructed signal (without the discarded frequencies) to determine whether the distortion is acceptable. If the distortion is acceptable, you should repeat the process with a lower amplifier bandwidth. If the distortion is too high, repeat the process with a higher amplifier bandwidth.

Before you panic at the thought of hours of tedious calculation, know that computer programs exist that considerably simplify the task. Some of the new math programs turn a laborious task into fun. Also, some test equipment breaks signals into a series of sine waves. I remember doing this procedure with a wave analyzer 30 years ago. I am thankful that the laws of physics—as they apply to electronics—haven't changed in my lifetime!

A neophyte might think that an NTSC video signal that fits into a 6-MHz bandwidth requires a 6-MHz-bandwidth amplifier, but the actual bandwidth you use ranges from 15 MHz in low-end video equipment to 300 MHz in studio equipment.

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