KONERU LAKSHMAIAH EDUCATION FOUNDATION

(Deemed to be University estd, u/s, 3 of the UGC Act, 1956)

(NAAC Accredited “A++” Grade University)

Green Fields, Guntur District, A.P., India – 522502

Department of Electronics and Communication Engineering

(DST - FIST Sponsored Department)

B.Tech. III ECE PROGRAM

A.Y.2023-24 ODD, Semester-I
21EC3061 Analog VLSI Design
CO2

Session 7: Simple current mirror, multi current mirror

1. Course Description (Description about the subject)

Analog VLSI Design is a course offered to create students who are academically and
practically skilled in designing analog circuits which are required as building blocks for the
real time applications. During this course, the students will learn about the process of
analyzing the different parameters of a circuit by varying different loads and different
configurations. This course provides insights to amplifiers, Op-amps and mixer circuits.

2. Aim

To learn the basics of current mirror and analyze their behavior while acting as load in analog
circuits

3. Instructional Objectives (Course Objectives)

 Apply the fundamentals of MOS transistors for the design of single stage amplifiers.
To learn CMOS process technology.
 Realize the active & passive current Mirrors and analyze the differential amplifiers
with qualitative and quantitative approaches.
 Analyze the CMOS Op Amps, and various types of Op Amps with qualitative and
quantitative approaches.
 Analyze the high frequency response of CS, CG and CD amplifiers and noise analysis
of various amplifiers and mixers.
 Design and analysis of various MOS analog circuits using Cadence/ LT-SPICE/
QSIM environment for real time applications
4. Learning Outcomes (Course Outcome)
CO2: Realize the active & passive current Mirrors and analyze the differential amplifiers
with qualitative and quantitative approaches.

5. Module Description (CO-2 Description)

Realize the active & passive current Mirrors and analyze the differential amplifiers with
qualitative and quantitative approaches.
6. Session Introduction

What is Current Mirror?

A Current Mirror is a circuit designed to copy current from a reference current source to other
parts of the circuit.
Current Mirror is used

To Bias current in circuits (i.e. fixing the current in circuit by copying it from reference current
source)
For processing signals i.e. it can be used to amplify the signal (current being ‘copied’ can be
sometimes a varying current signal)

As Active Load (component made of active devices such as transistors, having a high small

signal impedance yet not requiring a large DC voltage drop like large resistor)

Why current mirrors?

Current mirror can be used to model a more realistic current source since no ideal current
source exist.
As current source’s output current depends on many parameters i.e. it’s output resistance,
capacitance, temperature, etc.
So to bias MOSFET as stable current source, Current Mirror is used.

7. Session Description:

Application of Current Source

Fig. 1. Application of current source in current mirrors

MOSFET as Current Source
• MOSFET has to biased in saturation region to operate as current source.
• But MOSFET output current depends on many other parameters (supply, process,
temperature, output noise and matching with another current sources).

How MOSFET is biased so as to operate as Stable Current Source??

*Assuming MOSFET in Saturation

(1)

ID depends on μn, VTH, VGS and VDS which further depend on other parameters.
For Example

(2)

Iout depends upon

• Supply (VGS α VDD),
• Process (VTH can vary by 100mV from wafer to
wafer)
• Temperature (μn and VTH)
Fig. 2. MOS Biasing
So Iout is poorly defined
If device is biased with small overdrive voltage 200mV (to consume less headroom). Small
error in VTH i.e. 50mV will result in 44% error in output current.
Also if Gate-Source voltage of MOSFET is well defined and do not depend on power
supply still it’s Drain current is not well defined.
Therefore, we must seek other methods of biasing MOS current sources.
Current Copying Concept
• Designing of Current sources in Analog circuits is based on “copying” current from a
reference, with assumption that one precisely-defined current source is already
available.
• Complex circuit is required to generate reference current source which should be
independent of power supply, temperature and process variation.
• Hence for an integrated circuit we design reference current source only once and
copy it whenever we need it. It saves a lot of hardware and power.
Generation of copies from Reference Current Source
Copying current (Iout) must be equal to reference current (IREF) i.e. Iout = IREF
 Fig. 3. Reference current generator circuit
To ensure Iout = IREF
For a MOSFET
If ID = f (VGS) then VGS = f -1 (ID) i.e.
MOSFET bias at current ID will produce G-S voltage equal to V GS That is, if a transistor is
biased at IREF then it produce VGS = f -1 (IREF)
Thus, if this voltage is applied to G-S terminal of second MOSFET, the resulting current is
Iout = f f -1 (IREF) = IREF
Result : Two identical MOS devices that have equal gate-source voltages and operate in
saturation carry equal currents (if λ = 0).
The Basic Current Mirror structure consist of M1 and M2.
In general devices need not to be identical
Neglecting Channel length modulation

