Skip to main content

Ratioed Logic Circuits Inverter | Digital Electronics

Apart from the complementary CMOS inverter, there are other forms of MOS inverter that can be used to build logic gates. Figure 1(a): shows a generic nMOS inverter that uses either a resistive load or a constant current source.

For the resistor case, if we superimpose the resistor load line on the I-V characteristics of the pull-down transistor (Figure 1(b)), we can see that at o the output is some small V (Vo) Figure 1(c) e when v,-o, vo. rises to Voo. As the resistor is made larger, the Vo decreases and the current flowing when the inverter is turned on decreases.

Correspondingly, as the load r resistor is decreased in value, the να rises and the ON current: rises. Selection of the resistor value would seek a compromise between Vo the current drawn, and the pull-up delay that r increases with the value of the load resistor. Current sources al have high output resistance and thus offer sharper transitions.

Read More


Neither high-value resistors nor ideal current sources are readily available in most CMOS processes. A more practical circuit called a pseudo-nMOS inverter is shown in figure 2 (a). It uses a pMOS transistor pull-up or load that has its gate permanently grounded to approximate a constant of the current source. Pseudo-nMOS circuits get their name from an early nMOS technology (which preceded CMOS technology as a major systems technology) in which only ut nMOS transistors were available; the grounded pMOS R transistor is reminiscent of a depletion mode nMOS transistor that is always ON.

[caption id="" align="alignnone" width="636"]Ratioed Logic Circuits Inverter Ratioed Logic Circuits Inverter[/caption]

The transfer characteristics may again be derived by finding Vo for which L for a given Vi, as shown in Q Figure 2 (b) and Figure 2 (c). The beta ratio affects the shape of the transfer characteristics and the Vou of the inverter Larger pMOS transistors offer faster rise times but less sharp transfer characteristics. Figure 2 (d) shows that when the nMOS transistor is turned on, a constant DC current flows in the circuit.

[caption id="" align="alignnone" width="552"]Ratioed Logic Circuits Inverter Ratioed Logic Circuits Inverter[/caption]

The gates in this section are called ratioed circuits one is because the transfer function depends on the ratio of the opposing strength of the pull-down transistor to the pull-up device. The net resistor, current source, or ON transistor is sometimes called a static load. It is possible to construct other radioed circuits such as NAND or NOR gates by replacing the pull-up transistors with a single pull-up device.

Unlike complementary circuits, the ratio must be chosen so the circuit operates correctly despite any variations from nominal component values that may occur during manufacturing. Moreover, radioed circuits to dissipate power continually in certain states (e.g. when the output is low) and have poorer noise margins than complementary circuits.

Therefore, ratioed circuits tend to be used only in very limited circumstances where they offer critical benefits such as smaller area or reduced input capacitance.

Read More


Popular posts from this blog

Limitations of Terzaghi Theory

Limitations of Terzaghi Theory The value of the coefficient of consolidation has been assumed to be constant.  The distance d of the drainage path cannot be measured accurately in the field. The thickness of the deposit is generally variable, and an average value has to be estimated.  There is sometimes difficulty 1n locating the drainage face, sometimes thin previous seams that can act as good drainage face are missed in the boring operations. The equation is based on the assumption that the consolidation is one-dimensional. In the field, the consolidation is generally 3-dimensional. The lateral drainage may have a significant effect on the time rate of consolidation. The initial consolidation and secondary consolidation have been neglected. Sometimes these form an important part of the total consolidation. In actual practice, the pressure distribution may be far from linear or uniform. Read More Muller-Breslau principle

Price Guard Wire Method

Price Guard Wire Method Some form of  Price Guard Wire Method  is generally used to eliminate the errors caused by leakage currents over insulation. Fig. 3.14 illustrates the operation of This Method. In fig 3.14(a), a high resistance mounted on a piece of insulating material is measured by the ammeter voltmeter method. The micro-ammeter measures the sum of the current through the resistor (IR) and the current through the leakage path around the resistor. The measured value of resistance computed from the readings indicated on the voltmeter and the microammeter, will not be a true value but will be in error.   Figure 3.14 Application of  guard  circuit for measurement of high resistance In fig, 3.14 (b), the  guard  terminal has been added to the resistance terminal block. The  guard  terminal surrounds the resistance terminal entirely and is connected to the battery side of the micro-ammeter. The leakage current IL now bypasses t

Negative Booster

Negative booster A negative booster is employed to conform to the regulation that the potential difference between any two points of the rail return shall not exceed 7 V. Two boosters, positive and negative, are used which are mechanically coupled together and driven by a DC motor. The positive booster is connected to the trolley wire (near the generating station) and the negative booster (separately excited) is connected to the track rail.  The 'positive booster' adds voltage to the line while the 'negative booster lowers the potential of the point it is connected to. As we go along the trolley wire away from the generating station/sub-station, the potential drop increases, and the voltage of the trolley wire falls. Since the current returns via the track rail points away from the generating station acquire high potentials. This potential is brought down by the negative boost provided by the negative booster. When the load is sufficiently far away from the generating stati