Broskie auto-bias circuit

Posted on Feb 7, 2014

Considering that I had complained of a schematic typo from Siliconix, it is doubly embarrassing that that April 29`s blog held a few schematic typos of my own creation: in the variation on Morgan Jones`s circuit one of the resistor`s values was off by a decade and, more damning, one of the OpAmp`s inputs was reversed. Below, the corrected schemati

Broskie auto-bias circuit
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c is displayed. (The 4/29 blog has been updated to reflect these corrections. ) The generic DC servo circuit (shown below) works perfectly as long as the amplifier does not amplify asymmetrical signals and as long as the output stage`s current swings are within in the class-A window of operation. However, once the output stage jumps out of class-A into class-AB operation, the DC servo will see a net DC increase at its inverting input and it will strive to bring the amplifier back into line with its reference voltage, but at the same time the capacitor will begin to charge to higher voltage, which will throw the bias voltage off (negatively) until the input signal subsides enough to let it discharge back to its idle value, as shown in the graph below. The graph above shows the tube`s current conduction in hot pink, the grid bias voltage in blue, and the capacitor`s voltage differential in green. The signal burst excessively charges the DC servo`s capacitor and its output voltage is thrown off the mark by over half a volt (602mV). The principle behind the clipping-style auto-bias circuits is also simple: monitor the tube`s current conduction while it is operating in class-A; ignore its AC conduction beyond class-A, when the tube moves into class-B. The clipping diode shorts to ground error signals greater than its forward voltage. In other words, the clipping diode`s forwards voltage creates a window that extends from 0V to...

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