VBA Curve Tracer — Vol 1: What the VBA Curve Tracer Is

Paul Versteeg's open, fully analog semiconductor curve tracer, and where the name comes from

Figure 1 — The finished VBA Curve Tracer with the top lid off — transformer, heatsinked pass devices and 250 WVDC reservoir cap at the rear, the analog boards mid-chassis, and the labelled front panel (Step D… — Figure 1 — The finished VBA Curve Tracer with the top lid off — transformer, heatsinked pass devices and 250 WVDC reservoir cap at the rear, the analog boards mid-chassis, and the labelled front panel (Step Delay, Offset, Polarity, Steps, DUT Select, Current, Voltage, Voltage Ranges). Source: Paul Versteeg / paulvee, VBA-Curve-Tracer GitHub repo.

1.1 What it is, in one line

The VBA Curve Tracer is a home-buildable, fully analog instrument that draws the current-versus-voltage characteristic of a semiconductor part — a transistor, a diode, a FET, an LED — as a live curve (or a family of curves) on an ordinary oscilloscope running in X-Y mode. It is the modern, open-design answer to the classic bench transistor tracer: the Tektronix 575/576/577 iron and the Heathkit IT-3121. Paul Versteeg (“paulvee”) publishes the entire design — schematics, PCB layouts, bills of materials, build and calibration documents, and Gerbers — as a public GitHub project so anyone can build their own.

This is worth stating clearly up front, because it shapes everything that follows: there is no microcontroller, no firmware, and no host software in the VBA. Where the tube-tracer side of this bench (the uTracer6 and eTracer) fires a high-voltage pulse and lets a PC plot the result, the VBA is a classical analog machine. Op-amps, a 555 timer, a CMOS counter, discrete pass transistors and a bank of rotary switches do the work; your scope is the display. That makes it a genuinely different beast from the tube tracers documented elsewhere in this Curve Tracers category, and a good teaching instrument, because every function corresponds to a knob you can see and a circuit you can trace.

1.2 Where the name comes from

Do not read anything into the letters — “VBA” here has nothing to do with the Microsoft macro language. Per Versteeg’s own blog, the name is drawn from the surnames of the three main contributors: Paul Versteeg, Bud Bennett, and Mark Allie. Versteeg built the original prototype back in 2017 and drove the project; Bud Bennett — a former linear-IC designer — contributed the analog regulation approach; Mark Allie handled the schematic capture and PCB layout in Altium (his university held the full license). The GitHub README also credits community builders (Matt Web and Cory Lytle among them) who caught BOM errors and suggested protection and anti-oscillation fixes that folded back into the released design.

1.3 Design goals

The stated motivation is squarely practical: build a proper curve tracer for the home bench, and in doing so “bring curve tracers back into the foreground” so that a builder can verify a real device against its datasheet. A curve tracer answers questions a multimeter cannot — what a transistor’s gain actually is at a given operating point, how flat its output curves are, where it breaks down, whether two parts are close enough to call a matched pair. Versteeg’s blog notes the project drew university interest, with talk of building ten or more units for student labs, which pushed the design toward being reproducible and well-documented rather than a one-off.

The concrete goals that emerge from the write-ups:

Cover real, useful operating envelopes, not just tiny signal parts — hence ranges reaching 200 V and 2 A (see below).
Be safe to the part under test. Curve tracing pushes a device toward its limits; it is very easy to cook a transistor. The VBA leans hard on adjustable current limiting and an optional per-step delay to keep dissipation down.
Be reproducible by a competent builder from published files, with verification and calibration procedures included.
Use the scope you already own as the display, rather than adding a screen and the digital plumbing behind it.

1.4 What it can trace

The VBA is built for two- and three-terminal semiconductor devices. In practice that covers:

Bipolar transistors (BJTs) — NPN and PNP, small-signal through power devices — as a family of output curves (collector current vs. collector-emitter voltage, one curve per base-current step), from which you read DC gain (hFE/β).
FETs — enhancement-mode MOSFETs and depletion-mode JFETs — as a drain-current family stepped by gate voltage, plus threshold and breakdown behaviour.
Diodes — forward knee and reverse breakdown, including Zeners (measure the regulated voltage against current) and ordinary rectifier/signal diodes.
LEDs — the forward-voltage knee, which is what sets the different turn-on voltages you see across colours.
Composite and matched parts — Darlingtons, and matched-pair selection: the front panel carries a DUT Select switch and two device positions (Left/Right) so you can flip between two candidates and overlay their curves for matching.

The front panel exposes the operating vocabulary directly: a BJT/FET device-type switch, a Polarity switch (P for P-type, N for N-type) that sets the direction of the steps, a Steps selector (0–7), a Step Output range knob, Offset controls for biasing the steps, Current and Voltage level controls with a current-limit (CL) indicator, a Current Range multiplier, an X-Amp ×1/×10 switch, and a Voltage Ranges selector for the three supply envelopes.

The three DUT supply envelopes (voltage across the part × maximum current available) are:

Table 1 — The three DUT supply envelopes (voltage across the part × maximum current available) are

Range	Max voltage	Max current	Typical use
Low	0–35 V	0–2 A	Power BJTs/MOSFETs at real currents
Mid	0–70 V	0–1 A	Medium-power parts
High	0–200 V	0–100 mA	High-voltage parts, breakdown tests

These are selected by reconfiguring the mains transformer windings (parallel / series / plus an auxiliary winding), which is why the range switch is a deliberate, chunky front-panel control rather than a software mode.

1.5 How it shows you the curves

Figure 2 — The VBA drives two BNC outputs — X (voltage) and Y (current) — into a scope set to X-Y mode; the tracer itself is a self-contained signal source and the scope is the screen. Source: hand-authored SVG.

The instrument produces two analog outputs on BNC sockets on the back panel: X, proportional to the voltage across the device, and Y, proportional to the current through it (developed across a current-sense shunt and buffered by the Y amplifier). Set an analog or digital scope to X-Y mode, feed X and Y in, and the beam traces the device’s I-V characteristic in real time. A third BNC provides an optional Z (blanking) signal for CRT scopes that support intensity blanking, so the retrace and inter-step jumps can be hidden for a cleaner display. The X amplifier is scaled for a 1:1 relationship between device voltage and displayed voltage, and the Y amplifier can be switched ×1/×10 to trade sensitivity against noise. Vol 4 covers how to read the resulting pictures; Vol 2 explains how the sweep-and-step engine builds them.

1.6 Where it sits on this bench

Within the Curve Tracers category, the VBA is the semiconductor / component tool, opposite the tube tracers (uTracer6, uTracer NXT, eTracer). Against the vintage semiconductor tracers — the Tektronix 575/576/577 and the Heathkit IT-3121 — it offers comparable measurement capability (Versteeg positions it as doing “many of the measurements a Tektronix 577 can make”) in a modern, buildable, fully documented package, at the cost of needing your own scope as the display. The next volume opens the lid on how it actually works.