Kombinasyonel Mantık Devreleri / Combinational Logic Circuits

Digital circuits are called circuits that use their own unique voltage levels today. As we know when analyzing these circuits, we often use boolean alms. For this reason, we have detailed the boolean in our previous series. In short, we suggest that you take a look at our seriesbefore you start this topic. In this way, you will not have any difficulty processing combination logic circuits.

The outputs of combinational logic circuits are determined by logic "0" or logic "1", which is the logical state of the current input states at any time.

As a result, combinational logic circuits have no feedback. Any change in the signals applied to their input will have an immediate effect on the output. In other words, in a Combinational logic circuit, the output always depends on the combination of inputs. So we understand from here that combinational logic circuits are memoryless.

Kombinasyonel Mantık Devreleri / Combinational Logic Circuits

Combination Logic Circuits

Combinational logic circuits consist of basic logic NAND, NOR, NOT gates that are "merged" or connected to produce more complex switching circuits. These logic gates are the building blocks of combinational logic circuits. An example of a combination circuit is a decoder that converts binary code data contained in its entry into a series of different output lines that produce an equivalent decimal code at the output.

Combinational logic circuits can be very simple or very complex. Any combination circuit can be easily created with NAND and NOR doors, which classify them as "universal" doors,

The three main ways to specify the function of a combinational logic circuit are:

  1. Boolean algebra – Constitutes the algebraic expression indicating the operation of the logic circuit for each input variable. This is true or false, resulting in the output of logic "1".
  2. Accuracy table – The accuracy table defines the function of a logic door by providing a short list that shows all output states in table format for each possible input combination of the door.
  3. Logic diagram – A graphical representation of a logic circuit that shows the cables and connections of each logic gate represented by a specific graphical symbol that implements the logic circuit.

All three of the logic circuit representations are shown below.

Logic circuit representations

Combinational logic circuits consist of individual logic doors. Common combination circuits consisting of individual logic gates that perform a desired application include Multiplexers, encoders, decoders, full and half collectors, etc.

Classification of Combinational Logic

Some of the most common uses of combinational logic are Multiplexer and Demultiplexer type circuits. Multiple inputs or outputs are connected to a common signal line. Logic gates are used to select a single data input or output key.

A Multiplexer consists of two separate components, a logic decoder and solid state switches, but we need to understand how these devices use their "solid state switches" in designs before discussing multiplexers, decoders in more detail.

Solid State Switches

Standard TTL logic devices consisting of transistors can pass signal currents in only one direction. This makes them weak imitations of "one-way" devices and traditional electro-mechanical switches or relays. However, some CMOS switching devices consisting of FETs serve as excellent "duplex" switches, which become ideal for use as solid state switches.

Solid state switches appear in various types and degrees. There are many different applications for using solid state switches. They can be divided into 3 different main groups mainly for switching applications. In this combinational logic section we will only look at the analog key type.

Solid State Switch Applications

  • Analog switches – Used in data switching and communication, Video and audio signal switching, instrumentation and process control circuits.
  • Digital switches – Used in areas such as high-speed data transmission, switching and signal routing, Ethernet, LAN, USB and serial broadcasts, etc.
  • Power switches – Power Supplies and general" standby power are used in areas such as switching applications, greater voltage and current replacement, etc.

Analog Double Sided Switches

Analog switches are types used to change data or signal flows when in an open "state" and to block them when they are in a closed " state. The quick transition between the "on" and "off" state is usually controlled by a digital signal applied to the control door of the switch. An ideal analog switch has zero resistance when it is "on" (or off) and infinite resistance when it is "off" (or on).

Solid State Analog Switch

Solid State Analog Switch

It connects an N-channel MOSFET in parallel with a P-channel MOSFET, allowing signals to pass in both directions. This makes it a "two-way" switch. Whether the N-channel or P-channel device carries more signal current will depend on the ratio between input and output voltage. Two MOSFET are replaced as "on" or "off" by two built-in inverters and non-inverted amplifiers.