Buffer to protect the square wave generator. Scheme, description

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Buffer to protect the square wave generator. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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When conducting laboratory research, teaching students, developing new products, testing long cables, capacitive and inductive loads, there were cases of failure of rectangular unipolar pulse generators caused by output short circuits and its connections to high voltage sources. After the generators began to be connected through buffer amplifiers, it was they who were damaged by human errors, and the generators remained serviceable. This saved a lot of money.

Buffer to protect the square wave generator
Fig. 1

Often, not only a direct, but also an inverse output is required. The buffer amplifier, the circuit of which is shown in fig. 1 provides such an opportunity. It contains two identical blocks A1 and A2, a resistor R2, protective diodes VD1 and VD2, an on indicator - an HL1 LED with a current-limiting resistor R1.

The input signal through resistor R2 is fed to the input (pin 3) of block A1 - the parallel-connected inputs of all inverters of the 1DD1 chip. Although the diodes VD1 and VD2 are connected in parallel with the built-in protective diodes inside the 1DD1 microcircuit, according to the author, additional protection will not hurt. The inverter outputs of the 1DD1 chip are connected in parallel through current-limiting resistors 1R1 - 1R8. At the point of their connection, an inverse signal is formed, which is fed to the output (pin 4) of block A1 through the resistor 1R13 to another output (pin 5) and to the top output of the variable resistor 1R9 according to the circuit.

The 1R9 engine is connected to a voltage divider 1R10-1R12. Variable resistor 1R9 serves as a voltage regulator on the inverted outputs 1-3. Resistors 1R13 and 1R14 are current limiting. Diodes 1VD1 and 1VD2 also perform a protective function.

The output signal of block A1 from its pin 4 is fed to the input (pin 3) of a similar block A2, which produces similar output signals at pins 5-8. Since each block inverts the signal, the signals at the outputs of block A1 are inverse with respect to the input, and at the outputs of block A2, as a result of double inversion, signals are obtained that repeat the input.

The maximum operating frequency of the buffer amplifier depends on the chips used. For M74HC540 it is at least 25 MHz, for 74ALS540 - 50 MHz, for 74F540 - 100 MHz. The maximum output current is about eight times that for a single cell. The resistance of the current-limiting resistors 1R1 - 1R8 can be reduced to 10 ohms or increased to 47 ohms if necessary.

Rice. 2 (click to enlarge)

Separate adjustment of the amplitude of direct and inverse pulses may be inconvenient for some measurements. In order to carry out a joint adjustment of the amplitude, an adjustable supply voltage is applied. It is generated by the block, the scheme of which is shown in Fig. 2. The mains voltage through the SA1 switch and the FU1 fusible link is supplied to the primary winding (I) of the T1 mains transformer. An alternating voltage of 10 ... 12 V from the secondary (II) winding of the transformer T1 rectifies the diode bridge VD1-VD4 and smoothes the capacitor C2. A voltage stabilizer is assembled on the DA1 microcircuit, which is included according to a typical circuit, which can be adjusted by a variable resistor R3 within 2.6 V. The output voltage Uout is calculated by the formula

Uout \u1d (3 + (R4 + R2) / R1,25) -XNUMX V.

It is undesirable to change the resistance of the resistor R2, since it provides a minimum microcircuit load current of 10 mA. The lower control limit is set by selecting the resistor R4, the control interval - by selecting R3.

When conducting student laboratory work, the resistance R3 was chosen to be 510 ohms, and the resistor R4 was replaced by a jumper in order to obtain voltage regulation limits of 1,25 ... 6,6V. The operation of microcircuits at a low supply voltage was studied. For microcircuits 74ALS540 and 74F540, the interval is set to 4,5.6,5 V. In this case, the resistance of the resistor R2 is increased to 320 ohms, and R4 to 840 ohms.

In the absence of a variable resistor R3 of the desired nominal resistance, a larger resistance is selected and shunted with a constant resistor, the resistance of which is selected experimentally.

The AL360B (HL1) LED was selected with a low forward voltage so that its glow was noticeable even at a minimum supply voltage of 2 V. If there is no suitable replacement for another LED, a miniature incandescent lamp SMN-6,3-20 can be used instead, replacing resistor R1 with a jumper.

All fixed resistors - MLT, variables - SP-1A with a rated power of 1 W. Oxide capacitors - aluminum, non-polar capacitors - film or ceramic. In the buffer amplifier (see Fig. 1), diodes 1 N4148 can be replaced with 1N914 or other similar ones. In the power supply (Fig. 2), instead of diodes 1N4001 (VD1-VD6), you can use 1N4002-1N4007 or KD243A-KD243Zh. The voltage regulator chip LM317T (DA1) is replaceable by KR142EN12A. If the chip dissipates more than 1 W of power, it must be installed on a heat sink with a cooling surface area of approximately 20 cm2 per 1 W of dissipated power. Network transformer T1 - any with an overall power of 10.20 W and a secondary winding voltage of 10.12 V. You can use a transformer from lamp equipment by connecting two filament windings in phase in series.

Author: P. Petrov

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