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CMOS amplifiers ( complementary metal–oxide–semiconductor amplifiers) are ubiquitous analog circuits used in computers,
audio system Sound recording and reproduction is the electrical, mechanical, electronic, or digital inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording te ...
s, smartphones,
cameras A camera is an optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), with a ...
,
telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
systems, biomedical circuits, and many other systems. Their performance impacts the overall specifications of the systems. They take their name from the use of MOSFETs (metal–oxide–semiconductor field-effect transistors) as opposite to bipolar junction transistors (BJTs). MOSFETs are simpler to fabricate and therefore less expensive than BJT amplifiers, still providing a sufficiently high transconductance to allow the design of very high performance circuits. In high performance CMOS (complementary metal–oxide–semiconductor) amplifier circuits,
transistors upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
are not only used to amplify the signal but are also used as
active load An active load or dynamic load is a component or a circuit that functions as a current-stable nonlinear resistor. Circuit design In circuit design, an active load is a circuit component made up of ''active devices'', such as transistors, inten ...
s to achieve higher gain and output swing in comparison with resistive loads. CMOS technology was introduced primarily for digital circuit design. In the last few decades, to improve speed, power consumption, required area, and other aspects of digital
integrated circuits An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
(ICs), the feature size of MOSFET transistors has shrunk (minimum channel length of transistors reduces in newer CMOS technologies). This phenomenon predicted by
Gordon Moore Gordon Earle Moore (born January 3, 1929) is an American businessman, engineer, and the co-founder and chairman emeritus of Intel Corporation. He is also the original proponent of Moore's law. As of March 2021, Moore's net worth is repor ...
in 1975, which is called
Moore’s law Moore's law is the observation that the number of transistors in a dense integrated circuit (IC) doubles about every two years. Moore's law is an observation and projection of a historical trend. Rather than a law of physics, it is an empiri ...
, and states that in about each 2 years, the number of transistors doubles for the same silicon area of ICs. Progress in memory circuits design is an interesting example to see how process advancement have affected the required size and their performance in the last decades. In 1956, a 5 MB
Hard Disk Drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magne ...
(HDD) weighed over a ton, while these days having 50000 times more capacity with a weight of several tens of grams is very common. While digital ICs have benefited from the feature size shrinking, analog CMOS amplifiers have not gained corresponding advantages due to the intrinsic limitations of an analog design—such as the intrinsic gain reduction of short channel transistors, which affects the overall amplifier gain. Novel techniques that achieve higher gain also create new problems, like amplifier stability for closed-loop applications. The following addresses both aspects, and summarize different methods to overcome these problems.


Intrinsic gain reduction in modern CMOS technologies

The maximum gain of a single MOSFET transistor is called intrinsic gain and is equal to : A v_\text = g_m r_o, where g_m is the transconductance, and r_ois the output resistance of transistor. As a first-order approximation, r_ois directly proportional to the channel length of transistors. In a single-stage amplifier, one can increase channel length to get higher output resistance and gain as well, but this also increases the parasitic capacitance of transistors, which limits the amplifier
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
. The transistor channel length is smaller in modern CMOS technologies, which makes achieving high gain in single-stage amplifiers very challenging. To achieve high gain, the literature has suggested many techniques. The following sections look at different amplifier topologies and their features.


Single-stage amplifiers

Telescopic, folded
cascode The cascode is a two-stage amplifier that consists of a common-emitter stage feeding into a common-base stage. Compared to a single amplifier stage, this combination may have one or more of the following characteristics: higher input–output ...
(FC), or recycling FC (RFC) are the most common single-stage amplifiers. All these structures use transistors as active loads to provide higher output resistance (= higher gain) and output swing. A telescopic amplifier provides higher gain (due to higher output resistance) and higher bandwidth (due to smaller non-dominant pole at the cascode node). In contrast, it has limited output swing and difficulty in implementation of unity-gain buffer. Although FC has lower gain and bandwidth, it can provide a higher output swing, an important advantage in modern CMOS technologies with reduced supply voltage. Also, since the DC voltage of input and output nodes can be the same, it is more suitable for implementation of unity-gain buffer. FC is recently used to implement integrator in a bio-nano sensor application. Also, it can be used as a stage in multi-stage amplifiers. As an example, FC is used as the input stage of a two-stage amplifier in designing of a
potentiostat A potentiostat is the electronic hardware required to control a three electrode cell and run most electroanalytical experiments. A ''Bipotentiostat'' and ''polypotentiostat'' are potentiostats capable of controlling two working electrodes and ...
circuit, which is to measure neuronal activities, or DNA sensing. Also, it can be used to realize
transimpedance amplifier In electronics, a transimpedance amplifier (TIA) is a current to voltage converter, almost exclusively implemented with one or more operational amplifiers. The TIA can be used to amplify the current output of Geiger–Müller tubes, photo multipli ...
(TIA). TIA can be used in amperometric biosensors to measure current of cells or solutions to define the characteristics of a device under test In the last decade, circuit designers have proposed different modified versions of FC circuit. RFC is one of the modified versions of FC amplifier, which provides higher gain, higher bandwidth, and also higher
slew rate In electronics, slew rate is defined as the change of voltage or current, or any other electrical quantity, per unit of time. Expressed in SI units, the unit of measurement is volts/second or amperes/second, but is usually expressed in terms of m ...
in comparison with FC (for the same power consumption). Recently, RFC amplifier has used in hybrid CMOS–
graphene Graphene () is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure.
sensor array for subsecond measurement of dopamine. It is used as a low-noise amplifier to implement integrator.


