Working Principle of Transistor Characteristic Tracer





The working principle of the instrument is introduced below by taking the testing of "diode volt-ampere characteristics" and "transistor output characteristics" as examples.





1. Testing Principle of Diode Volt-Ampere Characteristics





The functional relationship between the current I flowing through a diode and the voltage U across the diode is called the "diode volt-ampere characteristic." This instrument quantitatively displays the "volt-ampere characteristic" of the diode under test by showing the "volt-ampere characteristic curve." From the diode volt-ampere characteristic curve (forward region), it can be seen that when the voltage U across the diode is gradually increased from 0, the current I in the diode will gradually increase according to the "diode equation."





In the diode equation: at an ambient temperature of 300K, UT≈26mV.





Repeating this process is called "voltage scanning." According to the quadrant where the characteristic curve is located, use the "shift" knobs of the "X-axis function" and "Y-axis function" of this instrument to adjust the origin of the scan to the lower-left or upper-right corner of the oscilloscope screen. When measuring the forward characteristic curve of a diode, since the curve is in the first quadrant, the origin should be adjusted to the lower-left corner of the screen. (For the reverse characteristic curve in the third quadrant, the origin should be adjusted to the upper-right corner, and the scan voltage polarity should be selected as "-".) The voltage U across the diode is amplified by the "X-axis amplifier" to control the movement of the oscilloscope spot in the X-axis direction. When the voltage gradually increases from 0, the spot moves horizontally to the right from the leftmost origin, and the length of the trace is proportional to the voltage value. At the same time, the current I flowing through the diode (converted to voltage) is amplified by the "Y-axis amplifier" to control the movement of the oscilloscope spot in the Y-axis direction. When the current gradually increases from 0, the spot moves vertically upward from the bottom origin, and the length of the trace is proportional to the current. The combined effect causes the oscilloscope spot to display the volt-ampere characteristic curve of the diode on the screen, and the voltage and current data can be quantitatively read according to the scale on the oscilloscope tube.





When testing the diode volt-ampere characteristic curve, the working principle of the instrument is shown in Figure 1.







Diode Test Circuit Principle





(1) Turn the "test selection" switch to the middle position ("off"), insert the diode under test into the "E" and "C" jacks on the left side of the test bench, and the diode is not powered at this time; after adjusting other options, turn the "test selection" to the "transistor A" side for powered measurement.





(2) When testing a diode, the base "step signal" does not work. The voltage applied to the diode under test is provided by the "collector scan signal" unit.





The "collector scan signal" unit outputs a pulsating DC voltage with a frequency of 100Hz, with a waveform of positive voltage or negative voltage, controlled by the "polarity" knob, which can be selected as "+" or "-"; the peak voltage is selected by the "peak voltage range," which can be "0-20V" or "0-200V," and then finely adjusted by the "peak voltage" knob to produce any value within the above range. Note: For measuring semiconductor devices, generally select "0-20V," while "0-200V" is used to test the reverse breakdown voltage of the device. The "power dissipation limiting resistor" is connected in series in the circuit for protection, to avoid excessive current flowing through the device under test.





When measuring a diode, adjust the control knobs of the "collector scan unit" of the JT-1 so that the "polarity" is "+", the "peak voltage range" is "0-20V", and the "peak voltage" is first turned to "0", and then increased to the required value during formal measurement. The "power dissipation limiting resistor" can be selected as a few ohms or tens of ohms when measuring high-current diodes, and tens of ohms to several kilohms for low-current tubes.





(3) The "X-axis function" is used to select the measurement object of the X-axis amplifier and the amplification factor of the X-axis amplifier. When switched to "collector voltage" 0.1 "volt/div", the input measurement object is the voltage between "C" and "E" on the test bench - that is, the voltage U across the diode, and each division in the X-axis direction represents 0.1 volt.





(4) The "Y-axis function" is used to select the measurement object of the Y-axis amplifier and the amplification factor of the Y-axis amplifier. When switched to "collector current" 1 "mA/div", the input measurement object is the voltage across the "current sampling resistor" - corresponding to the current flowing through the diode, and each division in the Y-axis direction represents 1 mA. The "current sampling resistor" is used to convert the collector current I into the voltage required by the oscilloscope.





(5) After setting the measurement conditions as described above, turn the "test selection" switch on the test bench to the "transistor A" side, and gradually increase the peak voltage from 0, then the forward region of the diode volt-ampere characteristic curve can be observed; if you want to observe the reverse region curve of the diode, you can insert the diode in reverse or turn the "polarity" of the "collector scan signal" to "-". And move the scan origin to the upper-right corner.





2. Measurement of Transistor Output Characteristics





The "transistor output characteristic curve" shown in Figure 3 can describe the collector output characteristics of a transistor. From the output characteristic curve of the transistor, the current amplification factor β, saturation voltage drop UCES, collector output resistance rce, and collector breakdown voltage BVCE0 can be measured, so it is very useful.





When testing the output characteristics of a transistor, first keep the base current IB of the transistor under test at a fixed value (for example, 20μA), use the collector voltage UCE as a variable, gradually increase from 0, record the collector current IC point by point, plot the relationship curve between UCE and IC, and obtain an output characteristic curve under this IB condition (in this example, IB=20μA); then change the value of IB, repeat the above operation to plot the output characteristic curve under another IB condition. Since the UCE-IC curve is plotted with IB as the condition, IB is called the "parameter variable."







Transistor Output Characteristic Curve