The transistor curve tracer circuit (Fig. 3) comprises power supply, ramp and clock generator, ramp buffer and offset null, current-to-voltage converter, counter, base current control, and switching sections.
1. The power supply section. The circuit operates on ±12V regulated power supply. The input AC mains supply is stepped down by transformer X1 to deliver a secondary supply of 15-0-15V AC at 1 ampere. The output of the transformer is rectified by a bridge rectifier. The 1000µF,
35V capacitors act as filters to eliminate ripples and provide unregulated DC out-put voltage.
The unregulated dual DC voltage is converted by three-terminal ICs AN7812 and AN7912 into ±12V regulated power supply. (Note. Connect 0.1µF decoupling capacitors between the supply terminals and ground of every IC in order to press unwanted noise signals in the sup-ply voltage.)
2. The ramp and clock generator section. The ramp and clock generator uses a constant current source (LM334) and a capacitor, in conjunction with timer NE555 (IC3) wired as an astable multi-vibrator, to generate a linear ramp. The control terminal of timer 555 (pin 5) is held at a reference voltage of 5 volts by a zener diode so that the upper threshold (VUTP) is at 5 volts and the lower threshold (VLTP) at 2.5 volts.
The output current from IC LM334 can be controlled with the help of poten-tiometer VR1. This current charges the capacitor linearly in the form of a linear ramp. As soon as the voltage across the capacitor exceeds the upper threshold volt-Fig. 3: Circuit diagram of transistor curve tracer
age (VUTP), the output of timer 555 changes its state and goes low. This activates the discharge terminal (pin 7) of timer 555 and hence the capacitor quickly discharges through the timer.
As the voltage across the capacitor drops below the lower threshold voltage (VLTP), the output of timer 555 changes its state and goes high to disable the dis-charge terminal and further discharging of capacitor stops. Once again the capacitor gets charged linearly through the constant current source and the sequence repeats.
Thus the potential across the capacitor is a positive linear ramp between 2.5 volts and 5 volts. The ramp frequency can be controlled by varying the charging current using potentiometer VR1. (EFY Lab note.
During lab testing, we used AD590 tem-perature transducer in place of LM334H as the constant current source, and the method of using the same is shown in Fig.
3 within dotted lines.)
3. The ramp buffer and offset null section. Since the output impedance of the ramp source is very high, we cannot load it. Also, a DC offset voltage equal to the lower threshold voltage (VLPT = 2.5V) is present in the ramp output. In order to nullify the offset voltage of the ramp and to source the current from the ramp, use a buffer amplifier. An op-amp in non-inverting amplifier configuration is used to achieve this function.
As the input impedance of the non-inverting amplifier is very high, it will not load the ramp source. Also, it is possible to nullify the DC offset voltage present in the ramp output with the help of ramp offset adjustment preset VR2.
By adjusting feedback preset VR3, the output of ramp buffer can be set to deliver a linear 0-5V ramp. This output is used as VCE for the transistor under test to source the collector current (IC).
To draw the input characteristics of the transistor, the base-emitter voltage (VBE) should be varied linearly. For this we require a linear 0-1V ramp with sufficient current sourcing capability. In order to achieve this, a ramp attenuator (voltage divider) and an amplifier are used.
The 0-5V ramp output of ramp buffer is attenuated by the potential divider network (comprising resistors R4 and R5) followed by an op-amp (IC5) connected in non-inverting configuration. The gain of the op-amp can be adjusted using preset VR5 connected in the feedback path.
In order to nullify the offset voltage of Fig. 4: Actual-size, single-side PCB layout for transistor curver tracer
Fig. 5: Component layout for the PCB
the op-amp, balancing preset VR4 is con-nected between the offset null terminals of the op-amp. The output of the op-amp is 0-1V linear ramp, which is used as the base-emitter voltage (VBE) for sourcing the base current (IB) of the transistor under test.
4. The current-to-voltage convert-er section. The spot on the CRO screen is deflected in proportion to the potential applied to its input. Hence in order to deflect the beam along y-axis, which is the current axis (collector current IC in
the transistor output characteristics and base current IB in the transistor input characteristics), the current component is to be converted into a proportional voltage.
The current to be measured is passed through series resistor R7 of 10-ohm,
±1% MFR (metal film resistor). Potential drop Vout across the resistor, according to the Ohm’s law, is proportional to current I through it and is given by the following relationship:
V = IR
where Vout = 10xI
Hence, there is a potential drop of 10 mV per mA of the current through the circuit.
We cannot apply this small floating potential directly to the CRO for a significant deflection.
Therefore we use a differential amplifier to have an output voltage with respect to the ground that is proportional to the current though the circuit. The differential amplifier has a gain of 100 that can be fine-tuned with the help of gain adjust preset VR7 in the feed-back path.
The current-to-voltage converter converts the current of 1 mA into a potential difference of 1 volt that can be applied to the CRO to deflect the beam in vertical axis. In order to nullify the offset voltage of the op-amp, connect a balancing preset to the offset null terminals of the op-amp.
