FOLDED CASCODE OPERATIONAL AMPLIFIER
Overview
The design of op amps continues to pose a challenge as the supply voltage and transistor
channel length scale down with each generation of CMOS technologies. Operational
amplifiers are the amplifier (controlled sources) that has sufficiently high gain so that when
negative feedback is applied, the closed loop transfer function is practically independent of
the gain of the op amp. This principle has been exploited to develop many useful analog
circuits and systems. The primary requirement of an op amp is to have an open-loop gain that
is sufficiently large to implement the negative feedback concept. Different architectures have
been used to obtain high gain bandwidth, output resistance and power supply rejection.
Two-stage Cascode op amps circuits are widely used in circuit designs at places where high
gain and high output impedances are required.
Folded cascode gives better performance even than the cascode op amp circuit. This
architecture has been analyzed, using CADENCE simulations. The difference between an
Operational Transconductance Amplifier (OTA) and an Operational Amplifier (Op-Amp) is
that the op-amp has got an output buffer so that it is able to drive resistive loads. An OTA can
only drive capacitive loads. Our goal is the design analysis and simulation of a High Gain
Folded-Cascode Op Amp using CMOS process in order to use it in wide applications [5 -8]
like in the design of high-order filters, signal amplifiers, analog-to- digital (A/D) and
digital-to-analog (D/A) converters, input and output signal buffers, and many more, and to
compare its some of the performance parameters with the two-stage cascode op-amp by
designing cascode op-amp for the almost same specifications.
1.1.1 FOLDED CASCODE CHARACTERISTICS
●Compared to the ordinary op amp, it has high gain.
●A single-pole operational amplifier with large output swing.
●Its small signal gain can be very large, which is very suitable for deep negative
feedback.
1.1.2 MERITS OF FOLDED CASCODE OP-AMP
1. Higher gain bandwidth for the same current as a two stage miller.
2. Higher output impedance and therefore dc gain.
