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Diode Recovery Screenshots
Diamp is an audio amplifier that uses power supply-type rectifiers in place of the usual transistors or vacuum tubes. It employs the phenomenon of charge-storage in order to produce power gain. Stored charge is normally characterized as a power supply rectifier defect; given that it increases the reverse-biased recovery time. However, Diamp converts this defect into a virtue! The first task involved in building a Diamp is to select rectifiers having a sufficiently long reverse-recovery time.

The waveforms shown on the right were obtained from a test circuit comprised of an AF signal generator driving the diode-under-test through a 1k Ohm (current-to voltage converter) resistor. An oscilloscope probe was placed across this resistor to obtain the plots shown on the right. The 20kHz waveforms shown (top-to-bottom) were produced by the following rectifiers: 1N4148, 1N4002, 1N4004, 1N4007 and 1N5401. The prominent negative charge recombination pulses produced in all but the top trace illustrate why these devices do not efficiently rectify AC frequencies much higher than several hundred Hz.    

Charge storage time is the length of time required for the stored minority charges to recombine when a forward-biased PN junction is suddenly reverse-biased. I measured (in the above diode order): 0uS, 5uS, 5.5uS, 6.2uS and 11.2uS.

A transistor or vacuum tube uses a DC collector or plate power supply. In contrast, the Diamp uses an AC power supply, or carrier. The lowest frequency required for the carrier is determined by both the signal frequency and the rectifier storage time. The carrier frequency ought to be at least ten times the signal frequency. A shorter storage time requires a higher carrier frequency in order to produce an efficient amplifier. In fact, the short carrier storage times associated with the germanium diodes used in the original (circa 1954-58) prototypes required a carrier frequency of 1MHz in order to produce a reasonable gain.

Some History

The idea of building an amplifier from a rectifier that exhibits charge-storage appears to have first occured to Arthur Holt, an engineer working at the National Bureau of Standards. In 1954, he applied for a patent which was granted in 1959. Two General Electric engineers, Harold Abbott and Lawrence Wechsler, came up with an improved circuit in 1958. They received three patents and presented their findings in the literature of the day. To the best of my knowledge, this circuit was never widely used. In fact, it appears to have been all-but-forgotten. This stands to reason, given the increased availability and performance of transistors.

I first came upon this circuit in a booklet of the, "101 Ways to Use Semiconductors" variety, some fifteen years ago. Unfortunately, I neglected to record the details at the time and promptly forgot everything; save the basic - albeit sketchy - idea.

Recently, I happened to be reading an old issue of IEEE Transactions, when I noticed a footnote mentioning a diode amplifier. The text led me to believe this wasn't tunnel diode or varactor-based parametric amplifier circuit. It turns out this was indeed the same amplifier that I'd happened upon a decade and a half earlier!

My Circuit

My circuit is based on Abbott and Wechsler's improved version. My first iteration used wide-band transformer coupling. My latest version uses peaked LC resonators having a modest loaded Q. A small DC bias is required at the charge-storage diode cathode in order to place the amplifier in Class-A operation. The optimum bias setting is fairly critical. I initially set each stage for maximum gain using a variable resistor in place of the grounded, bias-divider resistor.

To be continued...

This video clip presented below shows a three-stage Diamp working into a loudspeaker.

This clip shows the signal at the second-stage charge-storage diode anode. This two-stage amplifier drives a pair of headphones. It uses the same gain-less, RF front-end that was used in the above video. 


"Diode Amplifier,"Electronic Design Magazine, October 1954, pp. 24-25

"Now Diodes Amplify,"Radio-Electronics Magazine, November 1954, pp. 94-95 (thanks to Jack Ward, curator of the Transistor Museum!); View Page 94, View Page 95

"Diode Amplifer,"National Bureau of Standards Tech. News Bulletin, Vol. 38, October 1956, pp. 145-147

Solid State Circuits Conference, Digest of Technical Papers; IEEE International, February 1958, Vol. 1, pp. 57-59

"Semiconductor Diode Amplifiers and Pulse Modulators,"IRE Trans. on Electron Devices, Vol 6, Issue 3, July 1959, pp. 341-347

"Model and Experiments on Charge Storage," IRE Trans. on Electron Devices, Vol. 8, Issue 2, March 1961, pp. 123-131

"Diode Amplifier,"US Patent 2,879,409, Arthur Holt (National Bureau of Standards)

"Semiconductor Circuits,"US Patent 2,981,881, H. Abbott and L. Wechsler, (General Electric Co.)

"Semiconductor Circuits Utilizing a Storage Diode,"US Patent 2,976,429, Ibid

"Semiconductor Diode Amplifier,"US Patent 2,997,659, Ibid


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