The Circuit Designer's Companion - diagramas.diagram...

158 The Circuit Designer’s Companionand its compensation capacitor is 30pF, so its maximum slew rate is 0.67V/µs. Op-ampdesigners have the freedom to set both these parameters within certain limits, and thisis what distinguishes a fast, high-supply-current device from a slow, low-supplycurrentone. “Programmable” devices such as the LM4250 or LM346 make the tradeoffmore obvious by putting it in the circuit designer’s hands.If i out1 can be increased without affecting the transconductance g m , then slew ratecan be improved without a corresponding reduction in stability. This is one of the majorvirtues of the biFET range of op-amps. The JFET input stage can be run at high currentsfor a low g m relative to the bipolar and so can provide an order of magnitude or moreincrease in slew rate.Large-signal bandwidthSlew-rate limitations on dV out /dt can be equated to the maximum rate-of-change of asinewave output. The time derivative of a sinewave isd/dt [ V p sinωt ] = ω·V p cosωt where ω = 2πfThis has a maximum value of 2πf·V p , which relates frequency directly to peak outputvoltage. If V p is equated to the maximum dc output swing then f max can be inferredfrom the slew rate and is equal to the large signal or full power bandwidth,2π · f max = slew rate / V pSlewing distortionOperating an op-amp above the slew-rate limit will cause slewing distortion on theoutput. In the limit the output will be a triangle wave (Figure 5.10) as it alternatelyswitches between positive and negative slewing, which will decrease in amplitude asthe frequency is raised further. If the positive and negative slew rates differ there willbe asymmetrical distortion on the output. This can generate an unexpected effectequivalent to a dc offset voltage, due to rectification of the asymmetrical feedbackwaveform or overloading of the input stage by large distortion signals at the summingjunction. Also, slewing is not always linear from start to finish but may exhibit a fastrise for the first part of the change followed by a reversion to the expected rate for thelatter part.V diffV inV outV inV outV diffFigure 5.10 Slewing distortion5.2.8 Small-signal bandwidthThe op-amp frequency response shown in Figure 5.8(a) exhibits the same characteristic