292 3.3V Folded Cascode OTA

292 : 3.3V Folded Cascode OTA

Design render

TTGF0P3_LPCAS_FC_OTA

Overview

TTGF0P3_LPCAS_FC_OTA is a low-power Folded Cascode Operational Transconductance Amplifier (OTA) designed in the TTGF0P3 process. The amplifier is optimized for low-current operation while maintaining a large input common-mode range and output swing, making it suitable for a wide variety of analog signal-processing applications.

Key Features
  • Folded Cascode architecture
  • Low power consumption (~15 µA typical)
  • Typical Unity Gain Bandwidth (UGB): ~100 kHz
  • Wide input common-mode range
  • Output swing approximately 0.5 V to 3.0 V
  • Single-ended output
  • Bias current programmable through external resistor
  • Stable for capacitive loads up to approximately 50 pF
  • Suitable building block for amplifiers, filters, DAC buffers, sensor interfaces, and analog computation blocks

Architecture and Circuit Operation

Folded Cascode Topology

The OTA uses a PMOS differential input pair combined with an NMOS folded-cascode structure.

Input Stage

The differential pair consists of:

  • VINP (positive input)
  • VINN (negative input)

The differential pair converts the input voltage difference into differential currents.

Advantages of using a PMOS input pair:

  • Improved operation near ground
  • Larger usable common-mode input range
  • Lower flicker noise contribution

Folded Branch

The differential currents generated by the PMOS input pair are folded into NMOS devices.

This folded structure provides:

  • High gain
  • Increased output resistance
  • Better headroom compared to telescopic cascode architectures
  • Operation at relatively low supply voltages

The folded nodes transfer the signal current from the input stage into the output branch while preserving gain.

Cascode Gain Enhancement

Multiple cascode devices are used throughout the signal path.

Benefits include:

  • Increased output resistance
  • Increased intrinsic gain
  • Better PSRR
  • Improved isolation between stages

The gain enhancement is achieved without requiring additional gain stages, simplifying frequency compensation.

Current Mirror Load

The folded currents are mirrored and combined to produce a single-ended output at:

  • VOUT

The current mirrors perform:

  • Differential-to-single-ended conversion
  • Gain generation
  • Bias current distribution

Bias Network

The OTA uses an externally generated bias current.

The user must connect:

  • A resistor between VDD and VBIAS

Recommended value:

  • 2 MΩ

This resistor generates the reference current which is mirrored throughout the OTA.

Typical operating current:

  • ~15 µA

Changing the resistor changes the OTA bias current:

Bias Resistor Approximate Effect
2 MΩ Nominal operation
1 MΩ Higher bias current
500 kΩ Maximum recommended bias current

Increasing bias current generally results in:

  • Higher bandwidth
  • Higher slew rate
  • Higher power consumption

Pin Description

Pin Name Description
ua[0] VINN Inverting input
ua[1] VINP Non-inverting input
ua[2] VBIAS External bias resistor connection
ua[3] VOUT OTA output

Power Supply

Recommended operating supply:

Parameter Value
VDD 3.3 V
GND 0 V

Typical Connections

Biasing

Connect:

  • 2 MΩ resistor between VDD and VBIAS

Example:

VDD
 |
[2 MΩ]
 |
VBIAS

Capacitive Load

Maximum recommended capacitive load:

  • 50 pF

For characterization purposes:

  • External capacitor ≈ 20 pF
  • Probe/parasitics ≈ 30 pF

Total load:

CL ≈ 50 pF

Important Notes

Do NOT
  • Short VOUT directly to GND
  • Exceed recommended capacitive loading
  • Leave VBIAS floating
  • Apply input voltages outside supply rails
Recommended
  • Set current limits on laboratory SMUs
  • Start with a 2 MΩ bias resistor
  • Verify supply current before performing measurements

Basic Functional Tests

1. Voltage Follower (Buffer)

Connect:

VINP = Input Signal
VINN = VOUT

Expected:

  • Gain ≈ 1
  • Demonstrates stability and bandwidth

2. Inverting Amplifier

Connect:

           Rf
VOUT <----/\/\----+
                  |
                  |
                 VINN
                  |
                 Rin
                  |
                VIN

Gain:

A_v = -Rf/Rin

3. Non-Inverting Amplifier

Gain:

A_v = 1 + Rf/Rg

Useful for:

  • Sensor signal conditioning
  • Analog front ends

4. Comparator Experiment

Operate OTA open-loop:

VINP > VINN  → VOUT rises
VINP < VINN  → VOUT falls

Useful for:

  • Offset measurements
  • Switching behavior characterization

Advanced Characterization Tests

5. Open-Loop Gain Measurement

Measure:

  • DC gain
  • Gain roll-off
  • Dominant pole

Useful plots:

  • Magnitude response
  • Phase response

6. Unity-Gain Stability

Configure as a voltage follower.

