Point Contact Transistor Apply Small Signal Modulate Injection
# Apply a Small Signal Between Emitter and Base: The Device Stops Sitting Still
Every step from Step 17 through Step 20 set up a fixed condition and then held it there — a reference terminal, a steady forward bias, a steady reverse bias, a load waiting for a current that, until now, never changed. Step 21 is the first step to ask the device to do something other than sit at a constant operating point. A small signal applied between the same two terminals Step 18 already biased, measured against the same base reference Step 17 established, rides on top of the steady injection voltage and makes the rate of hole injection follow whatever that signal is doing from moment to moment. This is the step where the device stops being a fixed current source and starts being an amplifier.
"Small" means something new at this step, distinct from what it meant when Step 18 used the same word. Step 18's "small" bounded the steady bias voltage itself, against the forming current's limits and the device's linear region. This step's "small" bounds how far the signal is allowed to swing the instantaneous voltage away from that already-verified bias point — a different question, because a signal large enough to push the instantaneous voltage outside the linear region Step 18.4 confirmed produces a distorted injection current even if the average voltage, measured over time, still sits exactly on the correct bias point. A signal can be perfectly centered on the right bias and still be too large, in exactly the way a perfectly aimed shot can still miss if it swings too wide on either side of the target.
The injected current now carries information, not just magnitude, and that information has to survive a crossing this series already measured. Step 1 argued that diffusion length has to exceed point spacing for any carrier to survive the trip from emitter to collector at all; Step 13 fixed that spacing as an actual distance. Neither of those steps had to consider how fast a carrier crosses that distance, because a steady injection current has no timing information to preserve. A modulated injection does. The time it takes an injected hole to diffuse across the point spacing sets a real limit on how quickly this step's signal can vary and still have the collector current faithfully follow it — a signal that changes faster than holes can cross the gap arrives at the collector smeared out, carrying less of the original signal's shape than it had at the emitter.
Both failure directions for this step's signal amplitude connect to a risk an earlier step already identified for a different reason. A signal too small produces a collector current change too faint to distinguish from measurement noise once Step 20's load converts it to a voltage, wasting the sensitivity that load was sized to provide. A signal too large can momentarily drive the emitter voltage toward zero or below on its negative swing, briefly removing the forward bias Step 18 established and clipping the injection rather than merely modulating it — or drive it high enough on its positive swing to approach the same current-related risk to the formed point contact that made Step 18's bias magnitude a bounded quantity in the first place. The signal does not get a separate safety margin from the bias it rides on; it inherits the same one, and has to stay inside it on every instantaneous swing, not merely on average.
| Step | Process operation | Input | Output | Specification | Constraint |
|---|---|---|---|---|---|
| 21.1 | Determine the maximum signal amplitude that keeps the instantaneous emitter voltage within Step 18's verified linear region | Linear operating region confirmed in Step 18.4 | Maximum safe signal amplitude | Amplitude small enough that the bias point plus signal swing never leaves the confirmed linear range on either side | A signal centered correctly but swinging too far still distorts injection, even though the average bias is unchanged |
| 21.2 | Confirm the signal's frequency content against the point spacing's carrier crossing time | Point spacing from Step 13/14, signal source characteristics | Frequency-compatible signal | Signal frequencies kept well below the rate at which crossing time begins to smear the injected current | A signal varying faster than holes can cross the gap arrives at the collector with less fidelity than it had at the emitter |
| 21.3 | Apply the signal between emitter and base, superimposed on the bias from Step 18 | Verified amplitude from 21.1, verified frequency content from 21.2, biased emitter from Step 18 | Modulated emitter voltage | Instantaneous voltage equal to the Step 18 bias plus the applied signal, measured against the base reference from Step 17 | Measuring the signal against any reference other than Step 17's base reintroduces the ambiguity that step was meant to eliminate |
| 21.4 | Verify the modulated injection current follows the applied signal without clipping | Modulated voltage from 21.3 | Verified, unclipped modulated injection current | Current waveform proportional to the applied signal across its full swing, with no flattening at either extreme | Clipping at either extreme indicates the amplitude check in 21.1 was insufficient for this specific device |
| 21.5 | Confirm the collector-side load from Step 20 still operates within its own verified range under the now-varying current | Verified injection current from 21.4, load configuration from Step 20.4 | Confirmed, signal-compatible load condition | Load voltage swing stays within the bounds established in Step 20.3 as the current now varies rather than holds steady | A load correctly sized for a steady current can still be driven outside its safe range once that current actually varies |
| 21.6 | Record the applied signal's amplitude, frequency content, and verification results against the device's complete record | Confirmed modulated condition from 21.5 | Documented signal-applied configuration | Amplitude, frequency, and both verification results recorded against every provenance record since Step 2.6 | Without this record, Step 22's measured output cannot be tied to a specific, confirmed input condition |
Step 21 does not change what the device is biased to do; it is the first step that asks the device to do it differently from one instant to the next. Every requirement this signal has to satisfy was already established by an earlier step for a different reason — the linear region by Step 18, the crossing time by Step 1 and Step 13, the load's safe range by Step 20 — and this step's only real job is to respect all of them simultaneously while finally giving the device something to actually amplify. Nothing before this step produced a signal in the ordinary sense; everything before this step produced the conditions a signal would eventually need.