The Howland Current Pump Circuit

In August of 1950 I was hired to do research on speech recognition by Jerry Wiesner who thenheaded the MIT Research Lab of Electronics (RLE). While I made considerable progress indeveloping analog vowel recognition circuits, severe writer’s block resulted in no publication ofmy results. In 1951, without permission, I joined the group, also at RLE, of Dr. Jerry Lettvin, agenuine genius, who was also a neurologist, psychiatrist, neurophysiologist and certainly thewisest man I have ever worked with. One of my tasks was to design a circuit that could delivera known current stimulation to a nerve regardless of the impedance of the load. A monstrouscircuit using highly microphonic pentodes suspended by rubber bands, which Jerry dubbed“Mahatma’s Tomb” worked but was not esthetically satisfying.

It was the need for a better current stimulator that led to the later op-amp circuit. After being letgo from RLE in 1955, and a year off trying to develop an invention, I was hired by an early“artificial intelligence” group under the direction of Oliver Selfridge at the MIT Lincoln Lab, inLexington MA. Since our group had no clear objective, we were able to work on just about anyproject we wished. The great advantage of the military financed Lincoln Lab over RLE was thatwe had money to spare and I was able to acquire a set of eight Philbrick op-amps in a rack. Iused this to simulate and photograph on a scope the action of an automatic two-dial impedancebridge balancer which would have worked perfectly. The Philbrick module was so popular thatpeople were, in my absence, constantly disassembling my circuits to use the op-amps for theirown purpose, without first copying my circuit diagrams.

One day in 1958 or 1959 I called Mr. Philbrick, GAP himself (his initials appeared on the boxesthat op-amps came in), and I informed him that I had two promising new circuits. He kindlyinvited me to lunch at the elegant Copley Plaza Hotel, the only time I ever ate there. Two of hisassistants from their nearby Columbus Ave. lab accompanied him.

The first, and my favorite, circuit simulated the charge-storage properties of a contemporarysolid-state diode; this in conjunction with an inductor could generate large high harmonicenergy pulses. The action depended on the “kick” of the inductor when it was excited by a sinewave and was carrying a considerable current and the diode suddenly ran out of stored charge,the reverse current dropping to zero.

The above was my prize circuit, but my fallback excuse for a free lunch was the four-resistorcurrent-pump circuit. The resistors formed a balanced Wheatstone bridge and the output wastaken off one of the inputs to the op-amp. The need for such a circuit was inspired by my earlierflawed neuro-stimulator at RLE from five years before.

To my surprise and dismay, Mr. Philbrick showed little interest in my charge-storage diodesimulator but he was enthusiastic at my voltage-to-current bidirectional current pump circuit.He generously stated that while they were working on something quite similar, “ I’ll give youcredit for solving the problem”, he commented. He could plainly see that it would work.

Because of my indolent ways and my inability to evaluate my own work, I never publishedeither of the two circuits. And so I forgot about the current pump until it appeared later inPhilbrick’s famous Palimpsest under the name of “The Howland circuit”.

A later textbook by John I Smith (Wiley/ Interscience 1971, pp155-159) gives an extendeddiscussion of the original four resistor circuit. It is my fault for there being no “original”publication of this circuit.

However, I should like to state here a strategy which I recommend to all future inventors: Sinceit is very difficult to evaluate the worth of one’s ideas at the time, the only safe strategy is tomake a maximal effort to publish all of one’s ideas. An unorthodox method, which I later foundto be both useful and time saving, was to file a patent (with all expenses paid by the lab) anduse this as a refereed vanity publication with the lawyer doing all of the write-up.

I am, of course, greatly indebted to George Philbrick, a giant of the analog world, both forbringing the circuit to light and generously crediting me with it.

As a postscript I relate this following amusing incident: About ten years after my meeting withPhilbrick I had largely abandoned electronics and was working in optics for the Lincoln Labssatellite group when one of Prof. Roberge’s young, hot-shot, MIT trained analogue circuitdesigners asked me: “Are you the inventor of the Howland Circuit?” I said, “ Yes.” He replied,“Well, I don’t believe it. You just aren’t smart enough to have done it.” Happily, I did notanswer him and in the thirty-five years since I still haven’t been able to think of a suitable reply.