In the previous Part 4 of this series Build Your Own Curve Tracer, Part 4: TPA202 DVM and Supplies, the TPA202 circuit description was completed, describing what it is. In this Part 5, we look at how to use it and for what purposes.
The TPA202 is line-powered and has a rear power switch. Commercial power supplies can be substituted for the Power Supply of the circuit diagram. (For line-powered circuits, dangerous voltages are present and the usual safety precautions apply.)
The front-panel has the following controls:
Input Port Voltage (IVDR) – 10-turn pot that adjusts the INP voltage source. The INP range switch reduces by about ×10 the output voltage for more control resolution at low voltages (< 1 V).
Output Port Voltage (OVDR) – 10-turn pot that adjusts the OUT voltage source. The OUT polarity switch selects output voltage and current polarity.
DVM INP or OUT switch – selects either input or output port for DVM display.
DVM V or I switch – selects whether port voltage or current is displayed.
Input Port Current Sense Range – IISN ranges are selected. The range settings also select port source resistances.
Output Port Current Sense Range – OISN ranges are selected. The range settings also select port source resistances.
Input Port (INP) Output – BNC with COM ground outer conductor.
Output Port (OUT) Output – BNC with COM ground outer conductor.
LED analog output-port voltage indicator.
The front-panel outputs are the 3.3 decade LED display with sign (polarity), the three current unit LEDs of mA, μ A, and nA, and the analog output voltage LED indicator.
A simple transistor test fixture is constructed by connecting INP, OUT, and their COM terminals to the three transistor terminals of a transistor socket or 4-pin female header with connections ordered as EBCE. (For completeness, a 5-pin header with EBCEB is recommended.) Transistors with center base terminals are plugged into the left three pins as EBC; transistors with center collector terminals (such as some JFETs or MOSFETs) plug into the right three pins as BCE.
The OUT output of the TPA202 connects to the collector or drain (c ) and the INP output connects to the base or gate (b ). The outer-conductor grounds both connect to the emitter or source (e ) terminal which is also the common (COM) terminal. These various sockets can be mounted on a small box as shown below. (This particular TPA202 prototype is in a box instead of on rails and was found difficult to access in development – hence the change in packaging to a rail-mount assembly.)
To operate, power on, and before inserting the transistor or other DUT into the test fixture, select INP and OUT current ranges for the measurement. Insert DUT. Then select input port to set input conditions by setting the DVM INP to be displayed. Select current or voltage source. To set current or voltage, select the DVM input-port V or I display and adjust the front-panel input-port knob to the desired value.
Set the DVM display to output port voltage. Adjust the front-panel output voltage until the DVM output-port voltage display shows the desired voltage.
Several transistor parameters can be measured using the TPA202. Procedures for both bipolar junction transistor (BJT) and field-effect transistor (FET) measurements are given. Both JFETs and MOSFETs can be measured.
BJT Beta, β0
The first example of TPA202 operation is measurement of static BJT, β 0 , as follows. Set IISN to the 10 μ A range and OISN to the 10 mA range. Set the display to show output-port voltage (OVDR) by selecting DVM OUT and V switch settings. Adjust to set VCE at which β is to be measured, such as 10 V. Then set the display to input-port (INP) and current (I), which displays IISN. Adjust the input current for 10 μ A of input (base) current. Then switch the DVM display from INP to OUT. The number on the display is β 0 . The least-significant digit is 10 μA , the value of the base current. By measuring β 0 for different IB settings, a plot can be constructed of how β varies with current.
BJT Alpha, α0
Besides β 0 , α0 is derived directly from β 0 as
Alternatively, set IB , measure output current, IC and calculate IE = IB + IC . Then substitute IC and IE into the above equation for α 0 .
BJT and FET Transconductance, rm
BJT transconductance is a quasistatic (incremental or small-signal, low-frequency) parameter for which two measurements must be taken around a static (dc) operating-point (op-pt). It is defined as
Set the input current (II) to an op-pt value such as 100 μ A. Then display and record input voltage (IV) and output current (OI). Set II to a slightly different value and again note IV and OI. Use the above equation to calculate rm .
By changing the nomenclature in the above symbols (b → g , e → s , c → d ) the same procedure measures FET rm . For FETs, VG is typically larger than VBE and might require that the IVDR range switch be set to 10 V.
