Vacuum Tube Preamplifier Analysis and SPICE Simulation

The concept of an AC model for the triode is presented and the equivalent circuit technique is described. Theoretical calculations for amplifier gain and frequency response are derived and compared to simulation results in the SPICE3 environment where good agreement is shown. The characteristic properties of AC and DC signals are apparent from fun

damental circuit theory. As an example, a capacitor behaves as a DC block. However, it will pass AC signals in general, but the transfer will be contigent upon signal frequency, type of capacitor, and total capacitance. Here it should be noted that these differences are attributed to the mechanism of current flow in each case. DC signals move charge by conduction. Electrons from one part of the circuit travel continuously to another point due to an electric field that is created by a potential difference, or voltage. Once the capacitor is charged up by the conduction of charge to each plate, the capacitor appears as an open circuit to DC signals. By contrast, AC signals move through capacitors by the mechanism of displacement. Small amounts of charge are moved to and from the plates of the capacitor. They are not conducted through the capacitor. However, this displacement of charge is the means by which AC signals propagate through capacitors. Lastly, it is important to understand that the AC signal is superimposed upon the DC signal and thus the two components obey the mathematical laws of superposition: namely, the AC and DC components can be considered separately and superimposed. However, it should be noted that this is only true to a first order since AC nonlinearities in the tube will give rise to DC components, but this is beyond the scope of this paper. It is equally important to recognize that the two components...

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