Single ended input current amplifier circuits
In this tutorial, we will learn about one of the important circuits in analog circuit design: A Differential Amplifier. It is essentially an electronic amplifier, which has two inputs and amplifies the difference between those two inputs. We will see the working of a Differential Amplifier, calculate its gain and CMRR, list out some important characteristics and also see an example and an application. The Differential Pair or Differential Amplifier configuration is one of the most widely used building blocks in analog integrated-circuit design.
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Content:
- Power Op Amp
- Basic Knowledge, Circuit and Application of Operational Amplifier(op amp)
- Introduction to Ideal Op-Amp Circuit Characteristics
- A Low Power, Low Cost, Differential Input to a Single-Ended Output Amplifier
- Differential Amplifier
- Operational Amplifier Question & Answers
- Using a Differential I/O Amplifier in Single-Ended Applications
Power Op Amp
Today, digital circuit cores provide the main circuit implementation approach for integrated circuit IC functions in very-large-scale integration VLSI circuits and systems.
Typical functions include sensor signal input, data storage, digital signal processing DSP operations, system control and communications.
Despite the fact that a large portion of the circuitry may be developed and implemented using digital logic techniques, there is still a need for high performance analogue circuits such as amplifiers and filters that provide signal conditioning functionality prior to sampling into the digital domain using an analogue-to-digital converter ADC for analogue sensor signals.
The demands on the design require a multitude of requirements to be taken into account. In this chapter, the design of the operational amplifier op-amp is discussed as an important circuit within the front-end circuitry of a mixed-signal IC. The discussion will focus on the design of the op-amp using different compensation schemes incorporating negative Miller compensation and designed to operate at lower power supply voltage levels.
The simulation approach is focussed on the open-loop frequency response performance of the op-amp. Very-Large-Scale Integration. In this chapter, the focus of the discussion is on the design of the op-amp, which will act as an integral part of the on-chip analogue signal conditioning circuitry for the front-end section of a mixed-signal IC.
The performance requirements and design issues for circuit operation on a single-rail power supply and operating at 3. The op-amp architecture will be discussed, and the focus will be on the design of the compensation circuitry that will be required for amplifier stability purposes. In particular, the use of Miller and negative Miller compensation techniques, and the effects of different compensation techniques on amplifier operation, will be identified. The discussion will be supported using suitable simulation study results.
The chapter will initially consider the analogue circuit requirements before discussing op-amp design and compensation techniques. The concepts introduced and analysed will be accompanied by analogue circuit simulation results using Cadence Spectre simulator and the circuit design will be implemented using a 0. In order to provide a better understanding, the discussion will include the use of MATLAB for mathematical modelling the frequency response of the op-amp in open loop.
Today, electronic systems are embedded in everyday items such as smart phones, mobile computing, biomedical monitoring bioinstrumentation systems, entertainment systems and environmental monitoring systems.
In many cases, these systems are based on capturing sensor signals, processing and converting them to a suitable digital representation, undertaking digital signal processing DSP operations, storing values in local memory, interfacing to a user and finally providing wired or wireless communications to another electronic system. The basic idea is shown in Figure 1. The sensors can provide either analogue outputs such as voltage, current, frequency and impedance or digital outputs logic 0 and 1 levels with associated voltage values.
In general, the sensor output signals would require signal conditioning in order to create signal values that are in a suitable form to be captured by a digital processing module. This digital processing module would provide the necessary functions in hardware only or as a mixture of hardware and software operations. However, the alternative that involves the design of a custom integrated circuit would be based on application specific integrated circuit ASIC design techniques.
Designing such ASICs would enable a custom design to be created and higher levels of integration that result in physically smaller electronics and the integration of digital, analogue and mixed-signal circuits within a single packaged device. Considering the analogue sensor part of the system, the signal output from the sensor would normally need to be modified conditioned in order to provide signal levels that can be sampled by the digital signal-processing module via a suitable ADC, which converts the analogue signal to a digital representation.
