Prefix to the frequency meter for testing transistors. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The described device implements an interesting method for measuring the current gain of a transistor at a fixed collector current, which is important when selecting transistors for symmetrical cascades. Unlike simple meters of the small-signal parameter h21e described earlier, this device is direct-reading.

The prefix to the frequency meter allows you to check the performance of low-power bipolar transistors in the amplifying mode and measure the base current transfer coefficient in the small signal mode for a circuit with a common emitter - h21E. The measurement is carried out at a fixed collector current of 1 mA.

The electronic components of the set-top box operate in such a way that the pulse frequency at its output is proportional to the value of the h21E parameter. The gain measurement is as follows. The transistor outputs are installed in the sockets "E", "B", "C" of the console and turn on the power. A frequency meter is connected to the output of the device, set to a measurement limit of 10 kHz. In this case, the readings of the frequency counter, divided by 10, correspond to the value of the parameter h21Э.

The prefix (Fig. 1) contains a voltage comparator and an integrator, to the output of which the tested transistor is connected in the switching circuit with OE. All these components are connected in cascade into a ring and form a system for automatic control of the collector current of the device under test. The output voltage of the comparator controls the integrator so that the collector current of the transistor under test changes towards its nominal value - 1 mA. To maintain a continuous periodic oscillatory process in the control system, the comparator has a dead zone. The width of this zone determines the range of oscillations of the collector current of the transistor under test.

(click to enlarge)

The comparator is made on the operational amplifier DA2, for which the divider of the resistors R8, R9 creates an exemplary voltage. A positive feedback signal is introduced into the divider circuit through the chain R11, R10. The ratio of the resistances of the resistors R11 and R10 determines the width of the dead zone of the comparator (hysteresis). In the set-top box circuit, it is 100 mV.

The integrator is assembled on the OS DA1. The divider R1R2 creates a voltage at the non-inverting input of the op-amp, symmetrical with respect to the limits of the output voltage of the comparator, which have two values: upper - 10 ... 11,5 V and lower - 0,5 ... 1,5 V. To create a current source mode in the input circuit of the transistor under test, resistor R4 is connected, the resistance of which (300 kΩ) is many times higher than the input resistance of the transistor in the circuit with OE.

Elements R5-R7, C5, C6 create the necessary mode for measuring the parameter h21E. Resistors R5 and R7 determine the collector current (1 mA), resistor R6 determines the collector-emitter voltage.

The prefix works as follows. The base current of the transistor under test is constantly changing, linearly increasing or decreasing, since the integrator input receives either a positive or negative integration voltage relative to the midpoint of the divider R1R2, which changes the direction of integration.

Suppose, at some point, the base current of the transistor under test increases. The collector current also increases, but at the same time it is h21E times higher than the base current. Upon reaching the collector current of 1,1 mA, the comparator is triggered, which changes the direction of integration. The base current, and hence the collector current of the transistor under test, begins to decrease. But when it reaches a value of 0,9 mA, the comparator will again be triggered and the process will go into a phase similar to the original one. Since the rate of change of the base current in the circuit is constant, the changes in the collector current are directly proportional to the parameter h21E of the transistor under test. Therefore, the value of h21E determines the time interval between the moments when the collector current reaches values ​​of 0,9 and 1,1 mA, at which the comparator is triggered. Thus, the frequency of operation of the comparator is directly proportional to the value of the parameter h21E.

A slight deviation in the proportionality of the parameter to the frequency of self-oscillations is associated with a delay in the switching of the comparator and integrator, as well as a branch of the base current of the tested transistor for recharging the capacitances of p-n junctions and mounting. In amateur radio practice, the influence of these factors on the measurement accuracy turns out to be quite acceptable when the set-top box operates at frequencies of 200 ... 5000 Hz, corresponding to the range of h21E values ​​in the range of 40 ... 1000.

A frequency doubler is assembled on the elements DD1.1-DD1.4, therefore the output frequency of the set-top box is 10 times higher than the h21E value, which greatly simplifies the reading of the h21E value on the frequency meter scale.

