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1.5V High Performance OP AMP Using Self Cascode StructureKshitij Bhardwaj

Institute of Engg. &TechnologyDevi Ahilya VishwaVidayalay

Indore, M.P., India

[email protected]

S.S.RajputABV-IIITM

Morena Link RoadGwalior-474010, M.P., India

[email protected]

. Abstract

An operational amplifier (Op Amp) based on self cascodestructure is proposed that works with a supply voltage of ± .75 volt. It provides an open loop gain of 52dB with aunity gain frequency of about 17 MHz, with 145 microwattof power consumption. Simulation results are based on p-spice simulation for 0.25μm CMOS technology.

Categories and ubject escriptors

VLSI Circuits –high speed low power circuits

eneral Terms

Low Power, OP-AMP, Self Cascode MOSFET structure,Current Mirror (CM).

. ntroduction

Battery is the main source of power in all electronicsdevices, which adds volume and weight, So the portabilityrequire low power consumption and small size & long life .One of the techniques to meet the low power consumptionneeds to operate with sub volt supplies. This requirementcomplicates the design of high performance analog circuitstructures. There are some other techniques suitable to setnew analog structures for low voltage operations [1-4].

In this paper we examine the self cascode structure in thedesign of analog structures .A cascode is generally used toincrease the impedence and gain of analog structures;however the price paid for this high impendence is thelower voltage headroom for the output voltage swing .Toovercome this problem a self cascode structure is generallyused . A self-cascode is a 2-transistor structure as shown inFig. 1 (a). This structure can be treated as a single compositetransistor as shown in Fig. 1 (b). The composite structure hasmuch larger effective channel length and the effective outputconductance is much lower. The lower transistor M1 isequivalent to a resistor whose value is input dependent. For optimal operation, the W/L ratio of M2 is kept larger than thatof M1, that is, m >> 1. For the composite transistor, theeffective transconductance ( g m (effective)) will be gm2/m,which is equivalent to the transconductance of M1 ( g m1). Nowthe drain current (ID) through M1 and M2 will be effective (V in -

V T )2 / 2, where effective equals 1 2/ ( 1+ 2) which can be

approximated by 1 when m is large [1].The voltage betweensource and drain of M1 is small, and there is no appreciabledifference between the VDSAT of composite and simple transis-tors; and a self-cascode can be used in low voltage operation.For a self-cascode

VDSAT = VDSATM2 + VDSATM1.

. elf Cascode structure

The operating voltage of a regular cascode is much higher than

that of a self-cascode. The regular cascode operates with

higher output compliance voltage which is in general

VGS+VSAT, while it is simply ≈ 2VSAT for a self cascode. The

advantage offered by self-cascode structure is that it offers

high output impedance similar to that of a cascode structure

while output voltage requirements are similar to those of a

single transistor.

. elf Cascode CM

A current mirror developed using Self Cascode structure is

shown in Fig. 2. The output impendence of Self Cascode CM

is large. Output and Input Characteristics of Self Cascode

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current mirror structure is shown in Fig.3 and Fig.4

respectively. Frequency response of self Cascode current

mirror is shown in Fig.5 with a 3 dB frequency of 70MHz.

Fig.2 CM structure based on Self cascode MOSFETs

. Theoretical Analysis

The current transfer function of the Self Cascode current

structure is given by

in

out

I

I =

21

12

LW

LW , Rin=

1

1

m g

Fig. 3. Output Characteristics

Fig.4 Input Characteristics of self Cascode CM

Frequency Response of self cascode CM

-6

-5

-4

-3

-2

-1

0

1

0 500 1000

Frequency in MHz

C u r r e n t G a i n

i n

d B

Fig.5 Frequency Response of Self Cascode Current Mirror

. roposed structure

The self cascode building blocks described above facilitate

the building of an op amp to operate at very low voltage. In

the proposed op-amp (Fig.6) circuit a PMOS differential

pair with a diode connected load is used as a first stage.

