Transistor Test Sets

Author: Leonard Krugman

Several elaborate transistor test sets are available commercially. These testers are useful for large-scale experimental work, since they incorporate means for completely evaluating the characteristics of all types of point-contact and junction transistors, and do not require external test equipment and meters. The home experimenter and the lab technician, however, can get satisfactory results on breadboards, based on the techniques described on the previous pages.

In checking transistors during maintenance and repair, it is not necessary to check all the transistor parameters. A check of two or three of the performance characteristics will determine quickly whether a transistor needs to be replaced.

Fig. 4-19. Transistor tester for measuring α and I_co.

Figure 4-19 illustrates a transistor check circuit which will measure the current gain and saturation current with reasonable accuracy. The operation procedure and general functional description of the circuits follows:

1. With switch SW2 in the calibrate (CAL) position and switch SW1 in the current gain (α) position, adjust the signal gain of the audio oscillator for one volt across resistor R₁ Now throw SW2 to the current gain (α) position. The signal is now connected to the base of the transistor through resistor R₂ and the d-c blocking capacitor C₁-Since resistor R₂ is 100,000 ohms, the base and emitter resistances of the transistor are negligible; a-c base current i_b, 10 microamperes.
2. The d-c base current bias is controlled by resistors R₃ and R₄, which permit a variation of from about 1 to 100 microamperes. R₄ is adjusted until the collector d-c bias current, measured by meter M, is one milliampere.
3. Practically all of the a-c output appears across the 100 ohm resistor R5, because of the high impedance of choke coil L (over 60,000 ohms at 1,000 cps), and the high output resistance of the transistor (usually more than a megohm). The output voltage across R₅ is α i_bR₅, and since i_b = 10 microamperes, R₆ = 100 ohms, this voltage equals .001α. The value of α may vary from 10 to 100. The a-c voltage may, therefore, range from .01 to .1 volt. Thus, the current amplification can be taken directly on a low scale of a good voltmeter.

Due to the comparatively low value of R₅, the measured reading closely approximates the maximum current gain α = r₁₂/r₂₁. This value of current gain for the grounded emitter connection can be converted into approximately equivalent values for the grounded base and grounded collector circuits by means of the following conversion formulas:

where α_GE = maximum current gain for grounded emitter connection; α_GB = maximum current gain for the grounded base connection; and α_GC = maximum current gain for the grounded collector connection. These relationships are derived by neglecting r_e and r_b in comparison with r_m, r_c and (r_c - r_m). Error in this approximation is negligible.

For example, assume that a transistor is tested in the circuit of Fig. 4-19 and produces a reading of .022 volt on the a-c output voltmeter connected across R₅. The current gain

The saturation current is read directly on the milliammeter M if switch SW1 is now placed in the I_co position. This switch opens the base lead, removing the bias, and also shorts out the inductor L so that the six-volt battery is across the emitter and collector electrodes.

The circuit as shown is only suitable for N-P-N junction transistors, but can be modified easily for the P-N-P type by incorporating a switch to reverse the battery, the meter connections, and the d-c blocking electrolytic capacitors.

Eq. (4-21)

Eq. (4-22)