EL2003, EL2033 The easiest way to drive capacitive loads is to isolate them from the feedback with a series resistor. Ten to twenty ohms is usually enough but the final value depends on the op amp used and the range of load capacitance. DIRECT DRIVEDOUBLE MATCHED 10Ω is enough isolation and speed is determined by the isolation resistor and capacitive load time constant. OP AMP BOOSTER WITH CAPACITIVE LOADCLtROS 10pF 17ns 10% BACK MATCHED 470pF 20ns 50% Op Amp Booster 0.001µF 30ns 35% The EL2003 or EL2033 can boost the output drive of almost 0.005µF 80ns 0 any monolithic op amp. Because the phase shift in the 0.01µF 220ns 0 EL2003 and EL2033 is low at the op amp's unity gain frequency, no additional compensation is required. By 0.05µF 1.1µs 0 following an op amp with an EL2003 or EL2033, the buffered 0.1µF 2.2µs 0 op amp can drive cables and other low impedance loads directly. Even decompensated high speed op amps can take If the system requirements will not tolerate the isolation advantage of the EL2003’s or EL2033’s 100mA drive. resistor, then additional high frequency feedback from the op amp output (the buffer input) and an isolating resistor from the buffer output is required. This requires that the op amp be unity gain stable. OP AMP BOOSTER Driving capacitive loads with any closed loop amplifier creates special problems. The open loop output impedance This works with any unity gain stable OA. works into the load capacitance to generate phase lag which Snubber Circuit (51Ω 470pF) is optional. can make the loop unstable. The output impedance of the EL2003 or EL2033 is less than 10Ω from DC to about COMPLEX FEEDBACK WITH THE BUFFER TO DRIVE CAPACITIVE LOADS 10MHz, but a capacitive load of 1000pF will generate about 45° phase shift at 10MHz and make high speed op amps unstable. Obviously more capacitance will cause the same problem but at lower frequencies, and slower op amps as well would become unstable. 8