(3)

(4)

Fig. 4. Current Copying Mechanism

Fig. 5. Diode connected device providing current copying mechanism

Obtaining
(5)

Observation:
 • Allows precise copying of the current with no dependence on process and
temperature.
• Ratio of Iout and IREF is given by ratio of device dimensions, a quantity that can
be controlled with reasonable accuracy.
Current Mirrors have wide applications in Analog circuits
Current Mirror used to bias Differential Pair
• Differential Pair is biased by means of an NMOS mirror for the tail current source
and PMOS mirror for load current source.
• The device dimensions establish a drain current of 0.4I T in M5 & M6 reducing the
drain current of M3 & M4 and hence increase the gain.

•
Fig. 6. Current mirrors used to bias differential amplifiers
Note: Current Mirrors usually employ the same length for all of the transistors so to
minimize error due to side diffusion of the source and drain areas (LD).

L
eff = Ldrawn – 2LD (6)

In fabrication process, we can change Ldrawn while side diffusion length LD will remain same.

If Ldrawn is double, then Leff will not get doubled.

Therefore, current rationing is achieved only by scaling the width of transistor.

Also VTH of short channel device exhibits some dependence on channel length.

Current Mirrors can also be used to process signals

If IREF increased by ∆I, then Iout increases by ∆I(W/L)2/(W/L)1

That is, the circuit amplifies the small signal current

[If (W/L)2/(W/L)1 > 1]

Note: Amplifies small-signal current but bias current also multiplies proportionally.

8. Activities/ Case studies/related to the session

Determine the current that flows in M3 of figure shown. Neglect oxide encroachment and the
finite output resistance of the M3.

9. Examples & contemporary extracts of articles/ practices to convey the idea of the
session

Example 1: "Design and Analysis of Basic Current Mirrors for Low-Power Applications"

Abstract: This article presents the design and analysis of basic current mirrors for low-power
integrated circuit applications. The focus is on achieving high accuracy and stability while
minimizing power consumption. Various biasing techniques, such as resistor biasing and
diode biasing, are discussed, and their impact on the performance of the current mirror is
analyzed. Simulation results show the effectiveness of the proposed design in achieving low
power consumption and maintaining accurate current replication.

Extract: "The basic current mirror is a fundamental building block in integrated circuits,
widely used for current replication and biasing purposes. In low-power applications, it is
crucial to design current mirrors that consume minimal power while maintaining high
accuracy. In this study, we explored different biasing techniques and their impact on the
performance of the current mirror. Simulation results demonstrated that the diode biasing
technique provided better power efficiency and improved current replication compared to
resistor biasing. The proposed design offers a promising solution for low-power applications
where accurate current replication is essential."
10. SAQ's-Self Assessment Questions

