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Potentiometer tames current-feedback op amp
Leslie Green, Gould-Nicolet Technologies, Ilford, Essex, UK -- EDN, 5/11/2000
In Figure 1a, feedback resistor R1 strongly governs the bandwidth and pulse-response characteristics of the current-feedback amplifier. However, changing R1 also changes the gain. Thus, you have to change both R1 and R2 to adjust the pulse response while keeping the gain constant, which makes the design considerably difficult to adjust. The inclusion of the potentiometer allows you to adjust the pulse response independently of the gain, which makes the potentiometer a useful control.
Current-feedback amplifiers are available with closed-loop bandwidths much greater than 500 MHz, and they oscillate or give poor pulse responses if you wire them with 2-in. leads on all components and fail to use proper decoupling. You need skill to properly lay out these devices. However, using normal UHF circuit skills and the trick in Figure 1a, you can easily control these devices.
A typical value for the feedback resistor when using a current-feedback amplifier is approximately 150 to 900 W. The nominal value of the potentiometer, R3, is 50 to 500 W.
If you look at the manufacturers' specified time-domain, or pulse, response of current-feedback amplifiers, you may feel that the response is not flat enough for your application. Some data sheets show poor figures for overshoot and ringing. However, using Figure 1a's circuit, you can tune the response of the amplifier within a system to give acceptable performance. You can also put a capacitor across R3 to modify the response of the amplifier. These modifications are sometimes necessary to get the best response from an amplifier or a system. It is also not unusual to have to correct for an error that occurs in another stage.
The main signal path of an oscilloscope is a demanding application for an op amp due to the requirement for high bandwidth and excellent pulse response. However, the scheme in Figure 1a was useful in the design of the main ADC buffer amplifier of the 150-MHz Gould DataSYS 840 DSO. The design uses an HFA1130 current-feedback op amp and an R3 value of 100 W to adjust the overall bandwidth of the oscilloscope. (DI #2524)
