BalesCFBamp - 1470 IEEE JOURNAL OF SOLID-STATE CIRCUITS VOL...

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Unformatted text preview: 1470 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 32, NO. 9, SEPTEMBER 1997 A Low-Power, High-Speed, Current-Feedback Op-Amp with a Novel Class AB High Current Output Stage Jim Bales Abstract— A complementary bipolar low-power, high-speed, current-feedback operational amplifier is described. The ampli- fier incorporates a new Class AB output stage that enables high output current drive of 100 mA and large voltage swing within 1 V of the supply rails while operating at low quiescent current of 1.5 mA. The amplifier was fabricated in a junction-isolated complementary bipolar process with NPN/PNP f t of 4.5/3.8 GHz. The amplifier, configured for noninverting gain of two and 100- load, provides 3-dB bandwidth of 110 MHz and 2-V pulse rise time of 7 ns. Index Terms— Analog integrated circuits, bipolar analog inte- grated circuits, operational amplifiers. I. INTRODUCTION A N increasing number of video and communication ap- plications require line or cable driver circuits that can provide high peak output current with large bandwidth and low distortion while operating on low quiescent current and low supply voltage of 5 V or 5 V. This requirement is well served by complementary bipolar amplifier circuits. An excellent review of modern complementary bipolar high-speed monolithic operational amplifiers, including current-feedback op-amps, is given in [1]. A simplified circuit schematic of a typical modern complementary bipolar high-speed current- feedback op-amp is shown in Fig. 1. The basic elements of the architecture are unity gain input buffer, current mirror gain stage, and unity gain output buffer. Complementary emitter followers Q1–Q4 form the input buffer. The collector signal currents of transistors Q3 and Q4 are turned around and summed through Wilson current mirrors formed by Q5–Q7 and Q8–Q10, respectively. This signal current injected into the high impedance node sets the open-loop gain of the amplifier. The output buffer (Q13–Q16) provides near unity voltage gain and isolates the high impedance current summing node from the amplifier load. The single gain stage architecture provides high bandwidth at the expense of open-loop transimpedance. The loss of dc accuracy is usually acceptable in the high-speed applications in which these amplifiers are employed. Nearly all monolithic complementary bipolar high-speed current-feedback op-amps are a variation of the architecture just described, with variations and additions to meet design goals and accommodate the characteristics of the fabrication process. An overview of the design goals for the present op- amp design are given here to establish the motivation for the new output stage design that is the innovative feature of the Manuscript received January 29, 1997; revised April 11, 1997....
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BalesCFBamp - 1470 IEEE JOURNAL OF SOLID-STATE CIRCUITS VOL...

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