What do you do when you want to try out a “new and improved” fabrication process? Typically, you design a variation on an existing circuit that ought to show a useful improvement. You build it and see if the new process will make it work better. You build this as a test vehicle.
What if it doesn’t work well? You hope you learn something for the next try. What if it does work well? You test it—and run like heck and sell it! You have to plan for this as a best-case plan. You hope you have planned this circuit so people will want to buy it. As good old Jay Last of Fairchild used to say, “The only valid market survey is a signed purchase order.”
A REAL-WORLD EXAMPLE
That’s what National Semiconductor did with the basic 1500-MHz LMH6552 circuit: National souped it up and put in a silicon-germanium (SiGe) emitter process. And it worked at faster speed, as expected. So now, people who want a faster amplifier can buy the LMH6554, which can move twice as fast as the older design.
This improved circuit uses about twice as much current, but it sure does go fast, if that is what you want! It can provide low distortion out past 250 MHz, using its superior gain characteristics. Thus it has advantages per the figure of merit of MHz per milliwatt—it is a good PowerWise amplifier, running at a bare 1/4 W.
The LMH6554, like the LMH6552, uses National’s proprietary current-feedback architecture (CFA), so transient currents can flow into both the inputs, positive and negative, to get the outputs moving fast, whereas an ordinary CFA has a high input impedance at the positive input. The LMH6554 also has plenty of output drive, up to ±90 mA, to drive the time-varying input impedances of analog-to-digital converters (ADCs).
We spent a lot of the higher quiescent current to make the output followers stronger, richer, and healthier so they could drive a heavier resistive load and still hold very low distortion, even at 75 to 200 MHz—and drive switched-capacitor inputs, too.
Another important factor in getting low distortion is the coupling between signals and power-supply bond wires. Ordinary layouts in a small-outline package tend to allow a lot of coupling. When high pulses of power-supply current occur, they can couple magnetically into other signals.
We put this circuit in a more compact leadless leadframe package (LLP). The extra ground wires and short paths help you get less interaction. There’s still some cross-talk. But in a full-differential (push-pull) output, they can be designed to cancel out when seen by the differential input you’re driving. Even inside the chip and between the die and its lead frame, layout can be important. But you probably knew that.
TIME TO APPLY
The primary application of the LMH6554 is for amplifying fast sines and ac signals with low distortion out past 250 MHz and stability out to 2500 MHz. It can feed these sines to differential-input ADCs with high resolution and sample rates well past 100 MHz. These are popular for communication systems and digital scopes.
You may soon request applications note AN-2015 showing how this amplifier can put out fast, clean steps in the time domain. I like that. I’m a time domain man, myself. Guys who know how to optimize for clean sine response aren’t exactly the same as guys who can handle fast steps and clean settling in the time domain. Thus it can also drive differential lines for low-voltage differential signalling (LVDS).
The LMH6554 isn’t quite the fastest amplifier in its class. Some are a bit faster. However, the LMH6554 has significantly less noise density (less than 1/2 x) so it may actually work better for your needs. And, who needs a lot of excessive bandwidth if it is just going to amplify up a noisy noise floor?
This amplifier can be shut down to below 1 mA, with fast startup available. The package is a compact 14-pin LLP, which enables good heat transfer at about 60°C/W. These features may be useful if you need a realistic fast amplifier. Product planning takes in almost every real-world insight!
Comments invited! [email protected] —or:
R.A. Pease, 682 Miramar Avenue
San Francisco, CA 94112-1232