Integrated circuits efficiently enable individuals to process signals quickly and affordably when interacting with nature. Despite the fact that digital circuits are the stars of information processing, integrated circuits constantly struggle with the issue of how to gather data from the natural world. In this context, analog integrated circuits hold a steadfast position. Digital codes that are either 0 or 1 are used in digital circuits. They have a greater internal capacity and higher anti-interference capabilities, and they can execute operations swiftly and effectively with the help of numerous algorithms. Nature, on the other hand, uses analog signals, which must first be converted through analog circuitry into digital signals before being analyzed digitally.
Analog circuits must think about how to achieve more precise voltage, current, and charge values while transmitting analog signals in the natural world. For instance, the most popular link between natural and digital activities is an analog-to-digital converter. The oscillator is used to provide a frequency-stable clock signal to drive other digital or analog circuits. It combines the particle energy of nature., sound energy, etc. are accurately and quickly converted into the corresponding digital code, which is convenient for subsequent signal processing. The power management chip can provide reasonable power distribution and stable voltage that is not affected by factors like temperature and technology. Numerous additional functional modules are also present in analog integrated circuits. Analog integrated circuits are the best option for scientific research and engineering when it comes to the processing and creation of continuous signals.
Transistor
Utilizing transistors’ electrical characteristics allows analog integrated circuits to carry out these robust operations. You must have some familiarity with transistor operation in order to comprehend this. The MOSFET device’s gate voltage controls the drain-to-source current because it is a four-port device. Using an N-type MOS device as an example, the MOSFET device is turned on or off from the drain to the source when the gate voltage is at an absolute high level or low level. By using this principle in digital circuitry, digital signals can be transmitted and operated. However, a MOS device will experience a cut-off area, a saturation region, and a linear region if the gate voltage value fluctuates consistently from high to low. In the saturation zone, the device’s gate voltage, current between the drain and source, and I-V characteristics are as follows:
The transistor exhibits a higher transconductance, or higher gain, when this property is present. To obtain precise signal control while constructing an analog circuit, a substantial gain and feedback system are frequently required.
Bias Circuit and Reference Source
For bias and load, the majority of analog circuit blocks require current or voltage sources. Due to its high output resistance, the MOS tube’s drain output in the saturation zone can be viewed as the output of a current source by connecting the MOS tube’s gate to a constant bias voltage. A straightforward current source, however, frequently falls short of people’s expectations for performance in real-world applications, such as higher output resistance and a wider swing.
A bandgap reference circuit implementation is shown in the image below. The following is the expression for its output voltage:
A stable reference is often involved in analog circuits. For example, in a comparator, the stability of the reference voltage directly determines the accuracy of the comparison result.
Operational Amplifier
One of the most crucial components in an analog integrated circuit is an operational amplifier. A single operational amplifier can boost the weak signal at the input end, provide high gain, and simplify the processing of succeeding circuits. More significantly, the amplifier and the feedback network can combine to create a particular function module in the feedback system that can carry out operations like addition, subtraction, differentiation, and integration. An ideal amplifier in an analog circuit features infinite input impedance, infinitely low output impedance, virtual short and virtual break at the input end, among other properties. In analog integrated circuits, differential amplifiers represent a fairly diverse spectrum of amplifier architectures. They benefit from reduced bias requirements, higher output effective swing, enhanced linearity, and suppression of common-mode noise. The performance indicators of the module define the design structure, transistor size, power consumption speed, etc. in the actual design and application of operational amplifiers. The following indicators should normally be taken into account:
(1) Open-loop low-frequency gain: refers to the magnification of the amplifier under low-frequency conditions.
(2) Frequency characteristics: including 3dB bandwidth – the frequency at which the amplifier gain drops to 3dB of the low frequency gain, unity gain bandwidth – the frequency at which the gain drops to unity gain, and gain bandwidth product – the product of the low frequency gain and the 3dB frequency.
(3) Input resistance and output resistance: Generally, the input resistance can be regarded as infinite, and the output resistance should be impedance matched according to the driving load.
(4) Input offset voltage: The additional differential voltage at the output terminal due to the internal mismatch of the amplifier is generally equivalent to the input terminal for consideration.
(5) Input common-mode range: the common-mode input voltage range to ensure that the transistors inside the amplifier are in the saturation region.
(6) Common Mode Rejection Ratio: The amplifier’s ability to suppress changes in common mode voltage amplification.
(7) Power supply rejection ratio: the amplifier’s ability to suppress the amplification of changing power supply voltages.
Sleeve cascode amplifiers have a larger gain (approximately 60dB), but at the cost of slower speeds, a reduction in common-mode input range, and an increase in output swing. In addition to being able to give better gain and an enhanced output swing when compared to the conventional sleeve structure, the folded cascode structure also resolves the input common mode range limitation. However, this improvement comes at the cost of increased power consumption. Additionally, utilizing a two-stage amplifier and adding an output stage can also successfully raise the gain, but in order to increase the area and make the amplifier stable while operating, a compensating capacitor must be designed. As a result, there isn’t a perfect solution for operational amplifier design. The designer must be proficient in circuit analysis theory, comprehend the benefits and drawbacks of each construction, and make judicious decisions based on requirements.
The frequency characteristics of the amplifier must also be taken into account by designers in addition to its static specifications. The output signal will have a phase delay compared to the input signal whether driving a capacitive load, taking into account the parasitic capacitance of the transistor itself, or in the case of cascaded multi-stage amplifiers. if the feedback network’s amplifier is impacted by the delay. The entire system will be unstable in a negative feedback functioning state. This is the stable situation:
Analog Circuit System Structure
Through the feedback network, operations like addition, subtraction, multiplication, integration, and differentiation can be accomplished using an amplifier with adequate performance and stability as a module. The input and output impedance of the amplifier will alter in different ways as a result of various feedback techniques. It is important to remember that the stability and low frequency gain of the amplifier directly affect the overall performance after feedback. The slightly sophisticated systems, such as switched capacitor circuits, low dropout linear regulators, comparators, and other structures, must thus be designed on the basis that the modular design satisfies the criteria. It is also vital to take into account whether the clock regulates the internal modules, system-level static and dynamic faults, and the overall power consumption area in larger systems like analog-to-digital converters and power management processors. Each layer’s design meticulously complies with the specifications, ensuring the final analog chip’s high performance.
Layout Design
After the circuit is created, a layout must be drawn to represent the actual physical chip. Therefore, when designing the plan, it is necessary to consider the non-ideal aspects brought on by parasitic parameters. When compared to digital signals, analog integrated circuit chips are more susceptible to minor outside factors like interference. Assuming a fully differential circuit, offsets will be introduced due to the asymmetry of the differential circuit’s parts. The components’ locations are not the only area where the asymmetry is apparent; important trace surroundings also exhibit it. Redundant components are frequently positioned near passive components, like as resistors and capacitors, to isolate them and create a steady environment.
Simulation and Models
The device’s features are getting smaller and smaller as a result of technological advancements, and its electrical characteristics are becoming more complex. Based in large part on the simulation results of the simulator and the device model in the process library, engineers can confirm the accuracy of the planned circuit. The simulator can check whether an engineer’s instructions for debugging a circuit match their calculations, confirming the viability of the hypothesis.
Summary
Complex system engineering is required for the design of analog integrated circuits, and designers are held to rigorous standards. Designers must have a thorough understanding of devices, circuits, systems, and models in addition to having strong fundamental skills.
