Curve Tracers — Vol 4: Reading and Using the Curves

This is the volume the rest of the primer builds toward. The instrument draws a picture; here you learn to read it. We work through the bipolar transistor’s output family first — because everything else is a variation on it — then the diode, then the FET, then the specific job of matching devices, and finally the pitfalls that produce misleading curves.

4.1 Reading a BJT output family

The bipolar output family plots collector current IC (vertical) against collector-emitter voltage VCE (horizontal), with one curve per base-current step. Four features live on that picture.

Cut-off is the bottom of the plot: with IB = 0 the transistor passes only a tiny leakage current, so the lowest “curve” hugs the horizontal axis. A curve that sits noticeably above the axis at IB = 0 is telling you the device is leaky — a common failure you would never catch from a single β reading.

The saturation knee is the steep near-vertical rise at the far left, where VCE is only a few tenths of a volt. Here the transistor is fully on and VCE cannot fall any further; for a small silicon device the knee sits around VCE(sat) ≈ 0.2 V. The sharpness and position of the knee tell you the saturation voltage — important when the device is used as a switch.

The active region is the broad, nearly-flat plateau where the curves run left-to-right across most of the screen. This is where the transistor works as a linear amplifier: IC is set almost entirely by IB and barely depends on VCE. The vertical spacing between adjacent curves is the current gain. Because each step is a known increment of base current, β at any point is:

β = hFE = ΔIC / ΔIB

Concretely: if the step generator is set to 10 µA/step and two adjacent curves are separated by 1.8 mA of collector current, then β ≈ 1.8 mA / 10 µA = 180 at that operating point. Read the spacing low on the screen and again high on the screen and you can see β sag at the current extremes — the whole point of plotting it. Even spacing means constant gain; curves that bunch together at the top mean β is falling off as the device runs out of headroom.

The Early effect is the gentle upward slope of those “flat” active-region curves. Ideally the active curves would be perfectly horizontal (infinite output resistance); in reality they tilt up slightly, and if you extrapolate them backward to the left they converge on a single point on the negative voltage axis — the Early voltage, VA. A steeper slope means a lower output resistance ro = ΔVCE / ΔIC and a smaller Early voltage, which matters when the transistor is used as a current source or a high-gain amplifier stage. A tracer shows this instantly as a fan that is not quite parallel.

Breakdown is at the far right: push VCE high enough and the top curve suddenly turns upward and the current runs away — that corner is BVCEO, the collector-emitter breakdown with the base open. On a curve tracer you creep up on it deliberately, with the series/load resistor set high so the runaway current is limited and you read the breakdown voltage without destroying the part.

4.2 Reading a diode

A two-terminal diode gives a single curve. Forward-biased, it stays near zero current until the forward knee — about 0.6–0.7 V for a silicon junction, 0.3 V for germanium, higher for an LED — then current rises steeply. Reverse-biased, it passes only a tiny leakage current along the horizontal axis until, for a Zener or avalanche diode, it hits its rated reverse breakdown and the curve drops sharply and vertically at VBR~. A plain rectifier shows only the forward knee within its ratings; a Zener shows both corners, and the tracer reads its breakdown voltage directly off the horizontal scale. Leakage that is visibly above the axis, a soft (rounded) reverse corner, or a forward knee in the wrong place all flag a degraded part.

4.3 Reading a FET

A FET’s output family plots drain current ID against drain-source voltage VDS, stepped in gate voltage VGS (not current — the gate draws no DC current). The shape rhymes with the BJT’s but the vocabulary shifts. The steep region near the origin is the ohmic (triode) region, where the FET behaves like a voltage-controlled resistor — the slope there is RDS(on), the on-resistance that matters for a switching MOSFET. Past the pinch-off boundary the curves flatten into the saturation region used for amplification.

