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SSPD_Chapter 7_Part 3 is continued. Here we discuss the different pull up configurations we use to realize NMOS inverter. It is shown that CMOS is the best configuration from power conservation point of view though heat management remains a problem at high switching speed.

SSPD_Chapter 7_Part 3_Basic Electrical Properties_continued4.

7.3.11 Alternative forms of NAND Gates/Alternative forms of Pull-up configuration.

Just like RTL Logic, MOS Logic can have a passive load or an active load. From integration point of view we always prefer an active load in comparison to passive load.

Passive Load is a resistance and resistance occupies too much real estate on the chip as well as it has a limited options. Therefore we use an active device as the active load. In BJT Technology we have Current Mirror, Symmetrical Widlar and Widlar being used as the active load of differential amplifier which is the basic building block of an Op. Amp.

In Figure 7.3.11.1 we show the BJT Current Mirror being used as active load in differential amplifier.

Here Q3 and Q4 constitute the Current Mirror. The two together act as the active load of the differential amplifier. Q1 and Q2 are the drivers of the differential amplifier. In exactly the same manner MOS can also be used as an active load .

7.3.11.1. NMOS Inverter with (D)NMOS as pu transistor.

Passive load Pull-up configuration is technically unfeasible for IC technology. Hence we always go for Active Load Pull-Up configuration. The first configuration has already been discussed in Section 7.3.10. In this NMOS inverter with (D)NMOS as the Pull-Up transistor the biggest drawback is the standby power dissipation. When Vin = HIGH, Vout=LOW and both transistors are ON leading to rail to rail current flow hence power dissipated will be typically 5V×5mA = 25mW.

While switching the outut from ‘1’ to ‘0’, the actual switching starts when Vin has exceeded Vt [threshold voltage of (E)NMOS].

While switching the output from ‘1’ to ‘0’, load capacitance is rapidly discharged through the pd transistor which is in triode region. Pd transistor provides low resistance path hence time constant of discharge is short.

Similarly while switching from ‘0’ to ‘1’, load capacitance rapidly charges through pu transistor. Pu transistor also is in triode region for the latter part of switching from ‘0’ to ‘1’.

This is favourable feature which was encountered in Totem Pole configuration of TTL gates. This helps improve the switching speed but the standby dissipation under Vi = HIGH disfavours this circuit hence it was left out as IC Technology progressed.

7.3.11.2.NMOS Inverter with (E)NMOS as pu transistor

Figure 7.3.11.2 describes NMOS Inverter with (E)NMOS as pu transitor. Here also standby power dissipation is high when Vin= HIGH.

Vout never reaches V DD because of voltage equal to threshold voltage dropping along the channel.

By deriving V GG from a switching clock source, dissipation can be reduced. When Vin = HIGH we keep Gate of the active load LOW so that pu(T1) is turned off. So only driver (T2) is turned ON. T2 provides a low resistance path for the discharge of load capacitance when Vin = HIGH.

When Vin=LOW, T2 is OFF. During that period of Vin=LOW, we keep the gate of active load(T1) HIGH so that T1 is ON and low resistance path for charging the capacitive load. Hence Load Capacitance rapidly charges from ‘0’ to ‘1’ with a short time constant of charging.

This configuration is fast as well as it has low standby power but it has the added circuit complexity because of synchronized gate input of the active load. Hence this configuration has fallen into disfavour.

7.3.11.2.(E)NMOS Inverter with (E)PMOS as pu transistor- complementary transitor pull-up also known as CMOS Logic.

A NMOS Inverter with a (E)PMOS as pu transistor is shown in Figure 7.3.11.3. This complementary MOS configuration is known as CMOS. The permissible states are given in Table 7.3.11.1.

Table 7.3.11.1. The two permissible states of CMOS.

(E)NMOS (E)PMOS Vout
Vin LOW OFF ON(sourcing the current to the capacitive load) HIGH(O/P has a low resistance path through ON PMOS to V DD )
Vin HIGH ON(sinking the current from the capacitive load) OFF LOW(O/P has a low resistance path through ON NMOS to the GND)

Part (a) of Figure 7.3.11.3 gives the circuit configuration, Part(b) gives the transfer characteristics and Part (c) gives the current flow.

In Part(b) we see that full logical levels are realized i.e. O/P HIGH=V DD and O/P LOW = 0V.

In Part (c) we see that under standby the current from rail to rail is zero. Hence standby dissipation is zero. Hence CMOS is also known as NanoWatt Logic.

During the switching there is a current flow from rail to rail hence dissipation per gate is directly proportional to clocking speed. In Figure 7.3.11.4 we compare the dissipation curves of TTL Logic and CMOS Logic under standby as well as under switching condition.

For identical geometries PMOS is slower than NMOS on account of lower mobility of holes. Hence to achieve identical switching time from LOW to HIGH and from HIGH to LOW, geometries will have to be optimized as we have to optimize for different configuration of NMOS inverter.

From here onward we will be focused on CMOS Logic and its analysis.

Questions & Answers

what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
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Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Solid state physics and devices-the harbinger of third wave of civilization. OpenStax CNX. Sep 15, 2014 Download for free at http://legacy.cnx.org/content/col11170/1.89
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