Measure:

  • Phase margin
  • Settling time
  • Ringing

7. Common-Mode Sweep

Sweep:

VINP = VINN

Measure:

  • Input common-mode range
  • Output swing limits

8. Output Swing Measurement

Sweep differential input slowly and record:

VOUT_MIN
VOUT_MAX

Expected output swing:

~0.5 V to ~3 V

9. Monte Carlo Analysis

Characterize:

  • Offset voltage
  • Gain variation
  • GBW variation
  • Bias current variation

Useful for mismatch verification.

10. Process Corner Verification

Run:

  • TT
  • FF
  • SS
  • FS
  • SF

Verify:

  • Gain
  • UGB
  • Phase Margin
  • Current Consumption

11. Temperature Sweep

Recommended:

-40°C
25°C
85°C
125°C

Measure:

  • Offset drift
  • Gain drift
  • Bandwidth variation

12. PSRR Measurement

Inject ripple into:

  • VDD

Measure:

PSRR+ = ΔVDD / ΔVOUT

Useful plots:

  • PSRR vs Frequency

13. CMRR Measurement

Apply common-mode excitation:

VINP = VINN

Measure:

  • Common-mode gain
  • Differential gain

Compute:

CMRR = Ad / Acm

14. Noise Analysis

Measure:

  • Input referred noise
  • Output noise density
  • Integrated RMS noise

Useful for sensor applications.

Practical Application Examples

This OTA can be used as the active element in many analog circuits.

Filters

Active Low-Pass Filter

Applications:

  • Sensor conditioning
  • Audio filtering
  • Anti-aliasing
Active High-Pass Filter

Applications:

  • DC removal
  • AC coupling
Band-Pass Filter

Applications:

  • Communication systems
  • Tone detection
State Variable Filter

Applications:

  • Analog signal processing
  • Tunable filtering

Data Converters

R-2R DAC Buffer

Use the OTA as:

  • Output buffer
  • Reconstruction amplifier

Benefits:

  • High input impedance
  • Improved DAC drive capability
SAR ADC Front-End Buffer

Applications:

  • Sample-and-hold driving
  • Capacitive DAC buffering

Sensor Interfaces

Suitable for:

  • Temperature sensors
  • Resistive sensors
  • Photodiodes
  • Capacitive sensing systems

Signal Conditioning

Applications:

  • Instrumentation amplifiers
  • Differential-to-single-ended conversion
  • Analog preprocessing

Peak Detector

Build:

  • Precision peak detector
  • Envelope detector

Applications:

  • AM demodulation
  • Signal monitoring

Integrator

Applications:

  • Analog computation
  • PID controllers
  • Sigma-delta loops

Differentiator

Applications:

  • Edge detection
  • High-frequency enhancement

Oscillator Building Block

Can be used in:

  • Wien bridge oscillators
  • Quadrature oscillators
  • Relaxation oscillators

Current-to-Voltage Converter

Useful for:

  • Photodiodes
  • Current-output sensors
  • Electrochemical sensors

Suggested Characterization Results to Include

The following plots are highly recommended for documentation:

DC Characterization

  • Transfer curve
  • Input common-mode sweep
  • Output swing plot

AC Characterization

  • Open-loop gain
  • UGB
  • Phase margin

Transient Characterization

  • Step response
  • Settling behavior
  • Slew rate

Statistical Verification

  • Monte Carlo offset histogram
  • Gain distribution
  • Current distribution

PVT Verification

  • Corner sweeps
  • Temperature sweeps

Supply Rejection

  • PSRR vs frequency

Common Mode Performance

  • CMRR vs frequency

Noise

  • Input referred noise
  • Output noise density

Typical Operating Conditions

Parameter Typical Value
Supply Voltage 3.3 V
Bias Resistor 2 MΩ
Current Consumption ~15 µA
Capacitive Load ≤ 50 pF
Output Swing ~0.5 V to 3.0 V
Topology Folded Cascode OTA
Output Type Single Ended
UGB ~100 kHz

Authors

  • Nithin P
  • Pranay Patil
  • LPCAS Group, IIT Gandhinagar

This OTA is intended as a reusable low-power analog building block for educational, research, and mixed-signal integrated circuit design projects.

IO

#InputOutputBidirectional
0
1
2
3
4
5
6
7

Analog pins

uaPCB PinInternal indexDescription
0B612VINN
1B713VINP
2B814VBIAS
3B915VOUT

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