Using a Transistor Curve Tracer, September 1965 Electronics World

I have to admit to only having ever used a transistor curve trace in two instances. The first was while working as an electronics technician at Westinghouse Oceanic Division in Annapolis, Maryland, in the early and mid 1980s. We built sonar systems for the U.S. Navy, and when the contract levels were really low, some of us would get farmed out to other parts of the company rather than doing layoffs. On one occasion I spent about two months sharpening drill bits and end mill bits in the machine shop. Another time I worked at incoming inspection doing sample testing of resistors, capacitors, inductors, diodes, transistors, etc. That is where I first used a transistor curve tracer. The truth is at that point I didn't really know what the curves represented from a performance standpoint. Since it was in the days before digital test equipment, we had a plastic overlay on the display with upper and lower limits for the curve. Test engineers wrote procedures and did equipment setup so really all you needed to perform the tests was decent eyesight and a bit of good sense. The next time I used a curve tracer was during laboratory experiments at the University of Vermont (UVM) whilst working on my BSEE degree (graduated in 1989 at 31 years old - a late bloomer). This article describing the use of a transistor curve tracer appeared in a 1965 issue of Electronics World magazine. It was a lot of people's introduction to transistor parameter testing and characterization.

Operating principles of an important piece of test equipment which displays a complete family of diode or transistor characteristic curves. The method of interpreting curves to obtain parameters is covered.

Transistor curve tracer displays the dynamic characteristic curves of transistors and semiconductor diodes on the screen of a cathode-ray tube. This article will show some of the displays it is possible to obtain with a curve tracer and how the displays may be used to determine transistor and diode parameters. These displays are intended for measurements of transistor characteristics which exist well below alpha cut-off, commonly measured between d.c. and 1000 cps, hence, we will not consider any high-frequency measurements.

Fig. 1 shows a curve tracer being used with a thermocouple to check temperature effects on a medium-power p-n-p transistor.

We have chosen to give examples of measuring the four hybrid or h. parameters from the characteristic curves, although other parameters could be measured as well. Further, we have given examples predominantly in the common-emitter configuration. Measurements can also be made using common-base or common-collector circuits as well, with either p-n-p or n-p-n transistors being employed.

Fig. 2 - Functional block diagram of the Tektronix Type 575 transistor curve tracer is shown here.

Fig. 3 - Forward current transfer ratio h_fe(beta) is determined by measuring the change in collector current produced by a change in base current at a specified collector voltage. In this case at V_c = 10v., beta = Δ|c/Δ|b = 1.6 div. X (2 X 10^-1)/12 X 10^-6) = 1.6 X 10² = 160. Alpha = β/(β + 1) = 160/161 ≈ 0.994. Curves shown are for "n-p-n" transistor in common-emitter configuration.

Fig. 4 - Output conductance, h_oe, is determined by measuring the change in collector current produced by a change in collector voltage at a specified base current. At |_b=0.012 ma., h_oe = Δ|_c/ΔV_c = 0.2(2 X 10^-4)/(4 X 0.5) = 0.4 X 10^-4/2 = 0.2 X 10-4 = 20 μmhos. Curves are for "n-p-n" transistor, common-emitter circuit.

Fig. 5 - Input impedance, h_ie, is determined by measuring the change in base voltage resulting from a change in base current at a specified collector voltage. At V_c=5 v., h_ie=Δv_b/Δ|_b= 0.6(2 X 10^-2)/10^-5 = 1.2 X 10³ = 1200 ohms. The above curves are for "n-p-n" transistor in common-emitter configuration.

Fig. 6 - Reverse voltage transfer ratio h_re, is determined by measuring the change in base voltage resulting from a change in collector voltage at a specified base current. In this case at 1_b = 0.01 ma., h_re = ΔV_b/ΔV_c = 0.2(2 X 10^-2)/4 = 1 X 10^-3 or 1000 X 10^-6 "N-p-n" transistor in common-emitter circuit.

Fig. 7 - Collector family, "n-p-n" transistor, common-base circuit.

Fig. 8 - Measurement of |_cbo, the collector current that flows with emitter circuit open is shown on |_c vs V_c display for "n-p-n" transistor in common-base circuit. At V_c = 18 v., |_cbo = 0.1 ma.

Fig. 9 - Collector breakdown characteristic curves of |_c vs V_c for various values of |_b for "n-p-n" transistor in common-emitter configuration. Note sharp rise of current around 25-30 volts V_c.

Fig. 10 - The loops in the characteristic curves shown here are the result of the presence of collector-to-base capacitance. With reduced capacitance the loops shown will close up.

The visual display of a family of transistor characteristic curves using oscilloscopic techniques, employing synchronously stepped and swept voltage-current sources, offers certain advantages over the d.c. point-to-point measurement techniques:

1. Small irregularities in the characteristics, which may escape observation by the point-by-point method, are readily visible.

