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# Detailed Overview of Buffer Buffer and Open Drain Gate Od Gate Composed of MOS Transistor

What can you get from this article?

The buffer and open drain gate composed of MOS transistor are very important concepts of digital circuit. How to form them;

Inverter, line and logic how to play, how to use it?

According to the schematic diagram and truth table, apply typical circuits to fully understand the basic logic gates, and gates, or gates and NAND gates.

What's the matter with semiconductor SS, TT, FF?

1. As a hardware engineer, MOS logic circuit (and gate, or gate, not gate, etc.) must understand the chip; In order to understand the chip, various basic logic circuits composed of MOS tubes must be memorized, so that we can more skillfully understand the block diagram of the chip. Field effect transistors can form a variety of gate circuits through different combinations. As mentioned at the beginning, these most basic unit circuits may be the basis of modern IC. The following circuit forms exist in a large number of commonly used 74 series chips. The OD gate and buffer introduced later are common in the design of pins such as GPIO port of the chip.

MOS tube constitutes the basic and gate, or gate circuit and gate, which can be composed of six tubes. The working diagram of and gate can be seen more clearly through the schematic diagram, and then the corresponding relationship between input and output can be seen from the truth table. In this paper, the corresponding circuit of and gate is given. If you are interested, you can think about the circuit structure of or gate. In fact, there is a corresponding relationship between them.

2. Inverter the following figure shows the circuit diagram of the inverter. The input and output states are opposite, which is called inverter.

Circuit analysis: the input VI is at low level, the upper tube is on, the lower tube is off, and the output is at high level; Input VI is high level, the upper tube is off, the lower tube is on, and the output is low level.

NAND gate the following figure shows the circuit diagram of NAND gate. NAND gate means that the same is zero and the different is one.

When inputs a and B are at low level, tubes 1 and 2 are on, tubes 3 and 4 are off, the voltage at terminal C is consistent with VDD, and the high level is output. When a inputs high level and B inputs low level, tubes 1 and 3 are on, tubes 2 and 4 are off, and the potential at terminal C is consistent with the drain of tube 1 to output high level. When a inputs low level and B inputs high level, 2 and 4 are turned on, 1 and 3 tubes are cut off, the potential at terminal C is consistent with the drain of tube 2, and the high level is output. When inputs a and B are at high level, tubes 1 and 2 are cut off, tubes 3 and 4 are turned on, the voltage at terminal C is consistent with the ground, and the output is at low level.

4. Buffer CMOS buffer. The buffer is opposite to the inverter. The input and output of the buffer are the same, and the input and output of the inverter are opposite.

Circuit analysis: the first stage Q1 and Q2 form an inverter; The later stages Q3 and Q4 constitute an inverter, which is equivalent to reversing the secondary phase, so it is restored again. 5. Open drain gate open drain gate is a very classic and commonly used circuit, which is often used in the design of GPIO port of main chip or GPIO port of single chip microcomputer. The most important point is that the open drain is in a high resistance state, and the pull-up resistor is required for general applications.

[application of open drain gate - line and logic] z = z1z2z3

"Line and" logic is because the output triodes of multiple logic units share a pull-up resistor. As long as one logic unit outputs a low level, that is, the collector (drain) is open and the output tube is connected, the low level is output; Only all units are cut off, and the output end is set to high level by the pull-up resistance. This is a very practical circuit, which can be used in circuit systems such as logic arbitration.

Perhaps after working for a few years, I will generally feel that the diode triode circuit is very simple, which only shows that the research is not deep enough. Sometimes the simpler things are, the more complex the underlying things are. For example, from the perspective of technology, transistors are divided into TT, SS, FF and IC. The design can't get around these, and the foundation is not easy. Considering that it's more in-depth, I think I only learn the superficial.

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