Fast Composite Amplifier Circuit

An ultra-low-noise, low-distortion op ampthe AD797is combined with the ADS 11 op amp, which offers a high bandwidth and a 100-mA output drive capability. The composite-amplifier circuit serves quite well when driving high resolution ADC`s and ATE systems. The fast AD811 operates at twice the gain of the AD797 so that the slower amplifier need only slew one-half of the total output swing. Using the component values shown, the circuit is capable of better than -90 dB THD with a 5-V, 500-kHz output signal. If a 100-kHz sine-wave input is used, the circuit will drive a 600- load to a level of 7 V rms with less than -109 dB THD, as well as a 10-kQ load at less than -117 dB THD.
Fast Composite Amplifier Circuit - schematic

The device can be modified to supply ari overall gain of 5 by changing both the R/R-m ratio and R^R2 ratio to 4:1. This raises the gains of AD811 and the total circuit while maintaining the AD797 at unity gain. If only the R/Rm ratio is changed, the circuit might become unstable. In contrast, if only the RJR2 ratio is varied, the AD797 will then operate at gain. Subsequently, the circuit will have a lower overall bandwidth. Ri should be equal to the parallel combination of Rm and Rf.

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