But if we point out that those with master degrees make more money than those with bachelor degrees, who in turn make more than those with two year college degrees, who also in turn make more than those with only a high school diplomas …, an entirely different conclusion is reached. Therefore, the more education, the less income. (An analogous example of statistical lying by omission would be to only point out that those with PhDs make less money than those with only master degrees. With a 1V input signal, the bias voltage is off by only 5.95mV. Ah, how easy it is to lie with data! Given just these two data points, we might reasonably conclude that the error grows with the magnitude of the input signal it doesn’t. With a 2V input signal, the bias voltage is off by only 84µV. Starting at a 1kHz input signal of 10 volts peak, the bias voltage is thrown off by about 3mV, the same amount of voltage that capacitors, C1 and C2, differ in change of their charge after the tone burst. In SPICE simulations, these values worked best (and of course, real values may differ). In the schematic above, we see all the part values and voltages labeled. It’s much like docking a boat to a floating dock: no matter how much the tide rises or falls, the boat will remain in the same relation to the dock. In other words, the DC servo’s reference voltage will be thrown off in a matching way as the servo’s own capacitor is thrown off. Since capacitors C1 and C2 are equal in value, as are resistors R1 and R2, the rate of charging will be the same between both capacitors. Once the tube leaves the confines of class-A, however, its cathode voltage will exceed twice Vd and the diode attached to the cathode will begin to conduct, charging C2 as a result. What if we don’t clip the error signal? What if, instead, we use it to prevent the DC servo's capacitor's excessive charging from throwing the bias voltage off? Well, that’s the idea behind the Broskie auto-bias circuit. This plan works perfectly with no input signal well with large input signals that extend deep into class-B not so well with input signal just beyond class-A operation of the output tubes. In other words, the clipping diode's forwards voltage creates a window that extends from 0V to the cathode-to-anode voltage required to turn the diode on (silicon diodes also have cathodes).īy setting the DC servo’s reference voltage to half of the diode's forwards voltage, we ensure that the window of current encompasses the extent of class-A operation that the tube(s) can undergo, without one tube cutting off, while the other conducts beyond the window’s top edge. The clipping diode shorts to ground error signals greater than its forward voltage. 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 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 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.
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