2. The extremes in the variation of a parameter value may be observed without altering the operating conditions of the transistor.

3. The changing magnitudes of two parameters may be observed simultaneously, as well as the dependence of one upon the other.

4. The short duration and lower duty cycle of the peak sweeping voltages or currents applied to the transistor pro-duce less thermal rise than does the steady-state d.c. condition which occurs in point-by-point measurements. Inaccuracies due to thermal gradients are thereby minimized.

5. For the same reason noted in (4), the maximum ratings of the transistor may be observed without exceeding the safe limit of its power dissipation; thus incipient junction-break-downs are minimized.

6. Observation, comparison, or a permanent record by photographic techniques of the transistor characteristics may be made more rapidly than by the point-by-point plot, thus affording a savings in manpower.

7. A resistive load line may be constructed in the family of characteristic curves so that dynamic performance data for the transistor may be forecast.

Transistor characteristic curves may be found to deviate more widely from the published "nominal" characteristic curves than those for vacuum tubes. For this reason, the curves obtained on a curve tracer may not always agree with curves published in data sheets or transistor manuals.

A functional block diagram of the Tektronix Type 575 transistor curve tracer is shown in Fig. 2. In this curve tracer, two signals are developed to be applied to the transistor. One signal consists of either constant-current or constant-voltage steps, which are normally applied to the base (common-emitter configuration), or to the emitter (common-base configuration). The other signal is a variable amplitude half-sine wave, normally applied to the collector. The two signals are synchronized so that for the duration of one step, or one-half step, there is one excursion of the half-sine voltage from zero (ground) to maximum and back to zero.

The base step generator produces from 4 to 12 steps in selected amplitude increments of constant current or constant voltage. The duration of each step is either 1/20th or 1/240th of a second. The steps may be made repetitive, to occur once, or made inoperative and held at zero step level. The steps may be positive (for n-p-n) or negative (for p-n-p) in polarity with the zero step value adjustable by a small amount, positive or negative, from ground to start the stair-steps on the zero-volts or zero-current input curve.

The output of the base step generator is fed through a switch to the base (B) terminal of the test panel. The switch provides a means of disconnecting the base terminal from the base step generator to either open or short the terminal to ground.

The collector sweep is the source of the half-sine collector sweep voltage. The peak collector sweep voltage is variable in amplitude from zero to 20 volts at 10 amps, or zero to 200 volts at 1 amp. The polarity of the half-sine shaped voltage excursions may be made positive (for n-p-n) or negative (for p-n-p) from ground. The duration of sweep from zero to maximum and back to zero is always 1/120th of a second.

The collector supply may be fed directly to the collector terminal (C) of the test panel, or variable dissipation-limiting resistance values may be inserted.

The third terminal (E) of the test panel is ground. The test panel (not entirely shown in Fig. 2) has a switch for transferring all connections from the left-hand terminals and socket to the right-hand terminals and socket with a neutral "off" position. Another switch is provided to switch the sockets only from grounded base to grounded emitter.

The horizontal amplifier may be connected to display one of four signals on the horizontal axis of the cathode-ray tube: (1) collector volts; (2) base volts; (3) base current; (4) an external signal. In the same manner, the vertical amplifier may be connected to display one of four signals on the vertical axis of the CRT: (1) collector current; (2) base volts; (3) base current; (4) an external signal.

The most common transistor display is a "collector family," such as shown in Fig. 3. In this display, collector voltage V_c is plotted horizontally, collector current I_c is plotted vertically, and each curve or step represents a different value of base current I_b. Such a display is obtained by connecting the transistor under test for the normal operating polarities, that is, for a forward-biased emitter and for a reverse-biased collector.

However, with the curve tracer, the polarity of the base step and collector sweep signals may be selected independently and used to advantage. For instance, the emitter may be reversed-biased in order to measure the amount of current or voltage required to decrease the collector current, I_cbo or I_ceo, to minimum value.

A given parameter may be measured from anyone of the displays containing the current and voltage relationships involved in the parameter. The display chosen depends upon the particular parameter and the characteristics of the transistor under test. The display that should be used is, of course, the one which yields the most accurate measurement or vivid display of that parameter.

The user may sometimes prefer to measure other parameters that may be combined in a formula to produce a more accurate end result. A typical case is the relationship between a (alpha), the common-base forward current transfer ratio and β (beta), the common-emitter forward current transfer ratio. In the common-base configuration of Fig. 7, for example, it can be seen that the change in collector current for a given change in emitter current is very close to one. In a common-emitter configuration, however (Fig. 3), the change in collector current is considerably more than the change in base current. Since α and β are related by α = β / (β + 1), we may find β to a greater degree of accuracy as far as scaling is concerned, and solve for a.