Stability

In many applications, an amplifier drives a capacitor as a load. In some applications, like switched capacitor circuits, the value of capacitive load changes in different cycles. Therefore, it affects output node
time constant In physics and engineering, the time constant, usually denoted by the Greek letter (tau), is the parameter characterizing the response to a step input of a first-order, linear time-invariant (LTI) system.Concretely, a first-order LTI system is a s ...
and amplifier
frequency response In signal processing and electronics, the frequency response of a system is the quantitative measure of the magnitude and phase of the output as a function of input frequency. The frequency response is widely used in the design and analysis of s ...
. Stable behavior of amplifier for all possible capacitive loads is necessary, and designer must consider this issue during designing of circuit. Designer should ensure that phase margin (PM) of the circuit is enough for the worst case. To have proper circuit behavior and time response, designers usually consider a PM of 60 degrees. For higher PM values, the circuit is more stable, but it takes longer for the output voltage to reach its final value. In telescopic and FC amplifiers, the dominant pole is at the output nodes. Also, there is a non-dominant pole at the cascode node. Since capacitive load connected to output nodes, its value affects the location of the dominant pole. This figure shows how capacitive load affects the location of dominant pole (\omega_1) and stability. Increasing capacitive load moves the dominant pole toward the origin, and since unity gain frequency (\omega_\text) is A_v (amplifier gain) times \omega_1, it also moves toward the origin. Therefore, PM increases, which improves stability. So, if we ensure stability of a circuit for a minimum capacitive load, it remains stable for larger load values. To achieve greater than 60 degrees PM, the non-dominant pole (\omega_2) must be greater than 1.7\,\omega_\text.


Multi-stage amplifiers

In some applications, like switched capacitor filters or integrators, and different types of
analog-to-digital converter In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal. An ADC may also provide ...
s, having high gain (70-80 dB) is needed, and achieving the required gain sometimes is impossible with single-stage amplifiers. This is more serious in modern CMOS technologies, which transistors have smaller output resistance due to shorter channel length. To achieve high gain as well as high output swing, multi-stage amplifiers have been invented. To implement two-stage amplifier, one can use FC amplifier as the first stage and a common source amplifier as the second stage. Also, to implement four-stage amplifier, 3 common source amplifier can be cascaded with FC amplifier. It should be mentioned that to drive large capacitive loads or small resistive loads, the output stage should be class AB. For example, common source amplifier with class AB behavior can be used as the final stage in three-stage amplifier to not only improve drive capability, but also gain. Class AB amplifier can be used as a column driver in LCDs.


Stability in two-stage amplifiers

Unlike single-stage amplifiers, multi-stage amplifiers usually have 3 or more poles and if they are used in feedback networks, the closed loop system is probably unstable. To have stable behavior in multi-stage amplifiers, it is necessary to use compensation network. The main goal of compensation network is to modify transfer function of the system in such a way to achieve enough PM. So, by the use of compensation network, we should get frequency response similar to what we showed for single-stage amplifiers. In single-stage amplifiers, capacitive load is connected to the output node, which dominant pole happens there, and increasing its value improves PM. So, it acts like a compensation capacitor (network). To compensate multi-stage amplifiers, compensation capacitor is usually used to move dominant pole to lower frequency to achieve enough PM. The following figure shows the block diagram of a two-stage amplifier in fully differential and single ended modes. In a two-stage amplifier, input stage can be a Telescopic or FC amplifier. For the second stage, common source amplifier with active load is a common choice. Since output resistance of the first stage is much greater than the second stage, dominant pole is at the output of the first stage. Without compensation, the amplifier is unstable, or at least does not have enough PM. The load capacitance is connected to the output of the second stage, which non-dominant pole happens there. Therefore, unlike single-stage amplifiers, increasing of capacitive load, moves the non-dominant pole to lower frequency and deteriorates PM. Mesri et al. suggested two-stage amplifiers that behave like single-stage amplifiers, and amplifiers remains stable for larger values of capacitive loads. To have proper behavior, we need to compensate two-stage or multi-stage amplifiers. The simplest way for compensation of two-stage amplifier, as shown in the left block diagram of the below figure, is to connect compensation capacitor at the output of the first stage, and move dominant pole to lower frequencies. But, realization of capacitor on silicon chip requires considerable area. The most common compensation method in two-stage amplifiers is Miller compensation (middle block diagram in the below figure. In this method, a compensation capacitor is placed between input and output node of the second stage. In this case, the compensation capacitor appears 1+, A_, times greater at the output of the first stage, and pushes the dominant pole as well as unity gain frequency to lower frequencies. Moreover, because of
pole splitting Pole splitting is a phenomenon exploited in some forms of frequency compensation used in an electronic amplifier. When a capacitor is introduced between the input and output sides of the amplifier with the intention of moving the pole lowest in fre ...
effect, it also moves the non-dominant pole to higher frequencies. Therefore, it is a good candidate to make the amplifier stable. The main advantage of Miller compensation method, is to reduce size of the required compensation capacitor by a factor of 1+, A_, .The issue raised from Miller compensation capacitor is introducing right-half plane (RHP) zero, which reduces PM. Hopefully, different methods have suggested to solve this issue. As an example, to cancel the effect of RHP zero, nulling resistor can be used in series with compensation capacitor (right block diagram of the below figure). Based on the resistor value, we can push RHP zero to higher frequency (to cancel its effect on PM), or to move it LHP (to improve PM), or even remove the first non-dominant pole to improve Bandwidth and PM. This method of compensation is recently used in amplifier design for potentiostat circuit. Because of process variation, resistor value can change more than 10%, and therefore affects stability. Using current buffer or voltage buffer in series with compensation capacitor is another option to get better results.


See also

* FET amplifier * List of MOSFET applications


References

{{DEFAULTSORT:CMOS Amplifiers Electronic design Analog circuits Integrated circuits MOSFETs Transistors