Sensor signal sampling and digital signal processing. Such signal conditioning operations include signal amplification, DC level shifting and anti-aliasing filtering low-pass filtering to remove any high frequency signal components that would be aliased to lower frequencies. In general, these signal conditioning circuits are based on the use of the op-amp with negative feedback using external resistors and capacitors.
However, the operating conditions of the op-amp such as the power supply voltage level would need to be taken into account when either selecting an existing op-amp to use or when designing the op-amp itself.
The performance of the op-amp in these types of signal conditioning circuits would be a key factor in what performance could be achieved with the circuits used. In the past, the power supply voltage would not have been a major factor in determining the op-amp performance. The power supply voltage would have been at levels that enabled the op-amp circuitry to operate without encountering power supply voltage limitation issues.
With the move towards lower power supply voltage levels at, and below 3. The op-amp circuit architectures along with circuit design approaches must be reconsidered in order to enable these op-amps to be designed with appropriate characteristics for low-voltage operation.
The op-amp is a high-gain DC differential amplifier that is the core building block for many analogue circuits. In general, it consists of two or more amplification stages using transistors, integrated capacitors and in some designs, integrated resistors. The op-amp is designed to have certain characteristics that include a high open-loop differential gain A OL , a high gain-bandwidth product, a high input resistance, a low output resistance, a low output offset voltage, a high dynamic range minimum to maximum signal range and a high common-mode rejection ratio CMRR [ 1 ].
The op-amps shown in Figure 2 identify the circuits in open loop without any external feedback components from the output signal back to the input signal. The op-amp, therefore, would have a set of open-loop characteristics. In general, the op-amp would be designed to operate in closed loop where feedback components, primarily resistors and capacitors are used to provide either negative linear operations or positive non-linear operations feedback.
Single-ended output and differential output op-amps. The work presented here will focus on CMOS op-amp circuit design considerations, particularly the AC frequency response and stability. The standard topology for the single-ended output two-stage op-amp is considered, and the behaviour of an example case study design will be presented. It is a non-linear device that has four terminals: the drain, source, gate and bulk or body, substrate.
With these transistors, a voltage between the gate and the source v gs controls the flow of drain current i d. To design circuits using these devices, it is necessary to know their current-voltage IV characteristics.
In conventional circuit design, the transistor is usually modelled using two discrete models to mathematically describe the IV characteristics: a large-signal and a small-signal model. Each model would be used for different design and analysis purposes. A curve that describes the large-signal IV characteristic is shown in Figure 3.
The operation of the transistor is modelled using three different regions according to the values of the gate-source voltage v GS and the drain-source voltage v DS.
The three defined regions of operation are cut-off, linear and saturation where:. The gate-source voltage is less than the transistor threshold voltage V T in this region. A channel is created between the drain and source terminals, and there is current flow from drain to source.
The drain current will increase linearly with increasing drain-source voltage. Saturation region : In this region, the gate-source voltage is larger or equal to, the transistor threshold voltage, and drain-source voltage has reached or exceeds, v DSsat.
This occurs when the channel charge becomes pinched off at the drain-channel interface, and the transistor operation is now in the saturation region. In the simplest first order transistor model, increases in v DS do not cause an increase in i D and so i D becomes independent of v DS.
However, a more representative model includes an i D dependence on the value of v DS. Although the transistor is a non-linear device, for circuit analysis purposes when developing linear circuits, a linear model for the transistor operating in the saturation region at a specified DC operating bias point is initially created.
This then describes the behaviour of the transistor to small-signal changes around the bias point, and the small-signal model is then used to determine AC gain values. The signal changes are considered to be small so enabling the approximation that the transistor operation is linear around this DC operating point to be valid.
Moreover, defining the small-signal behaviour of the transistor as a transfer function, the transconductance g m , and output conductance g o is required model parameters. However, if the signal level is increased, the transistor operation becomes non-linear and will represent by the large-signal model. The conventional analogue design method for the op-amp considers the use of the transistor operating in the saturation region and the drain-source channel to be in strong inversion.
