This paper begins with background information. First, introductory
Topics on the basic principles of amplifiers are presented, including
the ideal op amp model, next the design, layout, simulation and noise theory of Operational Amplifier (Op-Amp) structures. Emphasis has been placed on the noise properties of the Op-Amp performance. The OP-Amp noise types and characteristics. Our results show that Experimental testing needs to be carried out for further evaluations.
History:
1941: First (vacuum tube) op-amp
Let's go back in time a bit and see how this device was developed. The term "operational amplifier" goes all the way back to about 1943 where this name was mentioned in a paper written by John R. Ragazzinni with the title "Analysis of Problems in Dynamics" and also covered the work of technical aid George A. Philbrick.
Today, and since that month in 1976, the types of op amps have increased almost daily. An op-amp, defined as a general-purpose, DC-coupled, high gain, inverting feedback amplifier, was first found in US Patent 2,401,779 "Summing Amplifier" filed by Karl D. Swartzel Jr. of Bell labs in 1941. This design used three vacuum tubes to achieve a gain of 90dB (decibells) and operated on voltage rails of ±350V. It had a single inverting input rather than differential inverting and non-inverting inputs, as are common in today's op-amps.
INTRODUCTION
What exactly is an OPerational AMPlifier? Let's define what that component is and look at the parameters of this amazing device. An operational amplifier IC is a solidstate integrated circuit that uses external feedback to control its functions. It is one of the most versatile devices in all of electronics. The term 'op-amp' was originally used to describe a chain of high performance DC amplifiers that was used as a basis for the analog type computers of long ago. The very high gain op-amp IC's our days uses external feedback networks to control responses. The op-amp without any external devices is called 'open-loop' mode, refering actually to the so-called 'ideal' operational amplifier with infinite open-loop gain, input resistance, bandwidth and a zero output resistance. However, in practice no op-amp can meet these ideal characteristics.
EXPLANATION
An operational amplifier is a differential input, single-ended output amplifier, as shown symbolically in Figure 1-1. This device is an amplifier intended for use with external feedback elements, where these elements determine the resultant function, or operation. This gives rise to the name “operational amplifier,” denoting an amplifier that, by virtue of different feedback hookups, can perform a variety of operations.
We now enjoy a variety of op amps that will provide the user essentially with anything s/he needs, such as high common-mode rejection, low-input current frequency compensation, CMOS, and short-circuit protection. All a designer has to do is expressing his needs and is then supplied with the correct type. Op-Amps are continually being improved, especially in the low noise areas.
Ideal Op Amp Attributes
An ideal op amp has infinite gain for differential input signals. In practice, real devices will have quite high gain (also called open-loop gain) but this gain may not necessarily be precisely known. In terms of specifications, gain is measured in terms of VOUT/VIN, and is given in V/V, the dimensionless numeric gain. More often, however, gain is expressed in decibel terms (dB), which is mathematically dB = 20 • log (numeric gain).
Also, an ideal op amp has zero gain for signals common to both inputs, that is, common-mode (CM) signals.
Or, stated in terms of the rejection for these common-mode signals, an ideal op amp has infinite CM rejection (CMR). In practice, real op amps can have CMR specifications of up to 130 dB for precision devices, or as low as 60 dB–70 dB for some high speed devices.
The ideal op amp also has zero offset voltage (VOS = 0), and draws zero bias current (IB = 0) at both inputs. Within real devices, actual offset voltages can be as low as 1 pV or less, or as high as several mV. Bias currents can be as low as a few fA, or as high as several ìA. This extremely wide range of specifications reflects the different input structures used within various devices.
The Noninverting Op Amp Stage
The op amp non-inverting gain stage, also known as a voltage follower with gain, or simply voltage follower,
Transfer expression of the network is defined as seen from the top of RF to the output across RG as β. This usage is a general feedback network transfer term, not to be confused with bipolar transistor forward gain. β can be expressed mathematically.
