Electronic Design

Superfast Op Amps Break 2-GHz Bandwidth Barrier

New complementary SiGe process yields ultra-fast amplifier family.

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.

Targeting Stability: Designing a very high-speed op amp is more challenging than just getting wider bandwidth and higher slew rates. One problem is the need to ensure stability under all operating conditions. Designers have to consider such electrical details as phase margin and compensation, as well as the end use. TI didn't want external feedback nodes available because the parasitic loading from the feedback could make the system unstable. Between that and the very high frequencies involved, the company decided to simplify the application of this gain function by creating a family of fixed-gain parts.

At gigahertz frequencies, the bonding wires and package pads combine to form a low-pass filter that's potentially well below the desired bandwidth of the parts. Normal package impedance characteristics are due to the bond pads, lead wires, and packaging. A 0.1-in. long, 2-mil diameter wire has an inductance between 2 and 3 nH, depending on the loop size. This inductance is a part of a pi-section low-pass filter comprising the bonding pad capacitance, the wire inductance, the lead bonding area, the package lead, and the pc-board pad. Depending on the actual parasitic values, the 3-dB roll-off frequency can be well below 1 GHz.

To solve these problems of package parasitics and op amp stability, TI developed new packaging and pin-outs for the amplifiers (see the figure). The new packaging is a leadless MSOP, a four-sided package with four pins on each side. The pins on the top and bottom are power-supply connections. Signal inputs and the power-down input are on the right, while outputs are on the left. This new package reduces inductance from bond wires and minimizes the package parasitics by putting four connections on the output and the supply leads. These parallel leads reduce lead and package inductance and resistance by a factor of four.

By developing a new package and keeping the feedback internal, TI created a device with much greater stability for easier use in systems. Potential applications include high-speed buffers and drivers for high-speed, high-precision data converters and DSP systems. These functions require the low distortion and noise characteristics of the THS430x parts for 16-bit or higher resolution.

The speed and low-noise characteristics should work well in medical imaging systems where the data accuracy requirements are very high. Communications systems could benefit from the devices' capabilities, especially in applications like basestations and other high-frequency signal-processing functions. The combination of speed and low noise will enable many new applications for these amplifiers.

One potential new application is in IF stages in communications receivers. Previously, designers had to use RF-type amplifiers or design special purpose IF stages for the multiple conversion receivers. The IF stages needed these specialized amplifiers because they were the only components available with appropriate gain and linearity. The new amplifiers open up the possibility for new architectures in software radios. By amplifying input signals and feeding them directly to a bank of fast converters, a DSP-based radio could capture and demodulate a wide range of signal bands and modulation types with very few RF components.

TI is starting to characterize the op amps with inductive and capacitive loads for this application. The company also is going to create application notes because this change in components will change the architecture of the RF and IF stages.

Price & Availability
The THS430x op amps are specified and priced at fairly competitive levels. For 1000-piece orders, the THS4302 costs $1.97 each. Samples and evaluation module assemblies are available now.

Texas Instruments Inc., www.ti.com; (800) 477-8924, ext. 4500.

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