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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Measurement of parameters of field-effect transistors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology The device for checking the main parameters of low-power field-effect transistors is based on inexpensive digital multimeters, possibly even with faulty measurement limit switches. This minimized labor costs for installation and fabrication of the structure. Digital readings make it somewhat easier to compare transistors and select pairs for differential stages. The steepness of transistors is determined by the simplest calculation. By the nature of my work, I often have to repair instrumentation with field-effect transistors. They are used in modulators, input stages of amplifiers in oscilloscopes and digital voltmeters, switching devices, etc. For example, about 7 KP38 series transistors are installed in the V30-301 voltmeter. These transistors are very sensitive to static electricity, and the slightest non-compliance with the installation technology leads to their failure. Most device malfunctions that are associated with the failure of field-effect transistors can be eliminated by a simple replacement, but if transistors are used in differential or "symmetrical" cascades, they must be selected according to the main parameters.
The main parameters of field-effect transistors include the initial drain current, cutoff voltage, and slope. It is possible to determine them, and therefore, to make a decision on the suitability of a field-effect transistor for use, using a device whose circuit is shown in Fig. 1. By changing the gate voltage and controlling the drain current, you can find out all three basic parameters. For transistors with p-n junction gate or insulated gate and built-in channel, the initial drain current ISnat is the drain current at zero gate voltage. The cutoff voltage U3uots is the gate voltage at which the drain current reaches a value close to zero. The slope of the characteristic is defined as the ratio of the change in drain current ΔIC (mA) to the change in voltage between the gate and source ΔUzi (V) that caused it: not difficult. The slope S of a field-effect transistor with a control p-n junction depends on the gate voltage U3i and has a maximum value Smax at a gate voltage of zero. If the values of the initial drain current ISnach and the cutoff voltage U3uots are measured. the steepness can be approximately estimated by the formulas: Smax \u2d XNUMXIsnach / Uziots S = √Isnach Ic/Uziots where voltage is in volts, current is in milliamps, slope is in mA/V [1]. For insulated gate transistors, the slope at drain current Ic and voltage Uzi can be calculated by the formula S = 2Ic/|Uzi - Uziots| where UZIots - cutoff voltage or threshold voltage (for transistors with an induced gate). On the basis of the layout of this device, a device was made for the operational measurement of the main parameters of field-effect transistors and monitoring their performance. Technical specifications Measured gate voltage, V ..............-12...+12
The device has protection of the tested transistor from damage. The meter circuit is shown in fig. 2. To change the gate voltage of the transistor, a variable resistor R2 is used, connected to a bipolar 2x12 V power supply, which makes it possible to obtain the characteristic of the slope of any low-power field-effect transistor with both n-channel and p-channel. Resistor R3 is needed to limit the gate current. The polarity of the voltage on the drain is changed by the switch SB1. To avoid overloading the milliammeter, a current limiter was used on the transistor VT1 and resistor R1. The limitation occurs at 25 mA because the maximum measurable current is set to 20 mA. Diode bridge VD1 provides the action of the limiter in any direction of the drain current. Relays K1 and K2 prevent the failure of the measured field-effect transistor from static electricity: until the "Measurement" button SB2 is pressed, the relay winding is turned off, and the contacts for connecting the transistor are closed to each other and to a common wire. During the measurement, the button is pressed and the transistor is connected to the measuring circuits through the relay contacts. The HL1 LED indicates that the measurement process is in progress. The main part of the device - RA1 milliammeter and PV1 voltmeter - is assembled from ready-made M890D multimeter assemblies. The basis of these multimeters is the well-known ICL7106 chip. These instruments were chosen for their convenient large housing to reduce labor costs in the manufacture of the parameter meter. The power supply of the analog-to-digital converter (ADC) of the multimeter is from a bipolar +5 / -5 V power supply, which is necessary for the ADC microcircuits and other parts of the device. The ADC chip has such an opportunity if the multimeter is modified as shown in the circuit fragment in Fig. 3 (numbering of elements is conditional).
In the main switch used with battery power, pins 30,32, 35 and 30 are connected together. With bipolar power, pin 30 (the low-level ADC circuit) is disconnected from this point. In this case, the microcircuit measures the potential difference between pins 31 and 2, while the ADC input is decoupled from the power circuits. The only condition is that the voltage in any of the measuring circuits must not exceed the ADC supply voltage relative to the common wire. Such a refinement is described in [XNUMX]. With minimal alterations, the microcircuit provides voltage measurement up to 200 mV without dividers. To build a voltmeter with a limit of 20 V, necessary to measure the gate voltage, a 1:100 divider consisting of resistors R5 and R6 was used. To build a milliammeter with a measurement limit of 20 mA, resistor R7 is used. At a current of 20 mA, a voltage of 200 mV drops across it, which is measured by the ADC. A milliammeter is installed in the source circuit and measures the current of the transistor. This decision is dictated by the impossibility of measuring the current in the drain circuit, because at the measuring terminals of the milliammeter there may be a voltage that exceeds the supply voltage for the ADC chip. The voltmeter is connected between the gate and the source, therefore, a current with a maximum value of not more than 5 μA will flow through the R6R12 divider, which will cause an error in the milliammeter readings of one least significant digit, which turns out to be insignificant. The scheme of the power supply unit of the device is shown in fig. four.
To lower the mains voltage to 12 V, a transformer T1 is used. Further, the alternating voltage is rectified by the diode bridge VD1 and filtered by capacitors C1, C2. Bipolar voltage stabilizers + 12 / -12V are microcircuits DA1, DA2. Bipolar voltage +5 / -5 V stabilizes the DA3 and DA4 microcircuits. The stabilizers are connected in series to reduce the voltage drop across the stabilizers DA3 and DA4. The scheme of a bipolar power supply can be any other; it is even possible to use autonomous power supply, for example, from Korund batteries. To do this, you will need to add a battery voltage converter to the one necessary to power the rest of the meter.
