|Check the Bias Current (Or How to Ensure that a Blessing is Heard)|
|Q. The bias current in modern amplifiers is very small. Do I need to bother about it?|
It was a moving coil microphone with a differential output to an SSM2019 preamplifier. The microphone was in perfect condition and delivering a signal to the amplifier inputs, but the connection from the cable screen to the metal microphone body was broken. A broken ground might cause hum (this barely happened), but why should it disable the amplifier?
When I examined the preamplifier circuit it became clear the microphone's center-tap, which was grounded to the case of the microphone, carried the bias currents of the two SSM2019 inputs. With the ground opencircuited, there was nowhere for bias currents to flow and the amplifier stopped working. When Father Aidan touched the microphone, his body provided a current path to ground and everything worked again, albeit with a little hum.
All amplifying devices bipolar transistors, JFETs, MOSFETs, and even tubes have dc currents, known as bias currents, in their inputs. For some JFETs and MOSFETs these are as small as 20 fA (2E-14 amps) about one electron every eight microseconds but the inputs of op-amps and inamps typically have bias currents in the range of pA to µA. A circuit design that does not accommodate these bias currents will not work properly, and may not work at all. But because they are so small, unintended leakage paths (the one through Father Aidan's body is an example) may often allow a badly designed circuit to work despite itself but it probably won't work well. Good analog designs do not leave bias current effects to chance and good luck, but allow for them and ensure that they affect neither performance nor functionality.
Not only did I repair the broken connection, I also added two equal grounded resistors to the amplifier inputs ensuring that future microphone ground faults might cause hum but would not silence Father Aidan.
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|James Bryant Offers Intrigue, Interest and Technological Troubleshooting Ideas... engineer, applications manager, philosopher, humorist, columnist and radio ham (G4CLF), only a man with such an eclectic dossier could make the often drab world of semiconductors come to life with such color and imagination.|
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