1. A current mirror is a circuit that is commonly used for:

a) Amplifying voltage signals

b) Generating reference currents

c) Reducing power consumption

d) Controlling power supply voltages

2. The basic principle behind a current mirror is to:

a) Match the current flowing through two or more branches

b) Amplify the current flowing through a branch

c) Control the voltage across a branch

d) Convert current into voltage

3. In an ideal current mirror circuit, the output current is equal to the:

a) Input current

b) Output voltage divided by the output resistance

c) Input voltage divided by the input resistance

d) Input current multiplied by the current gain

4. The most commonly used transistor configuration in a current mirror is:

a) Common-source

b) Common-drain

c) Common-gate

d) Common-base

5. The primary disadvantage of a simple MOSFET current mirror is:

a) Limited output resistance

b) High power consumption

c) Limited input resistance

d) Unstable output current

6. The cascode current mirror configuration is used to:

a) Improve output resistance

b) Improve input resistance

c) Increase power consumption

d) Increase output current

7. The Wilson current mirror is an improvement over the basic MOSFET current mirror as it:

a) Provides higher output resistance

b) Provides higher input resistance

c) Allows for better current matching

d) Reduces power supply voltage requirements

8. A current mirror with a cascode configuration typically consists of:

a) Two or more MOSFETs in series

b) Two or more MOSFETs in parallel

c) A MOSFET and a bipolar junction transistor (BJT)

d) A MOSFET and a voltage reference

9. In a cascode current mirror, the cascode transistor is typically operated in:

a) Saturation region

b) Triode region

c) Cutoff region

d) Reverse-active region

10. The purpose of a degeneration resistor in a current mirror circuit is to:

a) Improve output resistance

b) Improve input resistance

c) Reduce power consumption

d) Increase gain

11. The output resistance of an ideal current mirror is:

 a) Zero

b) Infinite

c) Equal to the input resistance

d) Equal to the load resistance

12. The current gain of a current mirror is defined as:

a) The ratio of the output current to the input current

b) The ratio of the output voltage to the input voltage

c) The ratio of the output resistance to the input resistance

d) The ratio of the load resistance to the input resistance

13. A Widlar current source is a type of current mirror that uses:

a) Bipolar junction transistors (BJTs)

b) Metal-oxide-semiconductor field-effect transistors (MOSFETs)

c) Junction field-effect transistors (JFETs)

d) Insulated-gate bipolar transistors (IGBTs)

14. A folded cascode current mirror configuration is used to:

a) Increase output resistance

b) Increase input resistance

c) Decrease power consumption

d) Decrease output current

15. The output current of a current mirror is affected by variations in

a) Temperature

b) Supply voltage

c) Process variations

d) All of the above

16. The body effect in MOSFET current mirrors refers to:

a) Variations in threshold voltage due to body bias

 b) Variations in the channel length due to body bias

c) Variations in the channel width due to body bias

d) Variations in the substrate doping concentration

17. The accuracy of a current mirror is often quantified by its:

a) Linearity

b) Voltage gain

c) Current gain

d) Input impedance

18. The ratio of the output current to the input current in a current mirror is often referred to
as the:

a) Transconductance

b) Transresistance

c) Transimpedance

d) Transfer ratio

19. In a Wilson current mirror, the output current is determined by the:

a) Difference in base-emitter voltages of two bipolar junction transistors

b) Difference in gate-source voltages of two MOSFETs

c) Difference in drain-source voltages of two MOSFETs

d) Difference in collector-emitter voltages of two bipolar junction transistors

20. The main advantage of a current mirror over a resistor is its ability to provide:

a) Higher output resistance

b) Lower output resistance

c) Higher input resistance

d) Lower input resistance

11. Summary
The basic concept of MOS based current mirror circuits, different biasing and small signal
modeling of the mirrors are discussed in this session

12. Terminal Questions

1 How can we reduce channel length modulation effect to have perfect copy of
current in small channel current mirror circuits?
2 Discuss the conditions that effect current copying mechanism in current mirrors.
3 How can we make a MOSFET to work as a perfect current source in saturation
region.
4 How does a basic current mirror circuit work?
5 What are some applications of current mirrors?
6 What are the trade-offs involved in the design of current mirrors?
7 What are some common techniques to improve the accuracy of a current mirror?
8 Why current rationing is achieved only while changing the width but not length of
the transistors.
9 Analytically show that two MOSFETs operating in saturation with same gate to

13. Case studies

1. Case Study: Designing a Temperature-Independent Voltage Reference Circuit

Objective: To design a voltage reference circuit that provides a stable output voltage
regardless of temperature variations.

