A Folded Cascode Circuit

Summary

The video script delves into the intricacies of cascode circuits, focusing on their unique construction and applications. It starts with a basic cascode setup, explaining how the transistors interact to control current flow and node voltage. The discussion then shifts to using a p-FET cascode to manage headroom constraints, a technique crucial for high-gain amplifiers. The script further explores the folded cascode differential amplifier, a topology that optimizes headroom and output resistance, making it a staple in analog circuit design. The video aims to provide a deep understanding of cascode circuits for those interested in analog electronics.

Takeaways

• 📚 The cascode circuit is a configuration where a transistor's source is connected to the gate of another transistor, typically used to improve performance.
• 🔄 The core function of a cascode is to allow current to flow through while fixing the voltage at a particular node, enhancing stability.
• 🔬 In a simple cascode, the overall transconductance is determined by the lower transistor (m2), with the output resistance influenced by both transistors (m2 and m3).
• 🤔 The concept can be challenging when considering a pFET cascode, where the source of the pFET is used to fix the node, requiring a current source for balance.
• 🔋 The cascode structure can address headroom constraints by pinning voltages near the power supply rails, which is crucial for circuit design.
• 🔄 The cascode configuration results in the same small-signal circuit regardless of whether nFETs or pFETs are used, assuming ideal current sources.
• 🎚️ The fold cascode differential amplifier is a specific topology that uses pFETs to cascode voltages, optimizing for headroom and maintaining symmetry.
• 🔗 The cascode amplifier is particularly useful in high-gain applications, often paired with transconductance amplifiers in various circuit designs.
• 🛠️ Current mirrors are an alternative to cascode structures, providing a different approach to managing current flow and headroom in circuit design.
• 🔄 The diode connection structure in cascode circuits provides a symmetrical current mirror, contributing to the overall balance and performance of the amplifier.

Q & A

What is a cascode circuit?

A cascode circuit is an electronic circuit configuration that combines a common gate amplifier with a common source amplifier. It is used to increase the output impedance and improve the overall performance of the circuit.

What is the primary purpose of using a cascode configuration?

The primary purpose of using a cascode configuration is to fix a specific node in the circuit, which helps in controlling the current flow and enhancing the output resistance.

How does a cascode circuit affect the transconductance of a system?

In a cascode circuit, the transconductance of the system remains relatively the same as the base transistor, represented by 'gm2' in the script, but the output resistance is increased due to the addition of the cascode transistor.

What is the significance of the source voltage in a cascode circuit?

The source voltage in a cascode circuit is significant because it helps in fixing the node, which is crucial for the operation of the cascode configuration. It allows the current to flow through while maintaining control over the node voltage.

Why might one consider using a pFET cascode?

Using a pFET cascode can be beneficial for fixing a voltage near the opposite power supply rail, which can help in addressing headroom constraints that might be challenging in certain circuit designs.

What is the role of the current source in a cascode circuit?

In a cascode circuit, a current source, such as transistor 'm4' mentioned in the script, is used to balance the currents through the transistors 'm2' and 'm3', ensuring proper operation and maintaining the desired node voltage.

How does the cascode circuit impact the small signal circuit?

The cascode circuit impacts the small signal circuit by providing the same small signal behavior regardless of whether an nFET or pFET is used, assuming a perfect current source, which helps in maintaining consistency in the circuit's performance.

What is the Folded Cascode differential amplifier and how is it related to the cascode circuit?

The Folded Cascode differential amplifier is a specific circuit topology that uses cascode configurations to pin voltages near the power supply rail. It is related to the cascode circuit as it employs the same principles of fixing nodes and controlling current flow for improved performance.

Why is the Folded Cascode differential amplifier topology useful?

The Folded Cascode differential amplifier topology is useful because it allows for better headroom management by moving the lower node up and down, which is essential for maintaining the operation of the main differential pair under various voltage conditions.

How does the cascode circuit contribute to high gain amplifiers?

The cascode circuit contributes to high gain amplifiers by providing a high output resistance, which, when combined with the differential pair and current mirror structures, results in a very effective high gain amplifier topology.

