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Amplifiers promise industry's best large signal bandwidth

Posted: 14 Apr 2005 ?? ?Print Version ?Bookmark and Share


Generally, you see many amplifiers touting GHz speeds, but you won't see an amplifier touting these speeds at large signals. In fact, most vendors will highlight their small signal bandwidth instead; for obvious reasons, which weren't so obvious to me until recently.

National Semiconductor's amplifier, however, offers solid bandwidth performance at both the small and large signal bandwidths, said Tushar Patel, he company's product marketing engineer.

Apparently, it looks a lot better on paper to look at the smaller bandwidth specificationand this is where National Semiconductor's very wideband DC coupled monolithic operational amplifier separates from the pack.

It's difficult to maintain a high large signal bandwidth and to keep the power at a reasonable low level. "We have managed to keep the large signal bandwidth up, without adding the extra power. There isn't an op amp out there at 750MHz, 2V peak to peak with only 11mA of current," Patel said.

It's also important to look at the large signal bandwidth in terms of a gain of two instead of one. For example, as you will see in the enclosed large signal bandwidth comparison chart, a competing device may provide 1.4-GHz speed at the small signal bandwidth, but the signal drops off to 520MHz at a gain of two with a large signal, Patel said.

Basically, the LMH6703 is the latest addition to National's existing portfolio of high-speed amplifiers. The LMH6703 is a follow on product to the 1.7GHz amp (LMH6702) that National developed two years ago. "We were able to make some significant performance improvements. Besides the bandwidth increase, we reduced the current, put in the shutdown pin, and made the frequency response smoother, cleaner and more stable," Patel said.

National was able to boost the large signal bandwidth at 2V peak to peak on the newer device to 750MHz, versus 720MHz on the LMH6702.

Meanwhile, the large signal bandwidth is crucial to video applications since these applications mainly deal with a 1V peak to peak signal. "Most manufacturers usually characterize their op amps at roughly 200 mV or 500mV peak to peak," Patel said. In fact, some companies don't even include the large signal bandwidth spec on the data sheet. "Video applications deal with large signals, not 200mV signals," he added.

One of the problems associated with all high-speed op-amps is that they become difficult to use and can be pc board-layout dependent. The LMH6703 was designed to help minimize the PC board layout dependency, and provide a nice, clean frequency response without peaking, Patel said.

The amplifier also offers lower distortion than competing parts, which is key when driving A/D converters. The second and third harmonic distortion is -75dB and -103dB, respectively, measured at 10MHz, compared to similar parts with the best third harmonic distortion available in the market currently at about -86dB also at 10MHz, according to Patel.

National Semiconductor will also be releasing the LMH6704, which is a programmable gain buffer. The buffer is similar to the LHM6703, except the feedback resistors are on the chip. "For the LMH6704 we have brought the feedback resistors on chip for setting the gain at -1, 1 or 2. This saves board space, and makes gain matching easier for engineers who don't want to fiddle around with resistors," Patel said. Therefore, the LMH6703 offers the user the flexibility of choosing the gain, while the LMH6704 has the gain internally.

- Bettyann Liotta


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