The key parameter is transconductance, gm — how much the drain current changes per volt of gate drive, read from the vertical spacing between curves at a fixed VDS:

gm = ∂ID / ∂VGS (at constant VDS)

For a JFET or depletion MOSFET you can also read the pinch-off / threshold voltage directly: it is the gate voltage at which the drain current just collapses to zero. This is exactly why the step-polarity/offset control from Vol 3 matters — an enhancement MOSFET does not conduct until VGS passes its threshold (often several volts positive), so the tracer must be able to offset the gate steps positive, which the original Heathkit IT-3121 could not do without modification and which the VBA Curve Tracer and djerickson’s modified Heathkit handle by design.

4.4 Matching devices — the job the tracer is uniquely good at

Why match? Differential pairs, push-pull output stages, current mirrors, and parallel power devices all depend on two (or more) transistors behaving identically. A single β or gm reading agreeing at one bias point does not make two parts a match — recall Vol 2’s caution about tube testers, which applies just as forcefully to semiconductors. Two transistors can share the same nominal gain yet diverge badly at the current or voltage where your circuit actually runs them.

How the tracer does it. Overlay one device’s family on the other’s — either by displaying them on the same screen or by photographing/capturing each and comparing. Matched devices produce superimposable families: the curves land on top of each other across the whole range, not just at one point. On a dual-trace-capable tracer you can flip between the two parts and watch the fan shift; on a digitizing tracer (uTracer, VBA) you overlay the captured data. This is the standard way to select complementary NPN/PNP pairs, matched quads for audio output stages, and matched tubes for a push-pull amplifier — and it is something no single-number tester can do honestly.

4.5 Common measurements, at a glance

The parameters you pull straight off the screen:

Table 1 — The parameters you pull straight off the screen:

Parameter	Device	Where you read it
β / hFE	BJT	Vertical spacing of active-region curves ÷ step size
VCE(sat)	BJT	Voltage at the saturation knee (~0.2 V, small Si)
BVCEO	BJT	VCE where the top curve runs away at the right
Early voltage VA / ro	BJT	Slope of the active curves (back-extrapolation)
Leakage	BJT / diode	Current above the axis at zero drive / reverse bias
VF (forward drop)	Diode	Voltage at the forward knee
VBR / Zener voltage	Diode	Reverse-breakdown corner
gm	FET	Vertical spacing of saturation curves ÷ VGS step
RDS(on)	MOSFET	Slope in the ohmic region near the origin
VGS(th) / pinch-off	FET	VGS where ID reaches / leaves zero

4.6 Pitfalls — how a tracer lies to you

• Thermal droop and runaway. Leave a power device dwelling at high dissipation and it heats up during the sweep; the curves drift (β climbs, curves creep) or, worse, the device thermally runs away. Keep the peak power / series-resistor control conservative and read quickly. This is a real-hardware effect, not a fault.
• Reading a parameter off the wrong region. β read down in the saturation knee or up near breakdown is not the β your circuit sees — read it in the flat active region at a representative operating point.
• Forgetting the scale factors. A curve is meaningless without knowing the current-per-division and volts-per-division. On a Heathkit-plus-scope or DIY-plus-scope rig you set those on the scope yourself; get them wrong and every number is wrong by that factor.
• Under-driving a MOSFET. Trying to trace an enhancement MOSFET without a positive gate offset shows a dead, non-conducting device — an instrument limitation, not a bad part (Vol 3’s step-offset control, and the reason for the common Heathkit MOSFET modification).
• In-circuit surprises on an octopus. A signature that looks “wrong” may just be other components on the same node loading the reading; the technique’s strength is comparison to a known-good board, not absolute reading.
• Exceeding the safe area. The tracer will happily drive a device past its safe operating area if you let it. Start high-resistance and low-voltage, and open up only as far as the measurement needs — the same discipline whether you are on 200 V of semiconductor sweep or several hundred volts of pulsed tube plate voltage.

With the reading skill in hand, Vol 5 turns practical: which actual instrument to reach for, and when to buy vintage iron versus build a modern one.