Heathkit IT-3121 — Vol 2: How a Semiconductor Curve Tracer Works

2.1 The one idea behind every curve tracer

A curve tracer answers a question a meter cannot: what does this device’s output current do as I sweep the voltage across it — and how does that change as I vary the input drive? To answer it, the instrument does three things at once and lets the scope superimpose the results:

1. Sweeps the main terminal voltage (collector, drain, or anode) from zero up to some maximum, over and over, many times a second.
2. Steps the control input (base current, or gate voltage) through a staircase of fixed values — one step held for each sweep.
3. Plots the device’s output current (vertical) against that swept voltage (horizontal) on an X-Y display.

Sweep once and you get one curve. Step the drive up and sweep again, and you lay a second curve above the first. Do it for a handful of steps and the scope shows the whole family of characteristic curves — the transistor’s fingerprint. Because the sweep is fast and repetitive, the family sits there as a stable, glowing picture you can read and adjust in real time. The IT-3121 does exactly this with 1970s analog hardware, and because its blocks are simple it is an unusually clear machine to learn on.

2.2 The swept collector supply

The horizontal axis is voltage across the device, and the IT-3121 generates it the cheap, clever way vintage tracers always did: it doesn’t regulate a smooth ramp — it uses the raw shape of rectified mains as the sweep. The line transformer’s secondary is rectified into pulsating DC, so the collector voltage naturally rises from zero to a peak and back, 100 or 120 times a second (twice per line cycle). Each of those rising humps is a voltage sweep from 0 to V_peak. Free of charge, the power line hands you a repetitive 0-to-max ramp at exactly the rate you want.

A front-panel voltage control — Erickson’s analysis of the design describes it as a 200 kΩ potentiometer driving a three-transistor Darlington buffer — sets how high that peak reaches, so you can dial the sweep from a few volts up to the range maximum. Two ranges cover the span: 40 V at up to 1 A, or 200 V at up to 200 mA. The low range is for pushing serious current through power devices; the high range is for reaching the breakdown voltages of higher-voltage parts.

Between the supply and the device sits the load resistor — a 12-position switch spanning 0 Ω to 1 MΩ. This is the current-limiting series resistance, and it does two jobs: it protects the device (and the instrument) by capping how much current can flow if the DUT conducts hard, and it sets the load line the device works against. Small-signal parts want a large load resistor and gentle currents; a power transistor on the 1 A range wants a small one.

2.3 The stepped base/gate drive

The other input to the picture is the control drive, and this is where a curve tracer earns its keep — it doesn’t apply one drive level, it marches through a staircase of them so every sweep is taken at a different, precisely known input.

For a bipolar transistor the control input is base current, so the IT-3121 uses a constant-current step generator. Erickson’s read of the circuit is a counter feeding a small transistor DAC that produces a staircase of roughly 1 V per step, converted to a base-current staircase whose size you pick on a 12-position switch running 0.002 mA to 10 mA per step (a 1-2-5 sequence). A separate control sets how many steps appear, from 1 to 10. So “0.1 mA/step × 5 steps” tells the instrument to trace five curves at base currents of 0.1, 0.2, 0.3, 0.4, and 0.5 mA. Because beta is the change in collector current per change in base current, laying the steps a known distance apart is what turns the vertical spacing of the curves into a direct read of gain.

For a field-effect transistor the control input is gate voltage, not current, so the same generator switches to a voltage staircase of 0.05 V to 1 V per step. A polarity switch flips both the step generator and the collector supply together so the instrument can handle NPN and PNP, and N-channel and P-channel, by reflecting the whole picture into the correct quadrant.

Here is the key timing subtlety, worth internalizing because it is what makes the family coherent:

The collector sweeps fast (100/120 Hz) and the base staircase advances slowly, holding each step long enough for a clean sweep before incrementing. Each step therefore paints exactly one curve, and after the staircase resets the whole family repeats — persistently, so the eye sees a steady figure.

2.4 Turning current and voltage into X and Y

The scope is a voltmeter on two axes, so the IT-3121’s last job is to hand it two voltages proportional to the quantities we care about.

• X (horizontal) is easy: it is the actual voltage across the device’s main terminals — collector-emitter, or drain-source — buffered out to a banana jack. The voltage-sensitivity switch (9 ranges, 0.1–50 V/div, 1-2-5) scales it so a chosen number of volts corresponds to one scope division.
• Y (vertical) is the device’s output current, which the instrument reads as a small voltage across a current-sense resistor in the collector/drain path, then scales with the current-sensitivity switch (9 ranges, 0.5–200 mA/div, 1-2-5) to so-many milliamps per division.

You set your scope to X-Y mode, feed X and Y from the IT-3121’s jacks, and read the family against the scope’s own graticule: horizontal grid lines are volts-per-division, vertical are milliamps-per-division, both exactly as the front-panel switches say. There is no calibrated internal graticule as on a Tek 576 — the scope you already own is the display, and its graticule is your measuring grid.

2.5 What each front-panel control does

Pulling the operator’s controls together, so the panel stops being mysterious:

Table 1 — Pulling the operator's controls together, so the panel stops being mysterious

Control	What it sets
Collector range switch	40 V/1 A vs 200 V/200 mA operating envelope
Collector voltage control	peak of the swept collector supply (0 → range max)
Load resistor switch (12-pos)	series current limit / load line, 0 Ω–1 MΩ
Step amplitude switch (12-pos)	base current per step (2 µA–10 mA) or gate volts per step (0.05–1 V)
Number of steps control	how many curves in the family, 1–10
Polarity switch	NPN/PNP, N-/P-channel — reflects step + collector together
Current sensitivity (9-pos)	Y scaling, 0.5–200 mA/div
Voltage sensitivity (9-pos)	X scaling, 0.1–50 V/div
A/B device select	swap between two sockets to compare/match parts
Cal switch	calibration check for the display scaling
X, Y outputs	banana jacks to the external scope’s X-Y inputs

Every one of those controls maps onto a piece of the story above: two of them shape the collector sweep, two shape the base staircase, two scale the axes, one sets polarity, and one lets you flip between two devices to compare them. Understand the block diagram and the panel reads itself. Vol 3 shows what a modern designer keeps and what he throws away when he rebuilds this architecture with a microcontroller; Vol 4 is the sit-down-and-drive-it procedure.