A word may be in order regarding small-signal hybrid, or h, parameters. In small-signal operation, linearity should exist over the operating range of the parameter and the value of the parameter should be independent of the signal amplitude. Ordinarily, the active region of a transistor without defects is quite uniform, and a cursory examination of the family of " characteristics will verify that linearity exists over the operating range of the parameter.

The versatility of the curve tracer provides for making "small-signal" as well as "large-signal" measurements. In order to verify that the parameter value, or operating region, of the transistor is independent of the signal amplitude, additional measurements of the parameter may be made adjacent to, or within, the intended operating region and the resulting values compared. For instance, the base step values may be reduced and the number of steps increased to examine a smaller segment within the same operating region, etc.

The waveforms, Figs. 3 through 6, illustrate some of the displays it is possible to obtain with a curve tracer and measurements that can be obtained from the displays. These show characteristics of a small-signal n-p-n transistor in the common-emitter mode, and how parameters may be obtained.

The parameters for the common-base or common-collector configurations may be handled in the same manner as just illustrated for common-emitter configuration. From observation of the collector family of output characteristics in common-base configuration, shown in Fig. 7, it is clear that h_ƒb or α, which equals ΔI_c/ΔI_e, and h_ob, which equals ΔI_c/ΔV_c, are difficult to scale from the graticule. This is quite normal and it may be anticipated that variations between transistors will not appear as vividly as they would in common-emitter operation.

Fig. 11 - Same display as shown in Fig. 10 except in this case a 1000-pf. capacitor was added externally between base and collector.

Fig. 12 - The zener diode forward conduction characteristics.

Fig. 13 - Zener diode reverse breakdown characteristics. The zener control voltage is shown to be approximately 8.7 volts.

The curves of Fig. 7, by themselves, are principally informative in the "saturation region," which appears to the left of zero collector volts. It will be recalled that in common-base configuration, unlike common-emitter, the polarity of the emitter is opposite to that of the collector. The apparent forward biasing of the collector in the "saturation region" results from emitter current and hence appears opposite in polarity to the applied collector voltage. Its magnitude may approach the magnitude of the emitter voltage; however, it can never exceed this value of voltage.

The collector current which flows with zero emitter current (common base) I_cbo, or with zero base current (common emitter) I_ceo may be measured directly from the collector family of curves. A typical measurement for I_cbo is illustrated above in Fig. 8.

As the voltage is increased on a reverse-biased collector of a transistor, a point will be reached where the collector current increases rapidly and may become essentially independent of the collector voltage. Several measurements of the break-down characteristics may be obtained with a curve tracer, but in this article we will not attempt to differentiate among the phenomena referred to as avalanche, punch-through, zener breakdown, or carrier multiplication, since they are largely dependent upon the configuration, type, and geometry of the transistor.

Collector breakdown is shown in Fig. 9, where I, increases almost without limit in the vicinity of 25 to 30 volts on the collector (V_c) .

Under certain conditions, some transistors will exhibit a negative-resistance characteristic in the breakdown region. Oscillation may also occur in some other region of the characteristic curve and may be recognized by the faint and ragged, or erratic, behavior of the trace on the oscilloscope dis-play on the curve tracer.

The effect of the collector capacitance may become quite pronounced in some transistors. The loop resulting from the collector-to-base capacitance causes a displacement current to add and subtract from the current of the transistor. The effect is most pronounced in common-emitter configuration with low base drive, high collector voltage, and the greatest expansion of the vertical collector-current display. The capacitance effect is more pronounced at the knee of the curve in the collector family because of the sudden change in collector current in the transistor. At the same time, the rate of change of the collector half-sine shape sweep voltage is maximum, that is, near the beginning and near the ending of the particular operating cycle. The equivalent steady-state d.c. value of collector current may be established by decreasing the collector voltage until only the peak of the collector sweep occurs at the point in question. The approximate magnitude of the capacitance may be measured by adding a capacitor of 10 to 1000 pf. between the base and collector terminals. The transistor capacitance is approximately equal to the capacitance of the external capacitor when the vertical size of the loop is doubled. A comparison of Figs. 10 and 11 illustrates the effects of the collector-to-base capacitance.

Forward-conduction or reverse-voltage breakdown of a diode may be displayed on a curve tracer by the simple expedient of using the collector sweep supply as a voltage source. The display will represent the E-I plot with anode current plotted vertically as I_c, and the anode voltage plotted horizontally as V_c. Forward and reverse zener diode characteristic curves are shown directly below in Figs. 12 and 13 respectively.

Using a Transistor Curve TracerSeptember 1965 Electronics World

Using a Transistor Curve Tracer
September 1965 Electronics World