Output of this stage is given to the NMOS self cascode

structure formed by M8, M9, M10, M11. A pmos self

cascode current mirror formed by M12, M13, M14 worksas a load to this stage. These two stages form the OTA

(operational transconductance amplifier) that has very high

output impedance. The (W/L) ratio of different MOS

transistors are chosen such a way that all transistor work in

saturation region. The output of first stage is connected to

an inverter with PMOS self cascode current mirror load by

which the gain of OP-AMP increased. C1 are used for the

frequency compensation to improve the phase margin.

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Fig.6 Proposed OP-AMP Structure using Self Cascode

.4 imulation esults:

A current mirror structure is shown in Fig.2. This structure

has been simulated by PSPICE on 0.25um technology for

W/L ratio of 4.2/.5,5/.5 and 50/.5for M1, M2 and M3.The

input characteristics of the current mirror is shown in Fig.3

along with the frequency characteristics shown in Fig.4.

The op amp of Fig.5 has been simulated on p-spice on 0.25

μm (Level 3) technology. Table 1 shows the different

parameters of the MOS transistor. The (W/L) values of

different MOS transistors are given in table2 .

Table 1 OP Amp Design Specification

arameters pecifications

Technology 0.25 μm

Threshold Voltages(VTHN,VTHP) 0.4238 V, -0.553 V

Transconductance (KP N,KPP) 250 μA/V2 ,51.9μA/V2

Power Supply (VDD, VSS) 0.75 V,-0.75V

Bias Current 30 μA

Table 2 (W/L) of MOS used in Op Amp

Transistor ize (/)μm

M1,M2,M9 40/0.5

M3,M4 4.2/0.5

M5,M13, 40/0.25M6,M14,M16 5/0.25

M7,M12,M15 4.75/0.25

M8,M10 5/0.5

M11 35/0.5

M17 35/0.25

An open loop characteristic of the designed op amp of Fig.6 is shown in Fig.7. The open loop gain comes out to be 52db. Unity gain bandwidth is about 16.8 MHz andresults are complied in table3.

-20

-10

0

10

20

30

40

50

60

0.0 0.1 10.0

G a i n i n d B

Frequency in MHz

OPEN LOOP GAIN

Fig. 7 Open Loop Gain of OP-AMP

.

Table.3 Op Amp characterization Results

arameters values

Open loop gain 52db

Unity gain frequency 16.8 MHz

Gain Margin 5.4dB

Phase Margin 51o

Power dissipation 145 microwatt

3-db frequency 36.46KHz

4. Conclusion:

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It is necessary to use low voltage low power circuits in all the portable electronics devices. Self cascode structure has proved to be an efficient technique. The Op Amp that is proposed in the paper using self cascode structure is suitable for the low voltagelow power applications as vindicated by the results. The proposedop amp is working at ±0.75 V supply voltage with a gain of 52db.The power dissipation is quite low (145 microwatt). The supply

voltage of the op amp may also be reduced by designing it with90nm or 65nm technology.

. eferences

[1] S. S. Rajput and S.S. Jamuar, “Low Voltage Analog CircuitDesign Techniques,” IEEE Circuits and Systems Magazine,Vol-2, No.2, pp 24-42, 2002.

[2] Elen,Analog & Mixed signal Center (AMSC), Texas A&MUniversity “Low Voltage Circuit Design Techniques”

[3] B.J. Blalock, P.E. Allen and G.A. Rincon-Mora, “Designing1-V op amps using standard digital CMOS technology”,

IEEE Trans. Circuits and Systems – II , vol. 45, No. 7, pp.769-779, July 1998.

[4] S.S.Rajput, “Low voltage, current mode Analog circuitstructures and their applications” IIT Delhi, Aug-2002.

[5] E. Sanchez- Sinencio and A.G. Andreou, “Low Voltage/lowPower Integrated Circuits and Systems,” IEEE Press (1999).

[6] Phillip E. Allen, Douglas R. Holberg, “CMOS AnalogCircuit Design”, Oxford University Press, 2004.

[7] Lisha Li,”High gain power operational amplifier design andcompensation techniques”Brigham Young University, April2007.

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