So, the feedback network returns a fraction of Vout to the op amp (–) input. Considering the ideal principles of zero offset and infinite gain, this allows some deductions on gain to be made. The voltage at the (–) input is forced by the op amp’s feedback action to be equal to that seen at the (+) input, Vin. Given this relationship, it is relatively easy to work out the ideal gain of this stage, which in fact turns out to be simply the inverse of β. This is apparent from a comparison of Equations.
The Inverting Op Amp Stage
The op amp inverting gain stage, also known simply as the inverter, is shown in Figure 1-4. As can be noted by comparison of Figures 1-3 and 1-4, the inverter can be viewed as similar to a follower, but with a transposition of the input voltage Vin. In the inverter, the signal is applied to RG of the feedback network and the op amp (+) input is grounded.
For clarity, these expressions are again included in the figure. The major difference between this stage and the non-inverting counterpart is the input-to-output sign reversal, denoted by the minus sign in Eq. 1-5. Like the follower stage, applying ideal op amp principles and some basic algebra can derive the gain expression of Eq. 1-5.
The inverting configuration is also one of the more useful op amp stages. Unlike a non-inverting stage, however, the inverter presents a relatively low impedance input for Vin, i.e., the value of RG. This factor provides a finite load to the source. While the stage gain can in theory be adjusted over a wide range via RF and RG, there is a practical limitation imposed at high gain, when RG becomes relatively low. If RF is zero, the gain becomes zero. RF can also be made variable, in which case the gain is linearly variable over the dynamic range of the element used for RF. As with the follower gain stage, the gain is ratio dependent, and is relatively insensitive to the exact RF and RG values.
The inverter’s gain behavior, due to the principles of infinite op amp gain, zero input offset, and zero bias current, gives rise to an effective node of zero voltage at the (−) input. The input and feedback currents sum at this point, which logically results in the term summing point. It is also called a virtual ground, because of the fact it will be at the same potential as the grounded reference input.
A special gain case for the inverter occurs when RF = RG, which is also called a unity gain inverter. This form of inverter is commonly used for generating complementary VOUT signals, i.e., VOUT = −VIN. In such cases it is usually desirable to match RF to RG accurately, which can readily be done by using a wellspecified matched resistor pair.
A variation of the inverter is the inverting summer, a case similar to Figure 1-4, but with input resistors RG2, RG3, etc (not shown). For a summer individual input resistors are connected to additional sources VIN2, VIN3, and so forth, with their common node connected to the summing point. This confi guration, called a summing amplifier, allows linear input current summation in RF.3 VOUT is proportional to an inverse sum of input currents.
Noise Gain (NG)
The first aid to analyzing op amps circuits is to differentiate between noise gain and signal gain. We have already discussed the differences between noninverting and inverting stages as to their signal gains, which are summarized in Eqs. 1-2 and 1-4, respectively. But, as can be noticed from Figure 1-6, the difference between an inverting and noninverting stage can be as simple as where the reference ground is placed. For a ground at point G1, the stage is an inverter; conversely, if the ground is placed at point G2 (with no G1) the stage is noninverting.
Note, however, that in terms of the feedback path, there are no real differences. To make things more general, the resistive feedback components previously shown are replaced here with the more general symbols ZF and ZG, otherwise they function as before. The feedback attenuation, â, is the same for both the inverting and noninverting stages:
Noise gain can now be simply defined as: The inverse of the net feedback attenuation from the amplifier output to the feedback input. In other words, the inverse of the â network transfer function. This can ultimately be extended to include frequency dependence (covered later in this chapter). Noise gain can be abbreviated as NG.
As noted, the inverse of â is the ideal noninverting op amp stage gain. Including the effects of fi nite op amp gain, a modifi ed gain expression for the noninverting stage is:
Open-Loop Gain:
Lets have a look how the 'ideal' amplifier would look like in Fig. 5-1. The search for an ideal amplifier is, of course, a futile exercise. The characteristics of the operational amplifier are good enough, however, to allow us to treat it as ideal. Below are some amplifier properties that make this so. (Please realize that these ratings are next to impossible to achieve).
Op Amp Noise Theory and Applications
The purpose of op amp circuitry is the manipulation of the input signal in some fashion.
