VBA Curve Tracer — Vol 2: How It Works — Principle and Circuit Design

2.1 The idea behind every curve tracer

A curve tracer is, at heart, two coordinated sources and two meters. To draw a transistor’s output characteristics you need to:

1. Sweep the collector-emitter voltage repeatedly from zero up to some maximum and back, fast enough that the eye (or the scope) sees a continuous line.
2. Step a second quantity — the base drive — to a series of fixed levels, holding each level for one full sweep so that each level draws its own curve.
3. Measure the voltage across the device (the horizontal axis) and the current through it (the vertical axis) simultaneously, and plot one against the other.

Stack those curves — one per base-current step — and you get the familiar fan of output characteristics. Because a scope in X-Y mode plots channel-1 against channel-2 instead of against time, it is the natural display: feed it “device voltage” on X and “device current” on Y and it draws the curve for you. That is the whole game. The art is in generating a clean sweep, generating clean steps synchronised to that sweep, and doing both without destroying the part under test.

2.2 Why a triangle, and why analog

Versteeg’s V3/VBA design makes a specific choice here: the collector/drain sweep is a triangle wave, not a rectified sine. The blog frames the regulated supply as “analog to a lab power supply, with the major difference that a lab supply only does this for a DC voltage, and we need to do the same for DC but also for a triangle-based waveform.” The triangle runs at about 160 Hz and is tunable via an internal trimmer over roughly 140–650 Hz, with an amplitude of about 7.5 V peak-to-peak at the generator before it drives the regulated DUT supply.

Two design reasons for the triangle are called out. First, a single-slope triangle per step avoids the “looping” artifact you get with half-sine drives at elevated power dissipation, where the up-sweep and down-sweep trace slightly different paths as the die heats. Second, the linear ramp maps cleanly onto the X axis. The whole thing is analog because the goal was a self-contained instrument with immediate, real-time response and no digitiser, sampling, or host in the loop.

2.3 The VBA’s building blocks

The design is split across four PCBs — Main Board, Front Board, Front Panel and Back Panel — but functionally it is five blocks: the triangle generator, the regulated DUT supply with its current limiter, the isolated step generator, the X/Y measurement amplifiers, and the multi-rail power supply that feeds them all. The Main Board and Front Board carry the active circuitry (the GitHub schematic sheets are named for these blocks: TriangleGenerator, DUTSupply, CurrentSource, IsolatedSupply, XYAmp, FaultDetect, D2A, StepGenBuffer).

2.3.1 The triangle generator

An op-amp integrator and a comparator (a 555 used as the comparator, not as a free-running astable) form a closed-loop triangle/square generator: the integrator ramps until the comparator flips, then ramps the other way. The integrator’s capacitor and a series trimmer set the frequency (the ~160 Hz nominal, 140–650 Hz range noted above), and a second trim synchronises the sweep to the step transitions across the isolation barrier, so each new step starts cleanly at the bottom of a sweep.

2.3.2 The regulated DUT supply and current limiter

This is the heart of the machine and the part Bud Bennett’s linear-design experience shaped. Rather than a plain op-amp voltage regulator, the supply uses a trans-conductance amplifier built from matched transistor pairs driving a MOSFET pass device (a main pass MOSFET plus a load-sharing slave MOSFET, with current-balance sensing so neither device hogs the load). The transconductance topology gives a smooth handover between constant-voltage and constant-current (current-limit) operation without the saturation glitches a naive op-amp regulator produces when it slams into its limit — exactly the behaviour you want when a curve crosses from its active region into current limiting.

The three voltage/current envelopes (0–35 V/2 A, 0–70 V/1 A, 0–200 V/100 mA) come from switching the mains transformer windings — paralleled for the high-current low-voltage range, in series for the mid range, and with an auxiliary winding added for the 200 V range. Current limiting is set by a precision 1 V reference divider whose rotary switch (S101) parallels resistors to scale that reference down, giving the front-panel Current Range multipliers ×1, ×0.5, ×0.2, ×0.1, ×0.05 and ×0.02. A small constant-current sink (~12.5 mA) is deliberately drawn from the DUT supply at all times, even at 200 V out, so the regulator always has a defined minimum load and stays in regulation at light loading.

2.3.3 The step generator (isolated)

The base/gate drive is generated on the Front Board and is galvanically isolated from the rest of the instrument (its own floating ±15 V rails), which is what lets the tracer drive P-type and N-type parts and float the step reference relative to the DUT supply. A binary counter advances one count per sweep; its outputs feed a resistor ladder that acts as a digital-to-analog converter, producing a clean staircase. A 555-based delay circuit optionally inserts a dwell between steps (the front-panel Step Delay), which is the thermal-protection lever: at high power you slow the stepping so the die is not held hot.

A rotary switch (S21) selects the step magnitude in a 1-2-5 sequence across current and voltage ranges, the Steps knob selects how many steps are active (0–7), and the Polarity switch chooses positive-going steps for N-type devices or negative-going for P-type. An Offset control adds a bias to the staircase — needed for FETs, where the interesting action sits around the threshold voltage rather than at zero. The step generator outputs are protected by back-to-back depletion-mode MOSFETs that clamp the output current under fault and by TVS diodes that survive the DUT supply being injected backwards up to 200 V — a fault-detect circuit trips the DUT supply off if a step output is driven outside a safe window.

2.3.4 The X/Y measurement amplifiers

Device current is sensed as the voltage across a ~1 Ω shunt (built from several parallel resistors) and buffered by the Y amplifier, switchable ×1/×10 so you can trade sensitivity against noise. The X amplifier buffers the device voltage at a 1:1 scale. Both drive back-panel BNCs into the scope. The optional Z (blanking) output, also on a BNC, lets a CRT scope blank the beam during retrace and step transitions.

2.3.5 Power supplies

The instrument runs several rails from a multi-winding transformer: the main DUT supply from the (reconfigurable) high-current winding feeding a large reservoir capacitor (a 250 WVDC electrolytic is visible at the rear in Vol 1’s photo), a separate isolated supply for the step generator (its floating ±15 V), and dedicated rails for the X/Y amplifiers and the triangle/control circuitry. Keeping the step generator on its own isolated island is what prevents ground-loop coupling between the low-level base drive and the high-current, high-voltage DUT supply.

2.4 The upshot

Everything the VBA does maps onto a physical control and a discrete circuit: the triangle sets the horizontal sweep, the isolated staircase sets the family of curves, the transconductance supply sets the envelope and enforces the current limit, and two buffer amplifiers hand the result to your scope. No sampling, no firmware, no host. Vol 3 turns to building it from the published files; Vol 4 to reading what it draws.