Inverted Shunt Regulator - Tube CAD Journal

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$Radiotron Designer's Handbook Tube CAD does the hard math for ...$

January 2001 ^^ TUBE <strong>CAD</strong> JOURNAL ^^ Volume 3 Number 1 >> NEW
Compliant Current Sources If by adding sufficient circuitry we can eliminate the coupling capacitor, then the added complexity might well be worth the effort. A DC servo loop can be wrapped around the current source, rather than the tube. This creates a compliant current source that tracks the tube's quiescent current and nulls a DC offset at the output. This circuit was covered in a GlassWare <strong>Tube</strong> circuit of the Month article, No Gain, No Pain and is quoted here: This trick consists of a Cathode Follower that is loaded at its output by a compliant current source, that is a current source that does not have a predetermined quiescent current. What is constant about it is that it strives to maintain a DC ground potential input regardless of the current flowing through it. Is it then really a current source? In AC terms, yes; in DC, no. It offers a very high impedance (roughly, 1 meg) to any AC signal it sees and in this respect it is identical to a typical active current source. In DC terms it works to adjust its quiescent current until its input is zeroed at ground potential (0 volts) over time (roughly, 3 Hz). Input +100vdc to +150vdc 100 1m IRFF220 < PREVIOUS 100 pg. 15 .1µf This piece of magic is the result of a DC servo loop that is wrapped around the input of the current source and the output of the Op-Amp. If the input moves toward the positive over a long period of time, that net DC drift is fed into the non-inverting input of the Op-Amp, which causes its output to go positive. This positive voltage will further drive the MOSFET into greater conduction, which will pull the output towards negative. On the other hand, if the output moves toward the negative over a long period of time, that net DC drift is fed into the same noninverting input of the Op-Amp, which causes its output to go negative. This negative voltage will move the MOSFET into less conduction, which will move the output towards positive. The subtlety here is that the MOSFET is within the DC feedback loop, but outside the AC feedback loop of the Op-Amp. As the time constant of the RC network made up of the two 1 meg resistors in parallel and the .1 µf capacitor is so long that no music can fall into it, the Op- Amp presents a virtually constant DC voltage to the gate of the MOSFET. This steady voltage sets the amount of current that flows through the MOSFET; if the tube’s idle current drifts over ½(5687 or 12AU7 or 7119 or 12BH7 or 12SN7) Rk 100-400 1k 0vdc LF412 1m 150k -6.3vdc -6.3vdc B+ 1m .1µf 1m 1m -12.6vdc 100 Diodes =1N4001 + + 1kµf -12v 1kµf Output -16v to -20v www.tubecad.com Copyright © 2001 GlassWare All Rights Reserved NEXT > 1µf
Page 1 and 2: < PREVIOUS This Issue The Triode Gu
Page 3 and 4: resistance, in parallel with a load
Page 5 and 6: Moving the series resistor to the b
Page 7 and 8: +250vdc 0vdc +1vdc Low bias DC and
Page 9 and 10: Portable Tube Headphone Amplifier (
Page 11 and 12: All of this assumes transformer or
Page 13: Now you can readily see why push-pu
Page 17 and 18: Hz and an impedance of 32 ohms, the
Page 19 and 20: Note the 140 volts power supply req
Page 21 and 22: 300 1M CS 70vdc 6922 Rk OTL single-
Page 23 and 24: The idle current of the push-pull o
Page 25 and 26: FET Preamps Allen Wright goes into
Page 27 and 28: Subject: Possible topic for next is
Page 29: Click to move to previous page Clic

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