Details and design. The following parts can be used in the device. Resistors R5-R7 - C2-29 or others with a tolerance of not more than ± 0,5%, although the ratings may differ from those indicated in the diagram; the main thing is the stability of resistance. The remaining resistors are any, for example MLT0.125. Variable resistor R2 - multi-turn, for example, RP1-53 or designed for precision adjustment (according to a coarse circuit) - SP5-35, SP5-40. If you can’t find one, resistors R2 and R3 can be replaced with an analogue - a node of two variables and two constant resistors, as is done in my design. The diagram of such a node is shown in Fig. 5. Resistor R1 sets the voltage coarsely, and R2 precisely. The LED can be replaced by others, for example, from the AL 102, AL307, KIPD series, better than the red color of the glow. Diode bridges - KTs407 with any letter, instead of them, you can use separate silicon diodes with a permissible average current of at least 200 mA in the rectifier and 100 mA in the current limiter. To simplify the design, integrated stabilizer microcircuits 7812, 7912, 7805 and 7905 are used, the domestic analogues of which are KR142EN8B, KR1162EN12A, KR142EN5A and KR1162EN5A, respectively. Relay - RES60 (version RS4.569.435-07) or similar with two contact groups for switching. Network transformer T1 - any one that provides output voltages of 2x 15 V and a current of at least 100 mA, it can be taken from a network adapter with a power of at least 6 watts. The secondary winding of such a transformer is rewound to obtain the desired bipolar voltage. The transformer and rectifier are placed in the adapter case, and the stabilizer elements are located in the device case. The device is connected to the adapter with a three-wire cable. The entire meter is assembled in the case of one of the multimeters. During the manufacture of the device, the multimeters were opened and, after removing unnecessary parts of the boards, they were combined in one case, as shown in Fig. 6.
Extra parts - divider resistors, switch, etc. - are removed (therefore, the reason for the manufacture of such a device may be a fatal defect in the switch of such a multimeter). They leave only part of the board with the ICL7106 chip, indicator, "strapping" elements of the chip and indicator, and power buttons that will act as switches SB1, SB2. The printed conductors leading to these switches must be cut. The bottom cover of the multimeter is not subjected to processing, and the top will have to be finalized. For one device, the lid is cut off so that only the part with the indicator and the button remains. At the second, the middle is cut out where the limit switch is located, and the cut-out part of the design of the first device is glued to this place. When cutting parts from the top covers, the racks are kept, into which self-tapping screws are screwed, fastening the top and bottom covers. Above, near the button, a resistor is attached that regulates the voltage at the gate. From below, a connector for connecting field-effect transistors is installed. A collet panel for microcircuits was used as a connector. The middle of the panel is cut out, and a number of contacts are glued together. The choice of a collet panel is due to its high wear resistance. In my design, a small board made of foil textolite was used, on which a panel, an LED and a relay are installed. In turn, the board is attached to the front panel with two screws. Extra holes on the front panel are sealed with a cut-to-size plastic or electric cardboard plate, on which the overlay printed on the printer is glued, its appearance is shown in Fig. 7.
Most transistors have a cylindrical body with a key label to identify the pins. The contacts of the connector for connecting field-effect transistors are interconnected according to their purpose in such a way that each type of transistor has its own place without the need to specify the pinout. In the proposed version, the transistors are installed with the key up. Connections of a separate output of the transistor case to the source, and the second gate of transistors of the KP306, KP350 series - to the drain are provided through the connector with jumpers between the corresponding sockets. The appearance of the finished device is shown in Fig. eight.
Before turning on the device for the first time, it is necessary to check the output voltage values of the stabilizer. Setting up the device consists in setting the current limiter and setting the exemplary voltages of the milliammeter and voltmeter. To set the limiter, you need to connect an exemplary milliammeter between the contacts "C" and "I" of the connector for connecting the measured transistor, press the "Measurement" button and select the resistor R1, achieving readings of 25 ... 30 mA. You can pre-select the transistor according to the current limiting parameter, then the resistor R1 is replaced with a jumper. Next, an exemplary milliammeter is connected in series with a variable resistor to the same contacts, a current of 10 mA is set, and the exemplary voltage setting resistor achieves the same readings of the milliammeter of the device. To adjust the voltmeter, an exemplary voltmeter is connected to the terminals "3" and "AND", the gate voltage is set to 10 V with the device's resistor, and the same readings are set with the device's voltmeter adjustment resistor. Due to the fact that FETs can be damaged by static electricity, the following procedure for operating the instrument may be recommended. Before connecting, all the outputs of the field effect transistor should be closed with a wire jumper between them. The type of channel conductivity is set on the device (n- or p-channel), the "Measurement" button is pressed. The field effect transistor is connected to its socket, the jumper is removed from the terminals, the "Measurement" button is pressed and its parameters are controlled. After measuring, press the button, close the transistor leads to each other and remove the transistor from the socket. With the help of the device it is easy to diagnose any kind of malfunction of field-effect transistors. As practice has shown, most transistor malfunctions come down to a large gate leakage current, a breakdown or open channel, or an internal break in one of the terminals. If, when you press the "Measurement" button, the voltage at the gate decreases compared to the set value, then there is a leakage of current from the gate. The milliammeter reading will not be zero at any gate voltage. In all other cases, the inability to measure the initial drain current and cutoff voltage indicates a malfunction of the measured semiconductor device. Literature
Author: V. Andryushkevich, Tula; Publication: radioradar.net
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