Modified Input Stage
This gain-bandwidth dependence arises because the input impedance looking into the inverting input of the op amp is nonzero. In the traditional CFB topology, a voltage buffer exists between the two inputs with a low-impedance path on the inverting node for the feedback-error current.
"As a result, this previously second-order ef-fect becomes a first-order gain-bandwidth dependence for any conventional CFB op amps operating at less than 2 mA of supply current," Steffes explains. "In this topology, the feedback transimpedance that determines the frequency response for a CFB op amp has a term that's the inverting input impedance times the noise gain. For low-power parts, this inverting input impedance becomes quite large. It becomes a dominant term in the loop-gain equation, giving all present low-power CFB parts gain-x-bandwidth dependence characteristics."
The new CFBplus amplifier transcends this limitation by adding internal feedback around the input stage buffer to drive its output impedance lower while retaining the benefits of very low total quiescent power. For example, a typical 2-mA CFB op amp may have 200 µA of bias current in the inverting node, giving a buffer output impedance of about 65 Ω. The new closed-loop input buffer employed in the CFBplus amplifier uses a similar level of quiescent current, but it provides only 5 Ω of impedance looking into the inverting input.
At very high gains, however, this bandwidth independence will be lost due to the loop-gain roll-off characteristics of the closed-loop buffer. The buffer's output impedance starts increasing at very high frequencies. One can overcome this problem by extending the buffer bandwidth at least five times the desired overall closed-loop bandwidth. But this improvement comes at the cost of more power consumption in the buffer stage.
An added benefit of the reduced impedance at the inverting node is the linear error-current sensing operation. Consequently, the new CFBplus op amps push distortion levels much lower than previous CFB amplifiers (see the table). This improvement is noticeable at lower frequencies where the output stage is no longer the dominant contributor to harmonic distortion.
Compared to other CFB op amps, the CFBplus offers over 10 dB of improvement in total harmonic distortion (THD) for a gain of 2 using a 100-Ω load at 2-V p-p output. This performance gap is even higher for higher gains under similar conditions.
In a conventional CFB op amp with a traditional topology, there's a dramatic drop in distortion performance at high gains due to high inverting-node input impedance. Also, as the load increases to 1 kΩ, the CFBplus' distortion performance significantly improves.
Similarly, noise characteristics of the new device are low. Maximum input voltage noise performance is specified at about 3.2 nV/—Hz at frequencies above 1 MHz. The slew rate is better than 800 V/µs. Simultaneously, the output stage of the new op amp has been improved to obtain sufficient headroom to handle large voltage swings at high output currents. Plus, this capability minimizes distortion at higher frequencies. Its dynamic performance has also been enhanced.
CFBplus op amps ensure that the amplifier can deliver adequate bandwidth and output voltage to drive short-loop ADSL lines. Consequently, a dual-channel version with few external passives can be configured as a low-power wideband, differential pre-emphasis line driver (Fig. 3). In conclusion, these improvements make the CFBplus op amps suitable for a variety of applications below 50 MHz.
Price & Availability
Both the single-channel OPA683 and OPA684 op amps are sampling now. Production is slated for the first quarter of 2002. The two amplifiers come in a SOT23-6 or an eight-pin plastic small-outline (SO) package. Dual-channel versions, the OPA2684 and OPA-2683, also are in the works. They will be supplied in SO-8 packages and should sample by the year's end. In quantities of 1000, the OPA684 costs $1.29 each and the OPA683 costs $1.15 each.
Texas Instruments Inc., Mail Stop #206, 6730 S. Tucson Blvd., Tuscon, AZ 85734; (520) 746-1111; www.ti.com.