A Folded Cascode Circuit
Summary
TLDRThe 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
🔬 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.
🔄 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.
Mindmap
Keywords
💡Cascode Circuit
💡Transconductance
💡Output Resistance
💡NFET and PFET
💡Headroom Constraints
💡Folded Cascode
💡Differential Pair
💡Current Mirror
💡Diode Connection
💡Transconductance Amplifier
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
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