Description: In this case study, students are tasked with designing a temperature-independent
voltage reference circuit using MOSFETs. They need to analyze different biasing techniques
such as PTAT (Proportional to Absolute Temperature) current generation and constant-Gm
biasing to generate a reference voltage that is insensitive to temperature changes. The
students select appropriate MOSFET sizes, resistor values, and biasing currents to achieve the
desired temperature stability. They simulate the circuit and evaluate its performance across a
range of temperatures to assess its temperature-independent characteristics.

2. Case Study: Analysis of Cascode Current Mirror

Objective: To analyze the performance of a cascode current mirror circuit and understand its
advantages over a basic current mirror.

Description: In this case study, students compare the performance of a basic current mirror
with a cascode current mirror. They are provided with the circuit schematics and component
values for both configurations. The students analyze the voltage and current characteristics of
each circuit and calculate parameters such as output impedance, voltage gain, and linearity.
They investigate the advantages of the cascode current mirror in terms of improved output
impedance and reduced sensitivity to variations in transistor parameters. The students
simulate the circuits and compare their performance to validate their analysis.

3. Case Study: Designing a Temperature-Compensated Differential Pair

Objective: To design a differential pair amplifier with MOS loads and active current mirrors
that maintains high performance over a wide temperature range.

Description: In this case study, students are given the task of designing a temperature-
compensated differential pair amplifier for an audio application. They need to select
appropriate biasing techniques, such as active current mirrors, to ensure stable operation and
minimize temperature-induced variations in performance. The students analyze the small-
signal characteristics, gain, and linearity of the amplifier. They simulate the circuit and
evaluate its performance across different temperature ranges to verify the effectiveness of the
temperature compensation techniques.

14 . Answers

1. b) Generating reference currents

2. a) Match the current flowing through two or more branches

3. a) Input current

4. a) Common-source

5. a) Limited output resistance

6. a) Improve output resistance

7. c) Allows for better current matching

8. a) Two or more MOSFETs in series

9. b) Triode region

10. a) Improve output resistance

11. b) Infinite

12. a) The ratio of the output current to the input current

13. a) Bipolar junction transistors (BJTs)

14. a) Increase output resistance

15. d) All of the above

16. a) Variations in threshold voltage due to body bias

17. c) Current gain

18. d) Transfer ratio

19. b) Difference in gate-source voltages of two MOSFETs

20. b) Lower output resistance

15. Glossary

Current Mirror: A Current Mirror is a circuit designed to copy current from a reference
current source to other parts of the circuit.
Current Source: MOSFET has to biased in saturation region to operate as current source
Small Signal: A small-signal model is an AC equivalent circuit in which the nonlinear circuit
elements are replaced by linear elements whose values are given by the first-order (linear)
approximation of their characteristic curve near the bias point.

16. References of books, sites, links

Text Books:

1) BehzadRazavi, “Design of Analog CMOS Integrated Circuits”, Tata Mc Graw Hill, (2005)

2) Jacob Baker, “CMOS Mixed Signal Circuit Design”, John Wiley, (2008)

Reference Books:

1) Neil H. E. Weste and David. Harris Ayan Banerjee, “CMOS VLSI Design” – Pearson
Education, 1999.

2) Gray& Mayer, “Analysis & Design of Analog Integrated Circuits”, 4th edition, Wiley,
(2001).17. Keywords

Web references:

1. https://www.coursera.org/learn/mosfet
2. https://nptel.ac.in/courses/108106068
3. https://www.coursera.org/learn/rf-mmwave-circuit-design#syllabus

17. Keywords

Current Mirror, Current replication, Biasing, Transistor matching, Load impedance,

Differential pair, MOSFET current mirror, BJT current mirror, Cascode current mirror,
Active current mirror