Go active or stay passive?

July 15, 2015 // By Bill Schweber
Bill Schweber discusses the pros and cons and ebb and flow of passive versus active circuit approaches at high frequencies.

Back in the Jurassic days of electronics, before Lee de Forest developed the three-element "grid audion" (a triode vacuum tube providing amplification) around 1906, circuits and channels were passive entities. If you needed to get a wireless signal from point A to point B but the received strength or SNR was too low, though the term SNR was not yet in use, your only option was to crank up the transmitted power.

The digital side was different, though, to some extent: you could boost signals by using relay-based repeaters where a small signal would drive a sensitive coil, while the relay contacts switched larger voltages and currents and so boosted the on/off signal. While it was slow and mechanical, it was also effective.

Active analog devices with gain and amplification made an unimaginable difference; it changed everything and began the electronic age as we know it today. Soon the basic vacuum tube morphed into more effective versions with multiple elements, improved noise performance, higher power, and application-specific designs. Passive was overtaken by active with good reason.

Still, passive components did not disappear. Even today, despite our wide selection of active devices and ICs, there is a place for passive functions such as basic RC, LC, and RLC filters in numerous topologies. This may be due to reasons of cost, internal noise, power levels, operating power (passives need no power supply), and other performance goals. Both passive and active components have pros and cons; otherwise, one would have wiped the other from the designer's resource kit and bill of materials.

In our fluid world of products and performance, devices such as RF mixers are an area where passive implementations have been the choice for many years, as active devices could not match their performance in noise and distortion at higher frequencies. As a result, many designers choose to use a passive mixer but then supplement it with a stand-alone amplifier to get the mixer's performance along with the needed gain. After all, there is no free lunch in design; if you stay passive, you'll incur signal-path losses that you'll likely have to make them up somewhere along the way.

That situation is changing. The growth of mass-market applications above 1GHz, along with IC process improvements has spurred the introduction of many active mixers to the market with performance as good as, and many times even better, than a passive mixer plus amplifier. The relationship between process and component advances, and market needs and volumes, is a case a beneficial positive-feedback loop. With the active mixer, designers get the traditional benefits of a single device such as reduced PC board real estate, plus guaranteed performance, as there are no layout surprises as there might be between two separate components.

Next: Passive refuses to lie down