9.5 Sspd_chapter 7_part 3_electrical properties of moscontinued3

SSPD_Chapter 7_Part 3_Electrical Properties of MOScontinued3

7.3.9 MOS as an Analog Switch- Pass Transistor.

Just as electro-magnetic Relay Switches can be used to form Logic Gates. In a similar fashion, MOS can be used as analog switches to form Logic Gates. The use of Relay Switch is shown in Figure 7.3.9.1 and use of MOS is shown in Figure 7.3.9.2.

When MOS transistors are used as analog switches they are called Pass Transistors.

Here Logic 1 is V _DD – V _th .

7.3.10. RTL Inverters.

RTL Inverters was the earliest building block of digital systems. It evolved to TTL gate. It further evolved to NMOS. Today CMOS Inverters have become the basic building block of most digital systems having much more superior performance characteristics as compared to the original RTL Inverter. A RTL Inverter is shown in Figure 7.3.10.1.

Table 7.3.10.1 The two Q points corresponding to I/P low and I/P high.

Part(C) gives the transfer characteristics and Part (D) gives the noise margin. As is evident from Figure (D), noise margin can be maximized by reducing the transition region and this precisely what happens in CMOS inverter. In CMOS Inverter, transition region is 0 V and

The central point of this brief review of RTL Inverter is to illustrate the principle by which Transfer Characteristics is arrived. We draw the load line over the family of output curves, determine the Q-points for the two binary states of Input and from the Q-points we arrive at the transfer characteristics of the Inverter. From the transfer characteristics we arrive at the noise margin. This is the procedure we will adopt to arrive at the performance characteristics of the subsequent inverters.

7.3.10.1. The NMOS Inverter.

MOS inverters are synthesized in the same manner as RTL Inverter in Figure 7.3.10.1. Since passive resistances are difficult to integrate as they occupy a large area on silicon die hence passive resistive loads are an anathema for ICs . The passive loads have been completely replaced by active loads. In Figure 7.3.10.2, an NMOS inverter is shown.

In the NMOS inverter, (E)NMOS acts as the inverter driver which is called the pull-down transistor and (D)NMOS acts as the active load of the driver also known as pull-up transistor. First we will examine how (D)NMOS acts as resistive active load of the logic circuit.