On the switching voltage hysteresis of the Schmitt trigger of the K561TL1 microcircuit. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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Radio amateurs widely use the K561TL1 chip and its analogues, both domestic (K564TJ11, K1561TL1) and foreign (for example, CD4093), which contain four Schmitt triggers with an input logic function 2I-NOT. One of the features of the elements is that the change in the output voltage Uout occurs abruptly with a relatively slow change in the input to the threshold values Uin (switching from state 1 to state 0) and (switching from 0 to 1), as shown in Fig. 1.
Fig. 1
Fig. 2
Fig. 3
Such a transfer characteristic of the Schmitt trigger is provided by the presence of internal positive feedback in it. Thanks to it, these elements are convenient for generating signals with steep fronts and decays when a smoothly varying voltage is applied to their input.
The second feature is the presence of output voltage hysteresis during switching, which increases the noise immunity of the Schmitt trigger when the input signal contains significant noise.
The transfer characteristic shown in fig. 1, corresponds to the element of the K561TL1 microcircuit at a supply voltage Upit = 10 V. The difference between the threshold values of the switching voltage determines the width of the hysteresis loop Ur = Uin10 - Uin01, and the threshold values (and hence the loop width in volts) depend on the supply voltage Experiments with the node, the diagram of which is shown in fig. 2 showed that at the supply voltage Upit = 5 V Ur = 0,6 ... 0,8 V, at 10 V - 2 ... 2,8 V and at 15 V -3 ... 3,5 V
Schmitt triggers are often used as voltage comparators in various automation devices, for example, photo and thermal relays. In some practical cases, the width of the hysteresis loop may need to be reduced. This can be achieved by introducing a negative feedback between the input and output of the element (Fig. 3). Here Roc and RBX are the feedback resistors, R and C are the resistance of the signal source. Through the resistor Roc, a part of the output voltage will flow to the input of the element - the feedback voltage
If the output current of the element DD1.1 is small, we can assume that Uout \uXNUMXd Upit. As a result, the width of the hysteresis loop in the presence of feedback ugos decreases: Ug os \uXNUMXd Ur - K * Uoc, where K is a correction factor. In this case, the switching voltage Uin10 decreases, and Uin01 increases. With a selection of resistors Roc and RBX, you can set the desired value of Uroc.
However, it should be borne in mind that as the width of the voltage hysteresis loop decreases down to zero, the operation of the logic element near the switching moments becomes unstable and it can switch to the mode of generating high-frequency oscillations.
The experiments were carried out with several copies of the K561TL1 microcircuit at a supply voltage of 10 V. Thus, for Roc = 560 kOhm, Ris->0 and with an element with Ur = 2,8 V, the width of the hysteresis loop UCOc was measured with different resistors RBX. At RBX \u22d 2,5 kOhm, Ugoc turned out to be 43 V, at 2,15 kOhm - 150 V, at 0,8 kOhm - 200 V, and at 0,07 kOhm - 0,8 V. With a further increase in the resistance of the RBX resistor at the moments of switching element was self-excited at a high frequency. As a result, for the studied specimens of microcircuits, the value of the correction factor K = XNUMX was determined.
Author: I. Nechaev
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Input impedance: 1 MΩ;
Possibility of serial connection - Daisy chain;
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Built-in ION: 4,096 V;
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Operating temperature range: -40...125°C.
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