5. The counter section. The base cur-rent (IB) is to be changed in discrete steps for every ramp to enable the transistor’s output characteristics for various IB values simultaneously on the CRO screen.
In the counter circuit, the output of timer 555 (IC3) from pin 3 is a square wave that intimates the end of ramp. This output is used as clock pulse for the counter wired around CMOS binary/decade, up/down IC MC14029B or CD4029B (IC7).
IC7 is wired as a 3-bit binary up-counter so that the output of the up-counter (Q2, Q1, and Q0) is incremented by bi-nary 1 for every clock pulse. The count sequence is 000, 010, 011, 100, 101, 110, and 111, i.e. 0 through 7 decimal. After 111, the counter is automatically reset to 000, and once again the count sequence repeats. Hence we get eight discrete logic levels, and accordingly we can set the base current (IB) using a base current control circuit.
Similarly, to draw the input charac-teristics of the transistor under test for various collector-emitter voltage (VCE) values, the collector-emitter voltage (VCE) is to be changed for each ramp. The least significant bit (Q0) of the counter is used to toggle the collector-emitter voltage (VCE) from 0 volt to 10 volts. Thus we can view the input characteristics of the transistor for VCE= 0 volt to VCE= 10 volts simultane-ously on the screen of the CRO.
6. The base current control section. This section receives the input from the counter circuit and varies the base current (IB) of the transistor. The output of counter IC7 in series with a high-value resistor acts as the constant current source.
Fig. 6: Waveforms at various points in the circuit
The high-level outputs of the counter are fairly constant at 10 volts.
When we connect a resistor of 100 kilo-ohms in series with Q0 output of the counter, it supplies a constant current of 100 µA during its logic 1 state. Similarly, when we connect a resistor of 50 kilo-ohms (two 100 kilo-ohm resistors in parallel) in series with Q1 output of the counter, it supplies a constant current of 200 µA during its logic 1 state. Using 25-kilo-ohm resistor in series with Q2 output we can get a constant current source of 400 µA.
When more than one current source are connected in parallel, the result is similar to having a current source equal to the sum of individual source currents.
If we use the base current (IB) setting as it is for a transistor with large current amplification factor (α), its collector cur-rent (IC) gets saturated for much smaller values of IB and only two or three traces appear on the screen of the CRO. To get the maximum number of traces, reduce the base current by increasing the series resistor values through IB SET potentiom-eter VR8. With the help of VR8, we can adjust the base current in incremental steps from 10 µA to 100 µA.
(Note. Connect two 100-kilo-ohm re-sistors in parallel to get 50-kilo-ohm resis-tor. Similarly, connect four 100-kilo-ohm
resistors in parallel to have 25-kilo-ohm resistor. This method has been shown in Fig 3.)
7. The switching section. Certain circuits are common in tracing both the output characteristics and input charac-teristics. The ramp and clock generator, ramp buffer and amplifier, and counter circuits are retained at their places for both output and input characteristics. But to trace the output characteristics the cur-rent-to-voltage converter is to be connected in the collector of the transistor under test and to trace the input characteristics it is to the connected in the base of the transis-tor (refer Figs 1 and 2 for output and input characteristics, respectively).
To have minimum complexity, the collector and the base circuits of the transistor are switched suitably using a changeover switch on the front panel. The switching details are obvious from the circuit diagram in Fig. 3.
Construction
Wire the circuit on a 2.5mm, IC-type gen-eral-purpose printed circuit board (PCB) as shown in Fig. 3. The use of glass-epoxy PCB is recommended. An actual-size, single-side PCB for the circuit is shown in Fig. 4, with its component layout shown in Fig. 5.
Carefully solder all the components and use sockets for ICs. All range resis-tors used should be stable, close-tolerance type (preferably MFRs). Preferably use linear-type IB SET potentiometer and mount it on the front panel of the instru-ment. Enclose the circuit board, power transformer, and other circuit compo-nents in a metal box having approximate dimensions of 22x17x7.5 cm. Extend input and output leads to the correspond-ing points in the circuit. Terminate the outputs for connection to the CRO in BNC(F) connectors.
Calibration
After construction, check the circuit thoroughly for short circuits, breaks, and open circuits on the PCB. After switching on the instrument, let it warm up for a few minutes before commencing with the calibration.
Calibration procedure of the circuit is as follows:
1. Check and ensure ±12V regu-lated voltage with respect to ground.
2. Connect a CRO to shorted pins 2
and 6 of timer 555 (ramp output). A lin-ear ramp with positive slope is observed on the screen of the CRO. By adjusting frequency control potentiometer VR1, set the frequency of the ramp at 1 kHz (refer waveform 1 in Fig. 6).
3. Connect the CRO to the output of ramp buffer. Adjust preset VR2 to nul-lify the DC offset voltage in the output of ramp buffer. Adjust preset VR3 to set the amplitude of ramp output to 0 to 5 volts (refer waveform 2 in Fig. 6).
4. Connect CRO at the output of ramp attenuator and amplifier. Adjust preset VR4 to nullify the DC offset voltage in the output of ramp buffer. Adjust preset VR5 to set the amplitude of ramp output to 0 to 1 volt (refer waveform 3 in Fig. 6).
