Misr International University
Faculty of Engineering
Electronics and Communications Department
Spring 2024
ECE345: VLSI Lab 3
Experiment Title: 2 Stage OP-AMP
Name: Rawan Ashraf 2019/13921
Instructor: Dr. Ibrahim Lotfy
TA: Eng. Esraa Nashaat
Table of Contents
Introduction ................................................................................................................................3
Schematic ...................................................................................................................................4
Basic Operation .......................................................................................................................4
Creating a Symbol for the Schematic...........................................................................................5
Test Bench ..................................................................................................................................5
DC Response ..............................................................................................................................7
AC Response ..............................................................................................................................8
Layout.........................................................................................................................................9
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Introduction
A two-stage operational amplifier (op-amp) is a versatile and widely used analog circuit in
electronic systems, particularly in signal processing, amplification, and filtering applications.
The two-stage design enhances the overall gain and performance of the op-amp. It typically
consists of a differential amplifier input stage followed by a gain stage (often a common-source
amplifier), and includes a compensation mechanism to ensure stability.
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Schematic
Basic Operation
Current Mirror

Purpose: The current mirror provides biasing currents to various stages of the op-amp.

Operation: It replicates a reference current (I_REF) to provide stable and consistent
biasing currents, ensuring proper operation of the transistors in the amplifier stages.
Common-Source Amplifier

Purpose: The common-source amplifier stage provides additional voltage gain to the
signal.

Operation: This stage typically follows the differential amplifier and amplifies the
voltage difference between its input and output.
Differential Amplifier (Inner Stage)

Purpose: The differential amplifier is the input stage of the op-amp. It amplifies the
difference between two input voltages (V_IP and V_IM) while rejecting common-mode
signals.

Operation: It consists of a pair of transistors whose gates are connected to the input
signals. The differential pair converts the input voltage difference into a current, which is
then mirrored and amplified by subsequent stages.
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Creating a Symbol for the Schematic
Test Bench
To verify the functionality of the two-stage op-amp, a test bench is created, which typically
includes:
Components and Connections

Differential Inputs (V_IP and V_IM): Apply differential sine wave signals to these pins
to test the op-amp's response.

Reference Current (I_REF): A current source connected to the current mirror to set the
biasing current.

Power Supply (V_DD): Provides the necessary supply voltage for the op-amp operation.
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
Ground (GND): Common ground reference for the circuit.

Output (V_OUT): The amplified output signal, often connected to a load capacitor for
stability and compensation.
Symbol and Connections
1. Differential Inputs:

VIP: Non-inverting input (positive sine wave).

VIM: Inverting input (negative sine wave).
2. Biasing:

IREF: Connect a current source to provide the reference current for the current
mirror.
3. Power Supply:

VDD: Connect to the positive supply voltage.

GND: Connect to the ground.
4. Output:

VOUT: Connect the output to a capacitor, often to ground, to provide stability and
compensation.
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DC Response
Checking if All Transistors are working correctly in region 2
1. Maximum Gain and Linearity:

Transistors in the saturation region exhibit maximum and predictable gain, crucial
for accurate signal amplification. This ensures the op-amp performs its intended
function effectively, maintaining signal integrity without distortion.
2. Stable and Predictable Performance:

Operating in saturation ensures that the transistor parameters (like
transconductance) are stable and less affected by temperature variations or
changes in operating conditions, leading to reliable and consistent circuit
performance.
3. Optimized Frequency Response:

Transistors in the saturation region have better high-frequency performance,
ensuring that the op-amp achieves its intended bandwidth and frequency
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characteristics. This is essential for applications requiring precise and stable
frequency response.
4. Minimized Distortion:

Ensuring transistors remain in the saturation region helps minimize non-linearities
and distortions in the output signal, crucial for high-fidelity analog applications
and maintaining the overall accuracy of the op-amp.
AC Response
Analyzing the AC response of a two-stage operational amplifier (op-amp) involves
understanding its frequency response, gain, bandwidth, phase margin, and stability. This is
crucial for applications where the op-amp will be used to amplify AC signals or in feedback
configurations.
Gain Plot: Shows the magnitude of gain in dB across the frequency range. Look for the midband
gain, gain bandwidth product, and any peaking which may indicate instability.
Key Observations

Low-Frequency Gain: The initial flat region of the gain plot, representing the DC gain
of the op-amp.

-3 dB Bandwidth: The frequency at which the gain drops by 3 dB from the lowfrequency gain.

Unity-Gain Frequency (f_t): The frequency at which the gain crosses 0 dB.
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Layout
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