BJT Emitter Incremental Resistance, re
The BJT incremental input resistance referred to the emitter is re. It is useful in calculating the gain of BJT amplifier stages. It can be derived from rm and α0 as
Alternatively, re can be measured from the procedure for rm by the additional step of adding the two values for IE from the measured and set values of IC and IB .
BJT and FET Early Voltage, VA, and ro
The BJT T model has a resistance from collector to emitter that causes the i-v curves to slope, as shown below, where the BJT output resistance,
As transistor current increases, the slope increases (and ro decreases) so that the linearly-extended current curves meet at a single point along the –VCE axis at negative the Early voltage , VA .
To find VA , ro is found first, then VA is calculated from the above equation for ro . The ro procedure measures IC for two different values of VCE . Then around a given IC op-pt,
Set the desired IC (output-port current) op-pt by adjusting input-port current. Then adjust the TPA202 output voltage and measure output current again. Calculate
The same procedure (with corresponding nomenclature) applies to FETs.
The accommodation of ro into a JFET model that corresponds to the BJT T model is shown below. No gate current flows because the current through rm determines iD of the drain dependent current source to be the same, and all of iD becomes iS .
Another parameter that is associated more with electron tubes and JFETs than BJTs and MOSFETs is
The appearance of μ in a transistor model is shown in the conversion from the BJT T model with ro to an equivalent model with a collector dependent voltage source, μ x vbe , and series resistance ro . In practice, the model can readily be converted to a FET model (as shown previously) for which the above equation for μ applies.
One port is sufficient for measuring the voltage-current pairs of diodes. The resulting data can be used to infer series resistance and emission efficiency as n in kB x T/n x qe . On a log-log plot of BJT IC (VBE ), the slope of the curve is usually near one, but for diodes it is closer to n = 2. Deviations from a linear log-log plot (or exponential function) indicate series resistance. The input port offers greater voltage resolution for forward-bias measurements while the output port has greater voltage for determining breakdown voltage of low-voltage diodes or b-e junctions.
All four types of amplifiers (voltage, current, transconductance, transrersistance) can have their gain measured because all combinations of voltage and current at the ports is possible, with both voltage and current input drive. If the amplifier is externally powered, then minimum measurable input voltage is 10 mV for voltage and transconductance amplifiers, and current is 10 nA for current and transresistance amplifiers (such as shunt-feedback BJT stages). The output port is connected to the output amplifier as the DUT after setting the current range to minimum for maximum (1.00 M Ω ) series resistance (in ISN).
Then output voltage is measured, and the ratio of input to output calculated. The current of current-output amplifiers flows with opposite polarity in the output port but can be measured up to +/-100 mA. The voltage range for measurement is not limited to the +/-35 V maximum output of the TPA202 but can measure up to the DVM range of +/-100 V. (The ISN current limiting is important for this application.) Thus, the maximum voltage gain resolution of the TPA202 is 100 V/10 mV = 104 , and current gain resolution is 100 mA/10 nA = 107 . Maximum transconductance is 100 mA/10 mV = 1/0.1 Ω , and maximum transresistance is 100 V/10 nA = 10 G Ω .
This completes the series of articles on the TPA202. If you have further interest in it, you have some options. The first is to obtain the complete TPA202 Manual from www.innovatia.com by making a request through the ordering webpage at innovatia.com/Inquiry.htm. The manual is free via email upon request and is in PDF. Additionally, if you want to build a TPA202 but would rather have a circuit-board, place a tentative request for a board through the email address given above. With requests for at least three boards, I will lay one out and do a small run of boards. The price per board will be marked up around 10 % over my total expenses to cover my time and effort in mailing them out. There are no limited-edition TPA202 units.
Beyond building one, another more ambitious option is to make and sell the TPA202 as a supplier. There is nothing preventing you from doing so, subject to the open-source license. Innovatia expects no license fees or royalties on your TPA202 sales. I would like to see a line of standard-design (open-source) instruments supplied by both established companies and start-ups just as Linux is available from various suppliers.
Beyond the TPA202, there is the more interesting (and more complicated) TPA204, a TPA202 with ATmega8515 μC and serial port. Before that project is completed, however, the TPA242 is next in line. (See www.innovatia.com under Instruments for tentative specs.) An article series on it awaits project design completion, with a working prototype meeting specs. Limited-edition TPA242s might become available and the TPA242 is also open-source technology.