Potentiometer tames current-feedback op amp

Leslie Green, Gould-Nicolet Technologies, Ilford, Essex, UK -- 5/11/2000

The circuit in Figure 1a includes a potentiometer in series with the inverting input of a current-feedback op amp. With a voltage-feedback op amp, a potentiometer in series with the inverting input would serve no useful function. However, with a current-feedback amplifier, the potentiometer controls the bandwidth without changing the gain. The function of this potentiometer is similar to a capacitor, C₁, in parallel with the feedback resistor of a voltage-feedback op amp (Figure 1b).

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)

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