Until now, very high-speed Operational Amplifiers (OP AMPS) have offered a top bandwidth of about a gigahertz. The addition of silicon germanium (SiGe) to a high-speed bipolar-CMOS process results in a family of fixed-gain, voltage-feedback op amps that changes everything. The new family from Texas Instruments (TI) uses a novel voltage feedback technique to achieve bandwidths of more than 2 GHz. Other performance parameters make these op amps suitable for a wide range of new applications, such as driving various capacitive loads like the inputs to high-speed data converters.
By using voltage-feedback topology, the THS430x series of op amps reduces the error-inducing parasitic properties of the previous speed champs—current-feedback amplifiers. Because a current-feedback amplifier has a fairly low input impedance, the input is highly sensitive to capacitance and loads down the input signals. These characteristics result in nonlinear frequency response characteristics and a high potential for oscillations.
Because of the voltage-feedback configuration, the input impedance of the new devices is much higher than current-feedback units, so it doesn't load down the input signal. The THS430x devices claim a speed record of 2.4 GHz at a gain of 5 into a 100-Ω load. This is actual bandwidth, not some fanciful gain-bandwidth product that is not entirely useful frequency response. As expected for a parts with this much bandwidth, slew rate is greater than 2500 V/µs, and settling time is about 1.5 ns.
Not only are these parts very fast, but they are also quiet. Input referred noise is 1.5 nV/√Hz. The parts have a low total harmonic distortion of 92 dBc (f = 30 MHz, RL = 100 Ω) or 75 dBc (f = 100 MHz, RL = 100 Ω). The third-order intermodulation intercept is specified at 80 dBc at 170 MHz with a 1-V output swing and a 5-V supply. This value decreases to 72 dBc at 300 MHz with the same output conditions. These specifications are better than many other high-frequency amplifiers at a test frequency that's three times higher than most other high-performance, high-speed op amps. The noise specifications exceed the requirements for 16-bit or higher-resolution converters.
Output current of ±80 mA is necessary for a part that's likely to operate in a low-impedance (50-Ω) system or be the driver for the inputs of an analog-to-digital converter or other data-acquisition system. The family has a range of fixed gains, from the THS4300 (+1) to the THS4301 (+2), the THS4302 (+5), and the THS4303 (+10).
In comparison, the LM6165 family of high-speed amplifiers from National Semiconductor exhibits a speed-power product of 725 MHz GBW (gain-bandwidth product, stable for gains as low as +25, and a slew-rate of 300 V/µs with only 5 mA of supply current). These amplifiers are built with National's VIP (Vertically Integrated PNP) process, which produces fast PNP transistors that are true complements to the NPN devices.
Intersil-Elantec's EL5191C is the company's fastest current-feedback amplifier to date. The highest-speed variant within this new product family features a 1-GHz bandwidth and a 2800-V/µs slew rate while operating on just 9 mA of supply current. The Maxim MAX4223/ MAX4225/ MAX4226 current-feedback amplifiers are optimized for a closed-loop gain of +1 (0 dB) or more and have a 3-dB bandwidth of 1 GHz. The MAX4224/MAX4227/MAX4228 are compensated for a closed-loop gain of +2 (6 dB) or more and have a 3-dB bandwidth of 600 MHz (1.2-GHz gain-bandwidth product).
New Process, New Amps: The fully differential THS430x op amps depend on a new complementary SiGe process for their speed and on high-precision resistors and capacitors for stability and accuracy. Tables 1 and 2 list some of the active and passive device characteristics of this BiCom-III process and clearly demonstrate the potential for further improvements in next-generation circuits (see "BiCOM-III Process Triples Speed, Halves Noise," p. 46). TI believes that the 430x series is the first complementary SiGe group of products to go into production.
The design of these very high-speed op amps requires innovative circuit topologies and very fast transistors. Both the BiCom-III process and the circuits demanded new approaches because neither by itself is sufficient to make this next-generation set of parts.
In general, designers had to trade gain or limited ranges of stability for bandwidth, because a unity-gain, stable amplifier must have sufficient phase margin to prevent oscillations. The previous alternative was to use a current-feedback architecture for very high-bandwidth parts. While TI has been developing high-speed amplifiers for a few years, the latest parts change the landscape for high-speed, high-precision, low-noise amplifiers.
To create these next-generation amplifiers, TI developed new topologies and its third semiconductor process for high-speed amplifiers since 1999. TI's first process, BiCom-I, was designed for ±15-V supplies. One product in the first process, THS 3001, is a low-distortion current-feedback amplifier. BiCom-II was the next generation. Designed for a ±5-V supply, it could handle up to 15 V. The BiCom-III process is specified for a nominal operating ±1.5- to ±2.5-V supply or a single +3- to +5-V supply.
Dave Wilson, system engineering manager for high-speed amplifiers, says that TI has leapfrogged other vendors with this process. Several companies have SiGe processes but they're not complementary. They currently only have high-speed NPN devices in their SiGe processes and use much lower-performance PNP devices. The PNP devices in the BiCom-III process are about three times faster than parts in other processes, including BiCom-II.
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