5. Calibrate the current-to-voltage converter by connecting a 1-kilo-ohm. 1%
metal film resistor between the collector and emitter terminals of the transistor under test. Connect the output of the current-to-voltage converter to a CRO.
By observing the ramp waveform on the screen of the CRO, nullify DC offset voltage using preset VR6 and adjust the amplitude of the observed ramp wave-form to 0-5 volts with the help of preset VR7. Calibrate the current-to-voltage converter to convert 1 mA of current into 1 volt (refer waveform 4 in Fig. 6). Then check the clock output by connecting the CRO to pin 3 of timer 555 (refer waveform 5 in Fig. 6).
6. Verify the outputs of the counter by using a dual-trace oscilloscope. Connect one input channel of the CRO with clock pulses at pin 3 of IC3 and the outputs at pins 6, 11, and 14 of counter IC7 to the other input of the CRO sequentially (refer waveforms 5, 6, 7, and 8 in Fig. 6).
7. Short-circuit the base-emitter ter-minals of the transistor under test. Select input/output characteristics switch S2 to output characteristics position and con-nect the CRO to the output of the current-to-voltage converter. By adjusting IB SET potentiometer VR8 on the front panel of the instrument, check proper operation of the base-current section by observing stair-case ramp of varying amplitude on the screen of the CRO (refer waveform 9 in Fig. 6).
operation
After calibration, the instrument is ready for use to trace the input and output characteristics of npn transistors. Follow the operating procedure given below every Fig. 7: Actual output curves on CRO (shown
without retrace)
Fig. 8: Actual input curves on CRO (shown without retrace)
time to get correct traces of input and out-put characteristics of the transistor:
1. Connect the x-axis and y-axis BNC pins of the transistor curve tracer to the
corresponding inputs of the CRO.
2. Plug in the AC cord of both the CRO and the transistor curve tracer and switch them on.
3. Set the CRO inputs to ground.
4. Allow warm-up time of at least 10 minutes for the circuit components to get stabilised.
5. Set the CRO for X-Y mode of opera-tion.
6. Adjust intensity and focus controls to get a sharp spot on the screen of the CRO.
7. Set the volts/div control of x-axis to 0.5 volt/div.
8. Set the volts/div scale of y-axis to 2 volts/div.
9. Adjust the position controls of the CRO to position the spot on the left bot-tom of the screen ((0,0) position in the graph).
10. Set the inputs for DC coupling to the CRO.
11. Connect the transistor whose characteristics are to be traced to the transistor curve tracer, ensuring correct pin configuration.
12. Set the selector switch for input/
output characteristics to the output char-acteristics position.
13. Release the CRO inputs from ground and switch them over to connect inputs.
14. Now view the output characteris-tics of the transistor. Fine-tune the IB set potentiometer to get eight traces on the screen of the CRO.
15. To trace input characteristics of the transistor, change the input/output characteristics selector switch to the input ParTs LisT op-amps can be used in place of µA741 with advantage)
IC7 - MC14029B/CD4039 binary/
decade up-/down-counter IC8 - LM334H/AD590
tempera-ture sensor
D1-D4 - 1N4007 rectifier diode ZD1 - 5V zener diode
Resistors (all ¼-watt, ±1% MFR, unless stated otherwise):
(A,B), R12(A-D) - 100-kilo-ohm VR1 - 1-kilo-ohm preset VR2 - 2.2-kilo-ohm preset VR3, VR4,
VR5, VR6 - 10-kilo-ohm preset VR7 - 150-kilo-ohm preset VR8 - 1-mega-ohm potmeter Capacitors:
C1-C4, C9 - 0.1µF ceramic disk C5, C6 - 1000µF, 35V electrolytic C7, C8 - 100µF, 25V electrolytic
16. Set the volts/div control of x-axis to 0.1 volt/div and observe the input charac-teristics likewise.
Figs 7 and 8 show a typical transis-tor’s output and input characteristics, respectively, on the CRO screen (without retrace).
Conclusion
To draw the characteristics of pnp tran-sistors, insert an inverter circuit in the ramp path of collector-emitter voltage VCE and base-emitter voltage VBE, and invert the output of the current-to-voltage converter.
By using a potential divider and buffer amplifier circuit in place of the base-current control circuit you can draw the characteristics of FETs and MOSFETs.
To trace the forward characteristics of diodes, connect the anode of the diode to the base terminal and the cathode to the emitter terminal. Set the transistor curve tracer to draw input characteristics, and the CRO screen displays the forward characteristics of the diode.
Similarly, with simple add-on circuits to the motherboard, you can draw the characteristics of UJTs, SCRs, TRIACs, etc.
Thin and faint retrace lines visible along with the characteristic traces can be removed by connecting a retrace blank-ing circuit to the Z-mod input of the CRO.
Almost all CROs exceeding 30MHz band-width have the Z-mod input facility.
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r.g. thiagraj kumar and s. ramaswamy