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Gustavo Castro ,
Applications Engineer, Linear Products Group, Analog Devices

Do I really need to match the impedance between both inputs of my amplifier?

In the 1940s, when television had started to catch up with households, TV sets were heavy, complex, and had many vacuum tubes. The large number of components, along with the heat generated by the tubes, made them quite unreliable and expensive to own. At this point, the stage was primed for an opportunistic entrepreneur to take over with several innovations that brought television to the mass market.

Earl “Madman” Muntz was a self-taught engineer and brilliant businessman who was often depicted in his advertisements wearing flannel underwear and a Napoleon hat. He learned to repair radios at age 8, and built his first car radio when he was 14. He dropped out of high-school to sell used cars, and started to make his own fortune. When he decided to go into the TV set business, he bought one, took it apart, and studied its operation. Muntz eliminated entire circuits until he had left the bare minimum for operation. He reduced the complexity, number of tubes, power, and cost, and engineered his televisions to be a little more reliable than the competition.

Later on, this practice became known as muntzing, when he walked around the design floor with diagonal cutters, eliminating components one by one until the TV stopped working “Well, I guess you have to put that last part back in.” His approach allowed him to sell TVs for under $100 for the first time ever. This made him one of the most successful consumer electronics manufacturers, selling over $50M per year at the peak of his business in 1953.

This lesson may not be a surprise to many electrical engineers: less is more. A simple, elegant design often results in higher reliability and lower cost. When the design is nearly finished, a good practice is to go back and remove the components that are not essential for operation. Could some components cause more harm than good? Where should you begin?

Most of us were taught that matching the source impedance seen by both amplifier inputs would reduce offsets induced by input bias currents. In an inverting amplifier configuration, for example, it is common to include a resistor from the noninverting input to ground that matches the parallel combination of the resistors connected to the inverting input. However, this only works when the input offset current (the mismatch between the input bias currents) is much less than the input bias current. Many dual-supply precision amplifiers, such as the ADA4077-2 and ADA4177-2, use input bias cancellation; and FET input (and some CMOS) amplifiers, such as the ADA4610-2, have extremely low bias currents. If you have added resistors to match the input bias on these amplifiers, you can probably take them out. They add noise and increase the chances for manufacturing defects: the circuit won’t work if the resistor is supposed to be there but isn’t; and even if it is there, it is another part with two more solder joints that can break.

This is one tiny example, but for sure not the only one. Muntzing your own designs—or those of others when you get to participate in a design review—can save cost, increase reliability, and keep the quality department (and an unhappy boss) away from your desk.

Note

Madman Muntz even muntzed his own advertisements. He used skywriting to advertise Muntz Televisions, but his name was blurred halfway through the second word, so he shortened the advertisement to Muntz TV, thereby coining the now widely used acronym.


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  • About The Author
  • Gustavo Castro
    Applications Engineer, Linear Products Group, Analog Devices

Gustavo Castro is a system applications engineer in the Linear and Precision Technology Group in Wilmington, MA. His main interests are analog and mixed-signal design for precision signal conditioning and electronic instrumentation. Prior to joining Analog Devices in 2011, he worked for 10 years designing high performance digital multimeters and precision dc sources at National Instruments. Gustavo received his B.S. degree in electronic systems from Tecnológico de Monterrey and his M.S. degree in microsystems and materials from Northeastern University. He holds three patents.