Vol 09, Article 0302; Jan-March 2018 International Journal...

Vol 09, Article 0302; Jan-March 2018 International Journal of VLSI and Embedded Systems-IJVES

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1722

Design and Analysis of High Gain CMOS Telescopic OTA in

180nm Technology Arti R. Pandya

1, Dr. Kehul A. Shah

2

1,2Department of Electronics & Communication, Sankalchand Patel University, Visnagar, Mahesana, Gujarat,

India [email protected],

[email protected]

ABSTRACT

The Operational Trans-conductance Amplifier (OTA) is the block with the highest power consumption in analog

integrated circuits and many applications. Low power consumption is becoming more important in miniature

device, so it is challenging to design a low power OTA. At a large supply voltages, there is a trade-off between

speed, gain and power for an OTA design since these parameters are contradicting each other.

The telescopic trans-conductance amplifier consume less power compared with the other Trans-conductance

amplifiers, so it is widely used in low power consumption and it also has the high speed characteristic. In This

Paper Telescopic OTA is designed for 180nm BSIM4 Technology using LT –Spice Orcad simulator, This

designed Telescopic OTA achieved gain 136db, Phase Margin 81 degree, UGBW 80MHz which are the basic

performance parameter of an OTA.

Keywords: OTA, Telescopic OTA, Gain, Phase Margin, UGB, CMRR

1. INTRODUCTION The OTA is a basic building block usually used in designing many analog circuits such as data converters and

Gm-C filters. Performance of Gm-C filters is related and used on to the OTA’s performance. The OTA is a

Transconductance device where the input voltage controls the output current; it means that OTA is a voltage

controlled current source device whereas the op-amp is voltage controlled voltage source electronic device. An

OTA is basically an op-amp without output buffer, so it can only drive loads. An operational trans-conductance amplifier (OTA) is a voltage input current output amplifier. The input voltage

Vin and the output current Io are related to each other by a constant of proportionality and the constant of

proportionality is the Trans-conductance “gm” of the amplifier. Io = gm V (1)

Where gm= Transconductance of OTA Vin = Differential input voltage Figure 1 shows how to represent OTA symbolically.

Figure 1: OTA symbol [4]

The trans-conductance gm of the OTA can be obtained by varying the value of the external controlling current Ic. gm = KIc (2)

Where K= suitable constant of proportionality Substituting equation (1) into equation (2), we get,

Io = KVinIc (3) Equation (3) depicts that output current is proportional to the product of Vin and Ic. In general OTA consists of a

differential transistor pair with a current mirror circuit acting as a load. As OTA operates on the principal of

processing current rather than voltage, it is an inherently robust device. As gm can be controlled by changing

the control Ic, the OTAs are suitable for electronically programmable function. 2. Different Configuration of OTA There are different configurations of OTA topologies.

1. Single–Stage OTA 2. Two Stage OTA 3. Telescopic Cascode OTA





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4. Regulated Cascode OTA (Gain boosting) 5. Folded Cascode OTA

2.1 Comparison of Different types of OTA

Table 1 Comparison between various parameters

Topology Gain Power

Consumption

Speed Noise

Single Stage Low Medium High High

Two Stage High Medium Low High

Telescopic OTA High Low High Low

Folded Cascode Medium Medium High High

Gain-Boosted High Medium High Medium

3. CIRCUIT IMPLEMENTATION 3.1 Design of Telescopic OTA A telescopic OTA as shown in Fig.7, typically has a higher frequency capability and consumes less power than

other topologies. Its high-frequency response stems from the fact that its second pole corresponding to the

source nodes of the n-channel cascode devices is determined by the trans-conductance of n-channel devices as

opposed to p-channel devices, as in the case of a folded cascode. Also, the parasitic capacitance at this node

arises from only two transistors instead of three, as in the latter. The single stage architecture naturally suggests

low power consumption. The disadvantage of a telescopic op-amp is severely limited output swing. It is smaller

than that of the folded cascode because the tail transistor directly cuts into the output swing from both sides of

the output.