This requires the circuit voltage levels and hence the power supply voltage to be of suitably high levels to ensure that the transistor remains in saturation and strong inversion for linear circuit operation. However, today, when developing circuit designs based on using MOSFETs at low-power and low-voltage, the small-signal and large-signal models are no longer suitable to define transistor operation. The op-amp circuit can be based on different architectures, and each architecture provides advantages in operation when compared to other architectures.
In the design considered in this chapter, the two-stage CMOS operational amplifier is used with a simplified architecture as shown in Figure 5. Two amplification stages are used, the first stage providing high voltage gain and the second stage providing additional voltage gain and a large output signal swing.
In addition, each stage uses negative feedback frequency compensation to improve stability and bandwidth. Negative Miller compensation is applied around the first stage using two identical capacitors C NM , and Miller compensation is applied around the second stage using two identical capacitors C M.
The circuit schematic of the selected op-amp architecture is shown in Figure 6. Note how the signals between the first stage and the second stage are connected and how the actual circuit connections differ from the simplified architecture Figure 5.
The first stage consists of a transconductance stage with differential input transistors PM1 and PM2 followed by folded cascode FC stage. The mirror connected transistors NM5 and NM6 in the folded cascode sum the input transistors differential current. The current sources PM8 and PM9 on the upper side must provide a current larger than the bias current for each input transistor. Two-stage op-amp case study design simplified architecture.
Two-stage op-amp design case study design schematic. The second stage is primarily used to provide a large output voltage swing rail-to-rail output with high DC voltage gain. These transistors are biased by two in-phase signal currents using the two cascode transistors NM8 and PM The compensation circuitry is split into two parts.
Miller compensation around the second stage provides op-amp stability. The op-amp has two Miller capacitors around the class-AB amplifier. Negative Miller compensation around the first stage is provided the extended the bandwidth increases the unity gain frequency and also uses two capacitors. The reason for considering stability in a circuit design is to ensure that the circuit remains stable under the required operating conditions. Instability occurs when the op-amp is configured with negative feedback, and under certain conditions, the negative feedback becomes positive.
In the unstable case, the circuit output then oscillates. Stability under any input condition is referred to as unconditionally stable, or absolutely stable [ 3 ]. However, if a system is not unconditionally stable, a margin of stability must be built-in to ensure stable operation under the required operating conditions. To achieve stable op-amp operation in closed-loop, the designer can add a capacitance between specific nodes within the op-amp that deliberately reduces the open-loop gain magnitude at higher signal frequencies.
This technique, referred to as compensation , is implemented by typically bypassing one of the internal op-amp gain stages with a high-pass filter. In the simplest sense, a capacitor is connected between the output and input nodes of a gain stage. The purpose is to decrease the gain magnitude to less than unity at frequencies where instability could occur. A single compensation capacitor implementation is widely used in two-stage op-amp designs.
However, there are several other techniques used for the op-amp compensation. Improvements to the op-amp performance using the single capacitor compensation approach include the inclusion of a series resistor, buffer or buffer and series resistor.
Other techniques, for example, use multiple feedback capacitors connected to different stages within the circuit. These techniques can be used with the two-stage op-amp. Additional techniques require the inclusion of more than two gain stages and, with decreases in integrated circuit process geometries, op-amps with more than two gain stages have become more common to achieve a sufficiently high open-loop gain.
Basic Knowledge, Circuit and Application of Operational Amplifier(op amp)
This is helpful for users who are preparing for their exams, interviews, or professionals who would like to brush up their fundamentals on the Operational Amplifier topic. An operational amplifier also called OP-Amp and is a basic building block of analog-type electronic circuits. IC is an op-amp invented by Karl D in The output obtained from an op-amp is an amplified value of the input signal. There are 4 types of gain in op-amps namely, voltage gain, current gain, transconductance gain, and trans resistance gain. Op-amp can perform operations such as logic and arithmetic. The advantage of an op-amp is it occupies less area, more reliable, low cost, low power consumption.
