[Product Innovation]
Wideband Op Amp Unshackles Gain From Bandwidth
Changing the input bufferto a closed-loop design aleviates the gain-bandwidth dependence in low-power CFB op amps.
Current-feedback (CFB) op amps have earned their reputation in many applications that need bandwidth, without compromising other noise and high-speed characteristics. They have served these needs adequately since the mid '80s, especially where dual supplies and higher voltages are easily available. Concurrently, op-amp suppliers have continued to polish this CFB topology to serve many emerging requirements where very high slew rates, coupled with wide bandwidth, are desired as the gain is increased. Nevertheless, CFB op amps are still far from perfect.
Many emerging portable systems demand higher performance with low power and single-supply operation. At supply currents of 2 mA and below, CFB op amps in particular have one major limitation: bandwidth depends on gain. Like their voltage-feedback (VFB) counterparts, CFB op amps with supply currents under 2 mA feature a bandwidth that varies inversely with gain.
Engineers at Texas Instruments' (TI's) Burr-Brown division have now come up with a novel design that virtually eliminates low-power CFB op-amp bandwidth dependence on gain. Key is modifying the op amp's input buffer stage. TI has used this technique to develop a new family of wideband CFB op amps. Called CFBplus, these op amps unshackle the gain from the bandwidth. Implemented in TI's 15-V complementary bipolar process with integrated CMOS, also called CBC, single-supply operation for the op amps is 5 to 12 V, while the dual supply range is up to ±6 V.
In the new modification, the conventional open-loop buffer, between the noninverting and inverting input nodes, has been transformed into a low-power closed-loop architecture (Fig. 1). A significant reduction of the inverting input impedance results, without sacrificing power.
This independence of gain and bandwidth will enable the CFB op amps to support a number of previously difficult applications. For in-stance, adding RC elements in the gain-setting path to provide gain peaking over frequency without interacting with the amplifier's bandwidth is now a reality. This is particularly useful in the differential driver or receiver stages to equalize the high-frequency attenuation for signals in the twisted pair lines.
Providing greater flexibility in gain settings will give amplifiers a greater number of channels. Likewise, an adjustable-gain stage will permit an amplifier to hold the bandwidth constant over gain. Ultimately, the designer decides which way to apply this new component.
The first two members of the CFBplus family include the OPA683 and OPA684. Internal measurements of the OPA684, which offers 1.8-mA/channel quiescent current, indicate that it achieves over 124-MHz small-signal 3-dB bandwidth from gains between 2 and 20. These results were measured for a load of 100 Ω and a 200-mV p-p output. For gains of 50 and 100, the 3-dB bandwidth drops to 93 MHz and 70 MHz, respectively (Fig. 2).
Bandwidth variation is about 2.5:1 for the 50:1 gain range with a fixed feedback resistor. "Though this still isn't a perfect gain-bandwidth independence, it's far superior to existing low-power CFB op amps," says Michael Steffes, TI's strategic marketing manager for high-speed signal-processing products. "By comparison," he adds, "to obtain similar performance at a gain of 100 from a VFB part, the designer would require a 7-GHz gain-bandwidth product."
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