Figure 2: Schematic of the CMOS Telescopic OTA

3.2 Design Steps of Telescopic OTA According to the design steps we get the values of NMOS and PMOS STEP I In 1

st step we have to find W/L of two NMOS transistors, M9 and M10





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Id = µn Cox/ 2 (W/L) [Vgs - Vth] 2 24×10-6 = 0.045 × 0.0045 (W/L) [1.0-0.7]2 (W/L) =2.7 (W/L) 9 = (W/L) 10 = 2.7 STEP II As per the 2nd step we have to find the W/L ratio of NMOS transistors M7 and M8 Id = µn Cox/2 (W/L) [Vgs-Vth]

2 15.7×10

-6 = 2.025×10

-4 (W/L) [0.56 - 0.42]

2 (W/L) = 3.9 (W/L) 7 = (W/L) 8 = 3.9 For transistor M5 and M6 Id = µnCox/2 (W/L)[Vgs-Vth]

2 15.7 ×10

-6 = 2.025×10

-4/ 2 (W/L) [0.56−0.42]

2 (W/L) = 7.9 (W/L)5 = (W/L)6 =7.9 STEP III After finding all NMOS transistors, we found The W/L ratio of M1, M2, M3 and M4. Which all are PMOS type Id = µn Cox/2 (W/L) [Vgs-Vth]

2 15.7 ×10

-6 = 0.0011×0.0045 / 2 (W/L)[−0.42 + 0.56]

2 (W/L) = 32.36 (W/L) 1 = (W/L) 2 = (W/L) 3 = (W/L) 4 =32.3 The respective aspect ratio values of all MOSFETs are shown in Table 1.

Table 2 Aspect Ratio

MOS Aspect Ratio

M9,M10 2.7

M7,M8 3.9

M5,M6 7.9

M1,M2,M3,M4 32.36

4. SIMULATION RESULT The CMOS Telescopic Operational Trans-conductance amplifier is simulated on LT-Spice software for a 180nm

Technology for obtaining different parameter such as UGB (unity gain bandwidth), Gain, Phase margin, etc.

These parameters shown below. Simulated results waveforms 4.1 Gain & Unit Gain Bandwidth The gain obtained for this telescopic operational Trans-conductance Amplifier is about 136dB. The unity Gain

bandwidth is 80 MHz. As per good UGB this system is quite accurate.

Figure 3: Gain of Telescopic OTA

4.2 Phase Margin The Phase Margin of OTA is 81 degree. Any of OTA requires the PM of minimum 60 degree to make system

stable. Less than 40 degree it causes problems by ringing effects at output.





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Figure 4: Phase Margin of Telescopic OTA

4.3 CMRR

Common mode Rejection Ratio (CMRR) is defined as the ratio of differential gain to common mode gain. CMRR

of this OTA is 176 dB.

Figure 5: CMRR of Telescopic OTA

Table 3 Summarized result of Telescopic OTA

Specification Results

Technology 180nm

UGB 80MHz

Supply Voltage(+ VDD,VSS) 1.8V

Gain 136dB

CMRR 176dB

Phase Margin 81dB

5. CONCLUSIONS In this paper, the basic concept of different OTA is described along with its advantage and dis-advantage. The

telescopic OTA is designed for a 180nm technology with the help of LT-Spice Orcad simulator. The unity gain

bandwidth achieved for the design is 80MHz, the gain is 136dB and Phase margin is of 81dB. Also the CMRR

is of 176dB.

6. FUTURE SCOPE

There is scope to improve gain further for enhanced output. For this purpose telescopic topology can be used as

gain boosting technique. Also there is improvement require in post layout work.

ACKNOWLEDGEMENTS I feel obliged to take this opportunity to thank the Professor Dr. Kehul A. Shah, Sankalchand Patel College of

engineering, Visnagar, for taking a keen interest and providing a base, encouragement in my project work. Also,

I would like to thank the teaching and non-teaching staff of SPCE, Visnagar for encouraging this research work.





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