I didn’t really like Cmos ICs when I was young, mainly because (at that time) they often broke down quickly due to static electricity. I preferred to use the more robust TTL ICs and in most computer circuits I also usually saw TTL instead of Cmos, except for the processors. On this page I tell you more about CMOS ICs. CMOS ICs contain digital circuits and can be used for more purposes than one might think at first glance. When CMOS circuits were first introduced in the 1970s, they were much more expensive than the other logic families, but this is no longer the case and they are almost the cheapest of all. Simple CMOS ICs are often cheaper than normal transistors.
Older TTL ICs have a few disadvantages, they have a relatively large power consumption, the supply voltage is quite critical (5 v 10%) and the input impedance is quite low, on the order of a few hundred ohms. CMOS ICs have none of these disadvantages and they consume very little power and simple gate circuits consume virtually no power when in a static state. The supply voltage for most CMOS ICs is between 3 Volt and 15 Volt. The input impedance of a CMOS IC is very high, on the order of 1,000,000 megohm. For most applications this can be considered infinitely high. CMOS circuits are therefore voltage rather than current controlled.
Construction of the CMOS circuit
The simplest CMOS circuit is the inverter, which consists of only two active parts. The output of this circuit also has only two stable states “high” and “low” and the operation of the circuit is easy to understand. The output must be at the opposite level as the input.
This inverter contains a P-channel IGFET (insulated gate field effect transistor) and an N-channel IGFET. The input is either ‘high’ when its potential is near the positive supply voltage, or ‘low’ when the potential is near the negative supply voltage. These states are also called logic “1 and logic 0 respectively. An IGFET has a very large drag-source resistance when the voltage on the gate is the same as that on the source. In this circuit, only one of the two IGFETs can be conductive and thus inverts the input voltage level.
This might give the impression that the circuit consumes no current at all, but this is not the case, transistors are not perfect switches, they have a resistance of a few hundred ohms when conducting and thousands of megohms when cut off. So a very small current also flows through a static inverter. Only a small current pulse is created in the circuit when it changes state. Although Cmos ICs can only supply a small current at the output, they still have a large fanout when using other Cmos ISs because the required input current is negligible.
Security of cmos circuits
As most readers will probably know, IGFETs are easily damaged by the high voltages of static charge. Even someone wearing a nylon shirt can be the cause of damaging an IGFET which could give the impression that Cmos ICs are very delicate and almost unusable. The reality is that all current CMOS ICs are internally equipped with protection diodes, which limit the voltage on inputs to a safe value. However, it is recommended to follow a few simple rules when dealing with CMOS ICs. The ICs are usually supplied with the connection pins inserted in a piece of conductive foam and it is best to leave the IC in this foam until it is actually needed. Never remove an IC from the base when the power supply is still on and preferably wear a grounded bracelet when working with CMOS ICs.