Unusual application of CMOS switches. Encyclopedia of radio electronics and electrical engineering

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Microcircuit switches of the CMOS structure are designed for switching analog signals. However, these devices, like many others, along with their main function, are capable of performing other, sometimes quite unexpected ones.

Several circuit options for the non-standard use of electronic switches are described in the published article.

Amateur radio experiments show that the electronic switches that make up the K176KT1. K561KTZ. 564KTZ and KR1561KTZ [1; 2], are universal elements that allow them to be used in various functional units - inverters, signal repeaters, etc. These switches can be used to build rectangular pulse generators, RS flip-flops, and Schmitt triggers with adjustable "hysteresis" width.

An example of using an analog switch as an inverter is shown in the diagram in fig. 1. When a low level signal is applied to control input C, the switch is in state Z and a high level signal is present at its output B due to the presence of resistor R1. When a high level is applied to input C, input A., at which a low level is fixed, is connected to output B. Therefore, the output will also be a zero signal. Thus, in relation to input C, the device works as an inverter.

You can assemble the inverter on any of the four switches that make up the microcircuit. In addition to K561KTZ. in this and other nodes described below, you can use K176KT1 chips. 564KTZ. KR1561KTZ.

On fig. 2 shows a signal repeater circuit. When a low level signal is applied at the input C, the switch DA1.1 is in the Z state, the output is a low level signal due to the resistor R1. When the low level is replaced by a high level at the input C, the "contacts" of the switch are closed and a high level is supplied from the input A to its output B. That is, in relation to the signal at input C, the node works as a repeater.

It should be noted that the transfer characteristic of the inverter and voltage follower on analog switches is quite smooth, which must be taken into account when designing devices using them.

An example of building a rectangular pulse generator based on analog switches is shown in the diagram in fig. 3. Switch DA1.1 works as a repeater, and DA1.2 as an inverter. At the initial moment after the power is turned on, the capacitor C1 is discharged, both switches are closed. A charging circuit for capacitor C1 is formed: positive power wire - R3 - R2 - C1 - R1 - common wire. As soon as the voltage at the input C of the switch DA1.1 reaches the threshold of its inclusion, it will open, and after it the switch DA1.2 will open.

Now the capacitor C1 begins to discharge through the resistors R1, R2 and the resistance of the open switch DA1.2. Subject to the conditions R1 < R2 < R3 < R2; Upit = const It was experimentally found that the oscillation period depends on the values ​​of the elements R2 and C1 as follows: if Upit = 5 V. then T = 0.6 R2-C1; 10V -0,5 R2-C1; 15V-0.4 R2-C1.

It is also possible to build an RS flip-flop on analog switches (Fig. 4). Let's assume that the flip-flop is in the zero state (Q=0, Q=1). switch DA1.1 is closed (a low level is present at its input C), and DA1.2 is open (a high level is present at its input C).

When you press the SB2 "S" button, a low-level voltage is supplied to the input C of the DA1.2 switch and it closes, and a unit voltage appears at the output Q of the trigger. Switch DA1.1 opens a single voltage at input C. and output Q goes to the zero state.

Similarly, when the button SB1 "R" is pressed, the switch DA1.1 closes and the output Q goes into a single state. Switch DA1.2 opens with a single voltage at input C and zero voltage acts at output Q.

An example of constructing a Schmitt trigger is shown in the diagram in fig. 5.

Here, the switch DA1.1 works as a voltage follower. By choosing the appropriate resistance values ​​of resistors R1-R4, you can set the upper Uv and lower U.. trigger switching thresholds. The threshold voltage values ​​can be determined from approximate dependences (we neglect the resistance of the signal channel of the open switch and the voltage drop across the open diode):

Usually take the resistance of the resistor R1 in the range from 10 to 50 kOhm. R2 and R3 - from 0.1 to 1 MΩ [3].

When using analog switches, keep in mind that their on-resistance depends on the supply voltage. Fluctuations in the supply voltage lead to corresponding changes in the frequency of the generated pulses, as well as the thresholds for the Schmitt trigger.

Literature

1. Pukhalsky G. I., Novoseltseva T. Ya. Designing discrete devices on integrated circuits (reference book). - m Radio and communications. 1990. p. 109. 110.
2. Biryukov S. A. Digital devices based on MOS integrated circuits (second ed.). - M. - Radio and communication. 1996. p. 81-83.
3. Zeldin E. A. Pulse devices on microcircuits. - M.: Radio and communications. 1991. p. 30.31.

Author: V. Oleinik, Korolev, Moscow Region

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