A REPORT ON

Design a two stage single-ended output OPAMP (Folded

Cascode [Differential amplifier + common gate stage] + gain

stage)

Shreyash Pratap Singh 2011A3PS223P

Srinivas Gopal B.V. 2011A3PS148P

Arvind Biswal 2011A3PS100P

Submitted in partial fulfillment of the course

EEE C443/ EEE F313/ INSTR F313 – Analog & Digital VLSI Design

Under the Guidance of Mr. Sachin Maheshwari

Birla Institute of Technology and Science, Pilani

August-December, 2013

1. Introduction

Operational amplifiers (op amps) are an integral part of many analog and

mixed-signal systems. Op amps with vastly different levels of complexity are used to realize

functions ranging from dc bias generation to high-speed amplification filtering.

As a part of analog assignment, in this report we present the design and

simulation of a two-stage single ended output OP AMP (Folded cascode [differential

amplifier + common gate stage] + gain stage) with the biasing circuit for the following

specifications

i. Gain ≥ 90 dB

ii. Unity Gain Bandwidth ≥ 100 MHz

iii. Phase margin ≈ 60°

2. Theoretical Expressions ( Details of Circuit realization)

3. Frequency Compensation

4. Transistor Level Schematic of the Operational Amplifier

Figure 2 Circuit Diagram of Two Stage Folded Cascode Single Ended Output

OPAMP

5. Simulation Setups and Plots

5.1. Gain and Unity-Gain Bandwidth (UGB)

Figure 3 Circuit Setup for Open Loop Gain Calculation

tt – Corner (27° C, +3.3 V Supply)

Gain = 105.4 dB

UGB = 122.6 MHz

ff- Corner (0° C , +3.63 V Supply)

Gain = 110.6 dB

UGB = 117 MHz

ss- Corner (100 0° C , +2.97 V Supply)

Gain = 88.66 dB

UGB = 61.2 MHz

5.2. Phase Margin

Figure 4 Circuit Setup for Phase Margin Calculation

The open loop phase margin was found to be 59.88° which falls in the range of stable

operation i.e. 45°-60°

Analysis to check the stability of the op-amp

Closed loop phase margin was found to be 59.23° during stability analysis which

falls within the range of stable operation i.e. 45 to 60 degrees. Consequently, the

opamp was found to operate in a stable manner during analysis at all corners

The Gain margin must be less than Phase Margin, also the Phase Margin should be

well above 50 degrees

tt – Corner (27° C, +3.3 V Supply)

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Stability Analysis ‘stb’ freq = (1Hz -> 10GHz)

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Gain margin = 9.538 dB at frequency = 207.5 MHz

Phase margin = 59.23° at frequency = 109.2 MHz

ff – Corner (0° C, +3.63 V Supply)

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Stability Analysis ‘stb’ freq = (1Hz -> 10GHz)

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Gain margin = 7.251 dB at frequency = 226.3 MHz

Phase margin = 40.64° at frequency = 145.7 MHz

ss – Corner (100° C, +2.97 V Supply)

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Stability Analysis ‘stb’ freq = (1Hz -> 10GHz)

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Gain margin = 16.98 dB at frequency = 195.2 MHz

Phase margin = 123.9° at frequency = 31.36 MHz

5.3 Input Common Mode Range

Figure 5 Circuit Setup for ICMR calculation

A DC sweep of the input to the non-inverting terminal was performed

The opamp was connected in unity gain feedback

tt – Corner (27° C, +3.3 V Supply)

ICMR: 3.2133 – 0.8439 V = 2.3691 V

5.4 Common Mode Rejection Ratio

Figure 6 Circuit Setup for CMRR Calculation

The op-amp was connected in unity gain feedback

A common mode AC signal is provided to both the inverting and non-inverting

terminals

tt – Corner (27° C, +3.3 V Supply)

CMRR = 108.9 dB

5.5 Power Supply Rejection Ratio

Figure 7 Circuit Setup for PSRR Calculation

The opamp was connected in unity gain feedback

An AC source was provided at the upper rail of the power supply

tt – Corner (27° C, +3.3 V Supply)

PSRR = 80.11 dB

5.6 Slew Rate and Settling Time

Figure 9 Circuit Setup for Settling Time and Slewrate Calculation

The opamp was connected in unity gain feedback

A Transient pulse was provided at the inverting terminal input

Slew Rate = 1.83 V/µs

Settling Time = 554.472 ms

tt – Corner (27° C, +3.3 V Supply)

5.7 Input and Output Offset Voltage

Figure 10 Circuit Setup for Offset Voltage Calculation

A DC sweep of the input to the non-inverting terminal was performed

The opamp was connected in unity gain feedback

tt – Corner (27° C, +3.3 V Supply)

Output offset voltage = 70.881 mV

Input offset voltage = 380.648 nV

5.8 Output Swing

Figure 11 Circuit Setup for Output Swing Calculation

A Transient pulse was given at both the inverting and non-inverting terminals

tt – Corner (27° C, +3.3 V Supply)

Maximum Symmetric swing = 2.9296V

5.9 Unity Gain Buffer

Figure 12 Circuit Setup as Unity Gain Buffer

The opamp was connected in unity gain configuration

An AC analysis was performed to find the gain and it was found to be

approximately 0 dB

tt – Corner (27° C, +3.3 V Supply)

The Gain was observed to be 0 dB for AC Analysis thus confirming the unity gain buffer.

A Transient analysis was also performed to confirm the unity gain buffer operation.

It was observed the output peak to peak voltage and the input peak to peak voltage were

almost equal giving a gain of approximately one, confirming the unity gain buffer operation.

Results

Op-AMP Specifications Results Gain 105.4 dB UGB 122.6 MHz Phase Margin 59.88° PSRR 80.11 dB CMRR 108.9 dB Settling Time 554.472 ms Slew Rate 1.83 V/µs ICMR 2.3691 V Output Swing 2.9295 V Power 2.8298 mW Input Offset 380.6481 nV Output Offset 70.881 mV

UGB gain 54.36 µdB