Outlines

00:00

🔬 Understanding the Cascode Circuit

The paragraph discusses the concept of a cascode circuit, starting with a basic structure that includes a cascode transistor over another transistor. The core idea is that the cascode transistor (m3) is used to control the source voltage of the lower transistor (m2), effectively 'fixing' the node voltage. This configuration allows current to flow while maintaining control over the node voltage, which is crucial for the circuit's operation. The paragraph then explores the idea of using different types of transistors, such as NFET and PFET, in a cascode configuration, and how this can affect the current flow and output resistance. The discussion also touches on the use of current sources to balance the circuit and the implications for headroom constraints, which are critical when designing circuits that operate close to the power supply rails. The Folded Cascode Differential Amplifier is introduced as a specific application of these concepts, where the cascoding is used to manage voltage levels and improve the amplifier's performance.

05:01

🔄 Common Applications of Cascode and Transconductance Amplifiers

This paragraph highlights the widespread use of cascode and transconductance amplifier topologies in various electronic designs. It suggests that these two configurations are fundamental and frequently revisited in different applications due to their versatility and effectiveness in managing signal amplification and control. The paragraph implies that these topologies are foundational in the field of electronics, often serving as the building blocks for more complex circuits.

Keywords

💡Cascode Circuit

A cascode circuit is a configuration in which a transistor's source or gate is connected to another transistor's drain or source. This setup is used to improve the gain and bandwidth of an amplifier. In the video, the speaker discusses the cascode circuit's role in fixing a node's voltage, which is essential for understanding the circuit's operation. The script mentions a simple cascode structure with a cascode transistor over another transistor, highlighting the importance of the cascode in controlling the voltage at node V1.

💡Transconductance

Transconductance (gm) is a measure of how much current a transistor can provide for a given change in voltage. It is a key parameter in the design of amplifiers. The video script refers to the transconductance of the transistors (gm2) in the cascode circuit, emphasizing its role in determining the overall system's transconductance, which is crucial for the amplifier's performance.

💡Output Resistance

Output resistance is a measure of how much the output voltage of an amplifier changes with changes in load. A lower output resistance is desirable for better driving capability. The script discusses how the cascode configuration affects the output resistance, with the speaker explaining that the output resistance is the combination of r02 and r03 from the top transistor, which is a critical aspect of the circuit's design.

💡NFET and PFET

NFET (N-channel Field-Effect Transistor) and PFET (P-channel Field-Effect Transistor) are types of transistors used in CMOS technology. NFETs allow current flow when the gate is more positive than the source, while PFETs allow current flow when the gate is more negative. The video script explores the idea of cascoding an NFET and a PFET, which is an unconventional approach that can help address headroom constraints in circuit design.

💡Headroom Constraints

Headroom constraints refer to the limitations on the voltage levels that can be used in a circuit due to the power supply rails. The speaker in the video uses the cascode circuit to address these constraints by fixing voltages near the power supply rails, which is a practical application of cascode circuits in overcoming design challenges.

💡Folded Cascode

A folded cascode is a variation of the cascode circuit that uses both NFETs and PFETs to improve the output resistance and gain of the amplifier. The video script describes the folded cascode differential amplifier, which is a specific topology that uses cascoding with PFETs to pin voltages near VDD, showcasing the versatility of cascode circuits in amplifier design.

💡Differential Pair

A differential pair is a pair of transistors that are connected in such a way that their outputs are opposites of each other. This configuration is used in differential amplifiers to amplify the difference in input signals. The script mentions a differential pair in the context of the folded cascode amplifier, highlighting its importance in the overall circuit topology.

💡Current Mirror

A current mirror is a circuit configuration that replicates the current in one branch to another, maintaining a constant current regardless of the load. The video script discusses using current mirrors in cascode circuits, where NFETs act as current mirrors, which is a clever way to maintain symmetry and ensure consistent current flow in the circuit.

💡Diode Connection

A diode connection refers to the configuration of a transistor where its gate is connected to its source, making it act like a diode. This is used in current mirrors to ensure that the current through the transistor is constant. The video script mentions a diode connection structure for the current mirror, which is an innovative way to achieve a high gain amplifier.

💡Transconductance Amplifier

A transconductance amplifier is a type of amplifier that converts input voltage to output current, with its gain determined by the transconductance of the input transistor. The script compares the cascode circuit to the transconductance amplifier, noting that both are commonly used topologies in analog circuit design.

Highlights

Introduction to cascode circuits and their basic structure.

Explanation of the core concept of a cascode circuit, which is to fix a node's voltage.

Discussion on the qualitative understanding of current flow and node fixing in cascode circuits.

Analysis of the transconductance and output resistance in a simple cascode circuit.

The surprising effect of cascoding a pFET and the confusion it causes.

How using the source of a pFET to fix V1 can be a starting point for understanding cascode circuits.

The necessity of a current source to balance the currents in a cascode circuit.

