Classification of Combinational Logic

One of the most common uses of combinational logic is in Multiplexer and De-multiplexer type circuits. Here, multiple inputs or outputs are connected to a common signal line and logic gates are used to decode an address to select a single data input or output switch.

A multiplexer consist of two separate components, a logic decoder and some solid state switches, but before we can discuss multiplexers, decoders and de-multiplexers in more detail we first need to understand how these devices use these “solid state switches” in their design.

Solid State Switches

Standard TTL logic devices made up from Transistors can only pass signal currents in one direction only making them “uni-directional” devices and poor imitations of conventional electro-mechanical switches or relays. However, some CMOS switching devices made up from FET’s act as near perfect “bi-directional” switches making them ideal for use as solid state switches.

Solid state switches come in a variety of different types and ratings, and there are many different applications for using solid state switches. They can basically be sub-divided into 3 different main groups for switching applications and in this combinational logic section we will only look at the Analogue type of switch but the principal is the same for all types including digital.

Solid State Switch Applications

§  Analogue Switches – Used in Data Switching and Communications, Video and Audio Signal Switching, Instrumentation and Process Control Circuits …etc.

§  Digital Switches – High Speed Data Transmission, Switching and Signal Routing, Ethernet, LAN’s, USB and Serial Transmissions …etc.

§  Power Switches – Power Supplies and General “Standby Power” Switching Applications, Switching of Larger Voltages and Currents …etc.

Analogue Bilateral Switches

Analogue or “Analog” switches are those types that are used to switch data or signal currents when they are in their “ON” state and block them when they are in their “OFF” state. The rapid switching between the “ON” and the “OFF” state is usually controlled by a digital signal applied to the control gate of the switch. An ideal analogue switch has zero resistance when “ON” (or closed), and infinite resistance when “OFF” (or open) and switches with R_ON values of less than 1Ω are commonly available.

Solid State Analogue Switch

By connecting an N-channel MOSFET in parallel with a P-channel MOSFET allows signals to pass in either direction making it a “Bi-directional” switch and as to whether the N-channel or the P-channel device carries more signal current will depend upon the ratio between the input to the output voltage. The two MOSFET’s are switched “ON” or “OFF” by two internal non-inverting and inverting amplifiers.

Contact Types

Just like mechanical switches, analogue switches come in a variety of forms or contact types, depending on the number of “poles” and “throws” they offer. Thus, terms such as “SPST” (single-pole single throw) and “SPDT” (single-pole double-throw) also apply to solid state analogue switches with “make-before-break” and “break-before-make” configurations available.

Analogue Switch Types

Individual analogue switches can be grouped together into standard IC packages to form devices with multiple switching configurations of SPST (single-pole single-throw) and SPDT (single-pole double-throw) as well as multi channel multiplexers.

The most common and simplest analogue switch in a single IC package is the 74HC4066 which has 4 independent bi-directional “ON/OFF” Switches within a single package but the most widely used variants of the CMOS analogue switch are those described as “Multi-way Bilateral Switches” otherwise known as the “Multiplexer” and “De-multiplexer” IC´s and these are discussed in the next tutorial.