Parallel connection of elements DD1.2 and DD1.3 increases the load capacity of the device. Resistor R17 protects the output of the set-top box from a short circuit. The output impedance of the set-top box is about 3 kOhm. The range of the output signal of the set-top box without load is about 11 V.

To power the set-top box, only a stabilized voltage source of 12 ... 13 V is needed, providing a current of 10 mA and a voltage ripple of not more than 10 mV.

The author uses a VR-11A multimeter as a frequency counter.

Details. In the device, you can use any resistors with a power of 0,125-0,5 W, for example, MLT, OMLT. It is permissible that the resistors R12-R17 have a deviation from the nominal value of not more than ± 20%, the rest - ± 5%. Resistors R1 and R3 will have to be selected when adjusting the console. Oxide capacitors - K50-16, K50-35 for an operating voltage of at least 15 V. Capacitors C3, C7, C8 - ceramic KM-5 or KM-6 groups H30-H90. Capacitor C2 - metal film, for example, K73-16 or K73-17. Any low-current switch or toggle switch can be used as the SB1 switch, P2K, PT2-1-1 are suitable. Chip K140UD6 will replace K140UD8A or similar. It is permissible to replace the K561LA7 chip with an analogue from other series - K176LA7 or K1561LA7.

On fig. 2 shows a drawing of the printed circuit board and the placement of parts. The terminal lugs of the "+" power leads are rigidly soldered to the board and with which it is fixed directly to the output terminals of the power supply. The design of the board may be different.

Briefly about setting up the console. After checking the correctness of the installation, the power source, the frequency meter and the transistor under test are connected, preferably with the parameter h21E measured in advance on an industrial device (it should not be confused with h21E, although their values ​​\u5b\u2bare practically the same in many cases). Observing on the oscilloscope screen the signal at the output of the comparator (pin 1 of the DA3 microcircuit), the resistor R21 is selected, achieving symmetry of both half-cycles of the signal (meander). Then, by selecting the resistor RXNUMX, the frequency meter readings are set corresponding to the value of the hXNUMXE parameter of the transistor under test.

If it is not possible to use a reference transistor, you can do this. Before installing parts on the board, measure the resistance of resistors R4 and R7 with an accuracy of three signs. Then, between the "+" and "-" terminals of the power source, turn on a variable resistor with a resistance of 22 ... 47 kOhm, to the engine of which connect one of the R4 terminals, and connect the other to the "B" socket of the set-top box. Install resistor R7 on the board. Install the transistor under test, for example, KT315G, in which the h21E value is in the range of 50 ... 300. Set the variable resistor slider to the middle position and turn on the power. By rotating the slider, set the voltage across the resistor R6 to 1,5 V, which will correspond to a collector current of 1 mA. Through a capacitor with a capacity of 1 ... 3 μF, apply a sinusoidal signal with a frequency of 1000 Hz (Uc) to the variable resistor engine. By smoothly increasing the amplitude of the applied signal Uc, set the signal voltage at the collector of the transistor under test equal to 100 mV. Using the formula h21E - 0,1R4 / UCR7, calculate the value h21E of the transistor under test. For example, the signal voltage on the variable resistor engine Uc \u0,95d 4 V, R309 \u7d 517 kOhm, R21 \u0,1d 309 Ohm, then h0,950,517E \u62,9d XNUMX-XNUMX / XNUMX \uXNUMXd XNUMX.

Having restored the original connections, by selecting R1 to achieve a meander at the output of the comparator, and then by selecting the resistor R3 set the corresponding frequency meter reading, which for our example is 629 Hz. This completes the set-top box setup.

Other op-amps without internal correction are also suitable for the comparator: K553UD1, KR544UD2, and also K157UD2, in which the second op-amp with a 30 pF correction capacitor can be used in the integrator. True, in this case, the layout of the board will have to be done differently.

Author: S. Permyakov, Sergiev Posad, Moscow region.

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