The revelation that both nFET and pFET cascode circuits result in the same small-signal circuit.

The practical application of cascode circuits to fix voltages near the opposite power supply rail.

The importance of output resistance in cascode circuits and how it combines with other resistances.

The Folded Cascode differential amplifier topology and its unique characteristics.

The role of headroom constraints in the design of cascode circuits.

The use of current mirrors in cascode circuits and their benefits.

The innovative topology of using nFETs as current mirrors in cascode circuits.

The significance of symmetry in current mirror design and its impact on circuit performance.

The high gain amplifier achieved through the use of cascode circuits and their widespread application.

The prevalence of cascode and transconductance amplifier topologies in various circuit designs.

Transcripts

so i'd like to talk about a cascode

circuit and particularly like to talk

about some unique ways of building a

cascode circuit

so if you might think about a very

simple casco circuit you would think oh

i would be starting with a structure

like this

which has a cascode transistor

over a particular say a second

transistor

and remember what the core of a cascode

computation is about it's about this

transistor m3 is cascoding m2

because it's making its source voltage

actually you know sort of cascoding this

v1 node and the whole point of a cascode

is to sort of allow that current to go

through

while fixing that note or or more fixing

that node is maybe a more precise way to

do it but if you think qualitatively as

fixing that node

that gets you in the right sort of

framework or mindset of thinking about

what you're doing here

and of course if you look at this and

you were to say from the input to the

current

it still looks like i have basically the

same transistor i have a certain

transconductance overall for this system

just relatively just gm2

but then i also then have an output

resistance

that is what i would expect is the

output resistance of this element

and then i get gs3 r03

from the top transistor

you kind of see that there's sort of a

pattern to this

well an interesting thing happens

i can you say well imagine i have an

nfed as a cascode no one has any trouble

but the meaning you say is imagine i

were to take a p fed and cascode that

node v1

and immediately your brain gets a little

confused

you're like okay see there's still this

vm transistor everything is as i would

expect but i'm now going to use the

source of the p-fet

to fix v1 and you think

okay that is exactly the right rough

starting point

and you can see that you get some

current coming through there

but i that still kind of mine bends a

little bit and what you need to be able

to do to make this work is you have to

have some sort of current source maybe

this transistor m4

to kind of balance things so i'm going

to get some current through m2 and some

currents for m3

and but it still turns out if you look

at the two of these you have exactly the

same small signal circuit

exactly the same well of course i'm

assuming m4 is a perfect current source

if not then you've got to put that into

it as well

and what it allows you to do why would i

ever do it with this well it allows me

to then fix say a voltage near the

opposite power supply rail and allows me

to work through some headroom

constraints that otherwise might be

difficult

so in this case my output resistance

i can do say still say this transistor

is cascading this node which means i'm

going to get a gs3 r3

as i come up here

and then as i look at this node i've got

well i've got a output resistance r02

and r04

so i can immediately just sort of say oh

yes my output resistance is this nice

combination

and you might say well that's an

interesting single circuit but i don't

see why i would use that very often and

in this particular form we don't use it

very often

but it is very much tied

to one particular circuit topology which

is called a folda casco differential

amplifier and sometimes it's drawn this

way there's some slight shifts of how

people draw this but the core of it is i

still have a differential pair and this

would look very familiar if you looked

at differential pairs before

and i have then what i'm going to do is

i'm going to casco these two voltages

with p-feds

m5 and m6

to be able to pin these voltages near

vdd

why would i use such a structure well it

turns out for headroom reasons

that this lower node is going to get

moved up and down by

the larger v1 and v2 that forces that i

have to have some amount of voltage

across m1 and m2 to keep it in

saturation

so i have some very interesting headroom

constraints

if i don't move the voltage off of out

of this sort of main differential pair

now one way to do that would be to just

use current mirrors and that's a really

useful topology

but another way to do it is to actually

just build current sources and then as a

result pull that current

uh the change of the current into some

other loops and so i have my casco

devices and then i have n fets

acting as a current mirror and actually

a very curious topology where i take the

two devices

the end fat part gets

set up as a cascode and then i end up

taking the output of this node and i

sort of do the diode connection

structure for the current mirror on that

side of it

which nicely says let me get a nice

symmetry in terms of my current mirror

on this side and then of course i get a

pretty good structure on the back end

and in fact this works out very well and

gives me a very nice high gain amplifier

and we see this get used all over the

place in many many different topologies

between this and the transconductance

amplifier topology these are really the

ones you see used again and again in so

many different places