Introduction to Ideal Op-Amp Circuit Characteristics
Year of fee payment : 4. Owner name : NXP B. Effective date : Year of fee payment : 8. Year of fee payment : A voltage converting multiplier circuit converts a single ended input voltage V gain into a differential output voltage V DO , and includes a differential input cell and a differential output cell, each biased by a respective control current. A current mirror ensures that the input current is supplied by the branches of the differential input cell, which current splitting is mirrored to the differential output cell. A temperature compensated voltage is achieved where I cin is a constant current and I cout is a temperature compensated current.
A Low Power, Low Cost, Differential Input to a Single-Ended Output Amplifier
Year of fee payment : 4. Year of fee payment : 8. Increasing the input common-mode range of a circuit which accepts differential signals as inputs. Such an increase may be attained by correcting an input signal at continuous levels or at 2 or more discrete levels without changing the strength represented by the input signal. In an embodiment, the common-mode component of an input signal is measured, and a correction voltage proportional to the difference between the measured common-mode component and a reference voltage, is generated.
Differential Amplifier
The operational amplifier is a direct-coupled high gain amplifier usable from 0 to over 1MH Z to which feedback is added to control its overall response characteristic i. The op-amp exhibits the gain down to zero frequency. Such direct coupled dc amplifiers do not use blocking coupling and by pass capacitors since these would reduce the amplification to zero at zero frequency. Large by pass capacitors may be used but it is not possible to fabricate large capacitors on a IC chip. The capacitors fabricated are usually less than 20 pf. Transistor, diodes and resistors are also fabricated on the same chip.
Operational Amplifier Question & Answers
Today, digital circuit cores provide the main circuit implementation approach for integrated circuit IC functions in very-large-scale integration VLSI circuits and systems. Typical functions include sensor signal input, data storage, digital signal processing DSP operations, system control and communications. Despite the fact that a large portion of the circuitry may be developed and implemented using digital logic techniques, there is still a need for high performance analogue circuits such as amplifiers and filters that provide signal conditioning functionality prior to sampling into the digital domain using an analogue-to-digital converter ADC for analogue sensor signals. The demands on the design require a multitude of requirements to be taken into account. In this chapter, the design of the operational amplifier op-amp is discussed as an important circuit within the front-end circuitry of a mixed-signal IC. The discussion will focus on the design of the op-amp using different compensation schemes incorporating negative Miller compensation and designed to operate at lower power supply voltage levels. The simulation approach is focussed on the open-loop frequency response performance of the op-amp.
Using a Differential I/O Amplifier in Single-Ended Applications
Definition : Differential Amplifier is a device that is used to amplify the difference in voltage of the two input signals. Differential Amplifier is an important building block in integrated circuits of analog system. It typically forms input stages of operational amplifiers. In simple words, we can say It is a device that amplifies the difference of 2 input signals.
Skip to Main Content. A not-for-profit organization, IEEE is the world's largest technical professional organization dedicated to advancing technology for the benefit of humanity. Use of this web site signifies your agreement to the terms and conditions. It allows setting the common-mode CM output voltage to a reference voltage vR while providing gain and a balanced output. The circuit, intended to work as sensor front-end for instrumentation applications, can be implemented by using available commercial devices, thus proving a solution for the board-level design of instrumentation systems.
Try out PMC Labs and tell us what you think. Learn More. This paper introduces a new current-controlled current-amplifier suitable for precise measurement applications. This amplifier was developed with strong emphasis on linearity leading to low total harmonic distortion THD of the output signal, and on linearity of the gain control. The presented circuit is characterized by low input and high output impedances. This circuit is supposed to be used in many sensor applications, as well as a precise current multiplier for general analog current signal processing.
The world is analog. With the invention of the transistor, electronics engineers gained competence in the digital processing of analog signals. However, in all signal processing systems, designers have to receive, amplify or attenuate, or process the analog signals using varieties of analog techniques.
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