Hardware: Analog Discovery 2, serial number 210321A1A229, with Transistor Tester module PB200-413 REV B and solderless prototype board. Configuration : Scope 2 x 8 k, Wavegen 2 x 4 k, Logic 16 x 4 k, Patterns 16 x 1 k, Others -.
I have used the Tracer instrument in WaveForms for a few weeks with great success in testing diodes, BJTs, JFETs and MOSFETs.
I noticed strange curves while testing 2N3904, silicon NPN BJT. I used the « Ic/Vce (Ib) » in the «measure» menu that displays the collector current, Ic, versus the collector to emitter voltage, Vce, for various discrete base current values, Ib.
The curves start out normally at low Vce then rise to the almost horizontal part then around 2 V to 3 V Vce, they curve downward. This happens only for collector current above 4 mA or so. See figure 1, 2N3904_biz_rev_20.PNG. For some transistor samples of the same model, the down slope starts at 7 V of Vce for others it starts as low as 500 mV. Some curves may show a fall and then an upward rise at higher voltage. Normal curves should be horizontal until breakdown voltage is reached.
Other transistors, PN2222 (a 2N2222 in TO-92 package) and a MPSA06, did not show this strange curving. Neither did JFETs or MOSFETs.
Changing to other 2N3904 samples showed the same type of dropping curves but different from sample to sample. I also tried a 2SS9018, silicon NPN BJT, it also showed the dropping curve.
Probing with my finger near the collector and base wires I could bring the curves back to normal. Such action usually means there are oscillations in the circuit.
I have found two solutions that remove the oscillations and produce normal curves.
Solution A
I placed a small valued capacitor, 2,2 pF, between the BJT collector and base and the curves went back to normal. Figure 2, normal curves with Ccb 2,2 pF, 2N3904_biz_rev_22.PNG.
Solution B
I removed the capacitor and inserted a ferrite bead between the Transistor Tester collector connector and the collector of the transistor under test and the curves became normal. The ferrite I used is a 2,5 turns Bourns Inc. (or J. W. Miller Magnetics) FB20010-3B. It as 520 ohms at 10 MHz and 780 ohms at 100 MHz. The wire that loops into the ferrite holes looks to be 22 AWG or 24 AWG and the DC resistance is probably less than 20 milliohms. An inductor will not work. A ferrite bead adds resistance at high frequency and it is that resistance that prevents oscillations from happening. Figure 3, with ferrite bead in series with the collector connection, 2N3904_biz_rev_21.PNG.
Other semiconductor curve tracers also have ferrite beads in the connections to the transistor under test, such as the Heathkit IT-2131, and the Tektronix models 575 and 576.
I recommend solution B with the ferrite bead. It will eliminate oscillations with the Transistor Tester module and also for those who use the Tracer function with external resistors and direct connection to the Analog Discovery 2. I hope this helps other users with this problem.
Edited by D. Dufresne Figures out of order, move cursor over figures for figure number
Question
D. Dufresne
Setup configuration description
Computer: Acer Spin 5, SP513-53N-55FB.
Operating System: Microsoft Windows 10 Home, Version 10.0.19044 Build 19044.
Application software: WaveForms 3.18.1, instrument: Tracer, Transistor Tester module calibration: done.
Hardware: Analog Discovery 2, serial number 210321A1A229, with Transistor Tester module PB200-413 REV B and solderless prototype board. Configuration : Scope 2 x 8 k, Wavegen 2 x 4 k, Logic 16 x 4 k, Patterns 16 x 1 k, Others -.
I have used the Tracer instrument in WaveForms for a few weeks with great success in testing diodes, BJTs, JFETs and MOSFETs.
I noticed strange curves while testing 2N3904, silicon NPN BJT. I used the « Ic/Vce (Ib) » in the «measure» menu that displays the collector current, Ic, versus the collector to emitter voltage, Vce, for various discrete base current values, Ib.
The curves start out normally at low Vce then rise to the almost horizontal part then around 2 V to 3 V Vce, they curve downward. This happens only for collector current above 4 mA or so. See figure 1, 2N3904_biz_rev_20.PNG. For some transistor samples of the same model, the down slope starts at 7 V of Vce for others it starts as low as 500 mV. Some curves may show a fall and then an upward rise at higher voltage. Normal curves should be horizontal until breakdown voltage is reached.
Other transistors, PN2222 (a 2N2222 in TO-92 package) and a MPSA06, did not show this strange curving. Neither did JFETs or MOSFETs.
Changing to other 2N3904 samples showed the same type of dropping curves but different from sample to sample. I also tried a 2SS9018, silicon NPN BJT, it also showed the dropping curve.
Probing with my finger near the collector and base wires I could bring the curves back to normal. Such action usually means there are oscillations in the circuit.
I have found two solutions that remove the oscillations and produce normal curves.
Solution A
I placed a small valued capacitor, 2,2 pF, between the BJT collector and base and the curves went back to normal. Figure 2, normal curves with Ccb 2,2 pF, 2N3904_biz_rev_22.PNG.
Solution B
I removed the capacitor and inserted a ferrite bead between the Transistor Tester collector connector and the collector of the transistor under test and the curves became normal. The ferrite I used is a 2,5 turns Bourns Inc. (or J. W. Miller Magnetics) FB20010-3B. It as 520 ohms at 10 MHz and 780 ohms at 100 MHz. The wire that loops into the ferrite holes looks to be 22 AWG or 24 AWG and the DC resistance is probably less than 20 milliohms. An inductor will not work. A ferrite bead adds resistance at high frequency and it is that resistance that prevents oscillations from happening. Figure 3, with ferrite bead in series with the collector connection, 2N3904_biz_rev_21.PNG.
Other semiconductor curve tracers also have ferrite beads in the connections to the transistor under test, such as the Heathkit IT-2131, and the Tektronix models 575 and 576.
I recommend solution B with the ferrite bead. It will eliminate oscillations with the Transistor Tester module and also for those who use the Tracer function with external resistors and direct connection to the Analog Discovery 2. I hope this helps other users with this problem.
Edited by D. DufresneFigures out of order, move cursor over figures for figure number
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