Unfortunately in the real world, the input signal has unwanted noise superimposed on it.
Noise is not something most designers get excited about. In fact, they probably wish the
whole topic would go away. It can, however, be a fascinating study by itself. A good understanding of the underlying principles can, in some cases, be used to reduce noise in the
design.
Noise is a purely random signal, the instantaneous value and/or phase of the waveform
cannot be predicted at any time. Noise can either be generated internally in the op amp,
from its associated passive components, or superimposed on the circuit by external sources.
Noise Floor
When all input sources are turned off and the output is properly terminated, there is a level
of noise called the noise floor that determines the smallest signal for which the circuit is
useful. The objective for the designer is to place the signals that the circuit processes
above the noise floor, but below the level where the signals will clip.
Signal-to-Noise Ratio
it is a ratio of signal voltage to noise voltage (hence the name signal-tonoise
ratio).
Types of Noise
There are five types of noise in op amps and associated circuitry:
1) Shot noise
2) Thermal noise
3) Flicker noise
4) Burst noise
5) Avalanche noise
Some or all of these noises may be present in a design, presenting a noise spectrum
unique to the system. It is not possible in most cases to separate the effects, but knowing
general causes may help the designer optimize the design, minimizing noise in a particular
bandwidth of interest. Proper design for low noise may involve a “balancing act” between
these sources of noise and external noise sources.
Noise Colors
While the noise types are interesting, real op amp noise will appear as the summation of
some or all of them. The various noise types themselves will be difficult to separate. Fortunately,
there is an alternative way to describe noise, which is called color. The colors of noise come from rough analogies to light, and refer to the frequency content. Many colors are used to describe noise, some of them having a relationship to the real world, and some of them more attuned to the field of psycho-acoustics.
White noise is in the middle of a spectrum that runs from purple to blue to white to pink
and red/brown. These colors correspond to powers of the frequency to which their spectrum
is proportional.
AMPLIFIER NOISE
Amplification of low level audio or RF signals is always accompanied by noise generated within the amplifier itself. The following article discusses the generation of this noise and some of the methods of assessing noise performance.
INTRODUCTION
One of the factors which governs the performance of any amplifier system is the noise in the system. Noise might be defined as signals in the system which are unwanted and which degrade the desired signal content in the system.
As far as the amplifier system is concerned, noise can be divided into noise it receives at its input and noise it generates itself. A good system is one in which the noise generated by the amplifier itself is small compared to noise from the incoming source. In a HF receiver, for example, atmospheric noise is high and it is not difficult to achieve this requirement. At VHF and UHF frequencies, atmospheric noise is low and performance is limited by the noise generated in the first stages of the radio receiver.
An operational amplifier, which is often called an op-amp, is a DC-coupled high-gain electronic voltage amplifier with differential inputs and, usually, a single output. Typically the output of the op-amp is controlled either by negative feedback, which largely determines the magnitude of its output voltage gain, or by positive feedback, which facilitates regenerative gain and oscillation. High input impedance at the input terminals (ideally infinite) and low output impedance (ideally zero) are important typical characteristics.
Op-amps are among the most widely used electronic devices today, being used in a vast array of consumer, industrial, and scientific devices. Many standard IC op-amps cost only a few cents in moderate production volume; however some integrated or hybrid operational amplifiers with special performance specifications may cost over $100 US in small quantities. Op-amps sometimes come in the form of macroscopic components (see photo), or as integrated circuit 'cells' or patterns that can be reprinted several times on one chip that is more complex, such as for a cell phone.
Modern designs are electronically more rugged than earlier implementations and some can sustain direct short circuits on their outputs without damage.
The op-amp is one type of differential amplifier. Other types of differential amplifier include the fully differential amplifier (similar to the op-amp, but with two outputs), the instrumentation amplifier (usually built from three op-amps), the isolation amplifier (similar to the instrumentation amplifier, but which works fine with common-mode voltages that would destroy an ordinary op-amp), and negative feedback amplifier (usually built from one or more op-amps and a resistive feedback network).
Categories:
Electrical Circuits,
Electronics,
Noise Theory,
Operational Amplifiers