Cmos ttl pdf

If the minimum feature size can be reduced, this means that the transistor length can be reduced effectively making the transistor smaller with the same electrical properties. This allows for lower current flow between the junction for the same purpose and lesser heat dissipation. Now the minimum line width can be calculated from equation 1. Important Points. Since we have the two MOS transistors connected in series, the resultant output will be the AND operation, with inverted output, i.

For this to occur, the top side transistors of the logic gate must switch current into the output of the logic gate at the same magnitude as the low side transistors. NMOS Configuration. PMOS Configuration. Hence, we taking NMOS configuration:. Static power is proportional to the static current, i.

Dynamic Power is related to the current that flows when switching takes place and is given by for CMOS as:. The two important characteristics of CMOS devices are high noise immunity and low power dissipation. So there is no direct path from the power supply to the ground. Hence, Power dissipation in CMOS is low in static operation but it has high power dissipation in dynamic operation.

The gate has an almost ideal voltage-transfer characteristic. The logic swing is equal to the supply voltage and is not a function of the transistor sizes. The steady-state response is not affected by fanout.

A CMOS digital input has a very high impedance. Consequently, when it is not driven it will float, creating an undetermined input logic level Neither 1 nor 0. The output signal swing is independent of the exact value of aspect ratio and other device parameters. However, the output currents are limited, often being barely enough to drive a couple of LEDs. Thanks to their smaller current requirements, CMOS logic lends itself very well to miniaturization, with millions of transistors being able to be packed into a small area without the current requirement being impractically high.

Field-effect transistors depend on a thin silicon oxide layer between the gate and channel to provide isolation between them. This oxide layer is nanometers thick and has a very small breakdown voltage, rarely exceeding 20V even in high power FETs. This makes CMOS very susceptible to electrostatic discharge and overvoltage. If the inputs are left floating, they slowly accumulate charge and cause spurious output state changes, which is why CMOS inputs are usually pulled up, down, or grounded.

TTL does not suffer this problem for the most part since the input is a transistor base, which acts more like a diode and is less sensitive to noise because of its lower impedance. Though TTL chips are still available, there is no real advantage in using them. Overall CMOS is the clear winner when it comes to utility.

Get Our Weekly Newsletter! Helena St. Although ICs belonging to the same logic family have no special interface requirements, that is, the output of one can directly feed the input of the other, the same is not true if we have to interconnect digital ICs belonging to different logic families. Incompatibility of ICs belonging to different families mainly arises from different voltage levels and current requirements associated with LOW and HIGH logic states at the inputs and outputs.

For such an arrangement to operate properly the following conditions are required to be satisfied,. In the present case, both ICs would operate from 5 V. As far as the voltage levels in the two logic states are concerned, the two have become compatible. It is the current level compatibility that needs attention. For a proper interface, both the above conditions must be met.

One such scheme is shown below. In this case, there is both a voltage level as well as a current level compatibility problem. The voltage level compatibility in the two states is a problem.

VOH min. When the two are operating on different power supplies, one of the simplest interface techniques is to use a transistor as a switch in-between the two, as shown below.