Lilienfeld 1925 Superimpose an Input Signal

# Superimpose an Input Signal: The Capacitor Only Passes What Changes

Step 12 fixed a steady offset on the foil, chosen to clear the worst point of the film's own sloping potential; this step adds a time-varying signal on top of that offset, and that addition is the first point where Step 3's oxide capacitance actually has to do the one job it exists for. A capacitor coupling two points together does not pass a steady potential difference as current — it passes *changes* in that potential difference, and only at a rate set by its own impedance relative to whatever resistance the signal source sees looking into the foil. A signal varied too slowly simply doesn't make it through; one varied fast enough does.

## 1. The Oxide Capacitance Sets a Floor on Usable Signal Frequency

Coupling an alternating signal through Step 3's oxide capacitance behaves like any capacitive coupling network: it passes high frequencies readily and attenuates low ones, with the transition set by the capacitance itself and whatever resistance the signal source presents:

Signals varied well above $f_c$ reach the film essentially undiminished; signals varied well below it are increasingly blocked by the same capacitor that Step 6 relied on to keep the foil from ever touching the film directly. This is a direct consequence of choosing a purely capacitive coupling path all the way back in Step 3 — it buys isolation from any direct current path, but it charges a cost in exchange: a lower limit on how slowly the input signal can vary and still be felt at all.

## 2. Real Diagram: Fast Signals Pass, Slow Ones Don't

## 3. The Signal's Swing Is Also Bounded by Step 12's Own Margin

Even a signal fast enough to pass through $C_{\text{ox}}$ cleanly still has to respect the bias requirement Step 12 established, because the foil's instantaneous potential is the fixed offset plus whatever the signal happens to be doing at that instant:

Step 12 chose $V_{\text{foil}}$ to clear the film's own worst point with some margin to spare; this step's signal amplitude is only free to use up that margin, not exceed it. A signal large enough to swing the foil's instantaneous potential below the film's highest local value at the trough of its own cycle momentarily inverts the field sense at that stretch of the foil, exactly the failure Step 12 worked to avoid under steady bias — now reintroduced dynamically, once per cycle, by a signal that simply swung too far.

## Real Diagram: A Signal That Swings Past Step 12's Margin

## Superimpose an Input Signal's Place in the Process Lineage

Superimpose an Input Signal follows Step 12, Apply Control-Electrode Bias, whose fixed offset and hard-won margin this step's own signal amplitude now has to respect rather than exceed; it precedes Step 14, Couple the Output Circuit, which depends on this step actually having produced a usable current variation to deliver somewhere. It is the thirteenth step of this concept's sequence and the point where every earlier choice about the control electrode — Step 3's oxide capacitance, Step 6's clearance, Step 12's bias margin — finally comes together into the one operation this entire device concept exists to perform: letting a small, time-varying voltage on an isolated electrode shape the current flowing through a completely separate conducting path.