The composite circuit in Fig 1 couples the output drive and slew rate of a current-feedback amplifier, IC2, with the low-noise and low-offset characteristics of a voltage-feedback operational amplifier, IC1. The circuit achieves gains as high as 1000, while showing little variation in frequency characteristics with gain. This circuit suits applications requiring high gain and wide dynamic range, such as ultrasound, radar, digital radio, and other high-noise applications.
IC1 has a gain-bandwidth product of approximately 1.6 GHz. Configured as an amplifier with a gain of 1000, it would exhibit less than 2 MHz of bandwidth. Further, the limit of the output swing of IC1 is less than ±3.8V typ. IC2 has over 50 MHz of bandwidth and ±13V output swings along with the gain-bandwidth independence for which current feedback is famous. Unfortunately, current-feedback amplifiers are not suitable for very high gains. To achieve high gain, a current-feedback amplifier's gain-setting resistor must become very small (for gain-bandwidth independence, the feedback resistor must remain relatively constant), and the signal gets lost in the current-feedback amplifier's noise.
In Fig 1, IC1 has a gain of 1000 and is inside IC2's feedback loop. IC2 is a gain block with A=10. Thus, IC1 is actually providing a gain of only 100, allowing it to maintain in excess of 10 MHz of bandwidth and requiring it to develop relatively small output swings.
Because IC2 has a gain of only 10, it has reasonable gain-resistor values, and their noise contribution is not significant. For this circuit to remain stable, IC1 must have sufficient phase margin to allow for IC2's phase lag. Adding the resistor from IC1's RP pin to VEE reduces the current IC1 draws, compensating the amplifier. A 250(ohm) resistor stabilizes the circuit, allows for a 10-MHz large-signal bandwidth, and still offers good gain flatness.
If you need similar frequency characteristics at another gain, change the gain of IC2. The current-feedback amplifier's frequency characteristics are independent of gain. For example, to reduce the circuit's gain to 500, reduce RF to 25 k(ohm) and increase R1 to 160(ohm), so that IC1's gain is still 100. Increasing gain simply by increasing RF proportionally decreases the circuit's bandwidth. (DI #1399)