19-12-2012, 03:29 PM
Operational Amplifier (Op-Amp)
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ABSTRACT
The operational amplifier is one of the most useful and important components of analog electronics. They are widely used in popular electronics. Their primary limitation is that they are not especially fast: The typical performance degrades rapidly for frequencies greater than about 1 MHz, although some models are designed specially to handle higher frequencies.
The primary use of op-amps in amplifier and related circuits is closely connected to the concept of negative feedback. Feedback represents a vast and interesting topic in itself. We will discuss it in rudimentary terms a bit later. However, it is possible to get a feeling for the two primary types of amplifier circuits, inverting and non-inverting, by simply postulating a few simple rules (the \golden rules"Ñ. We will start in this way, and then go back to understand their origin in terms of feedback.
Operational Amplifier (Op-Amp) Basics
Symbols and Schematic Diagram:
Below is the symbol used to represent an operational amplifier. The two inputs are the inverting (V-) and non-inverting (V+) terminals, and the output is Vout. The supplies are discussed further in the pages ahead.
The circuitry that makes up an op-amp consists of transistors, resistors, diodes, and a couple capacitors. In general, these components are combined to achieve within the op-amp two stages of differential amplifiers and a common-collector amplifier.
In an effort to simplify the operational amplifier, one must not forget that the internal circuitry of an op-amp is more than just a “black box”. All operational amplifiers are integrated circuits (ICs), and Figure 4 illustrates the components that work together to achieve what we know to be an op-amp.
Frequency Response:
The frequency response of the op-amp is pretty straight forward. Basically, as the operating frequency of the op-amp increases, the voltage gain decreases. Actually, it is only after the cutoff frequency is reached that the attenuation of voltage gain starts happening. The cutoff frequency is defined as the frequency at which the open loop gain equals 70.7% of its maximum gain, or, equivalently, down 3 dB from the maximum gain. All frequencies lower than the cutoff frequency, even 0 Hz, see the max gain because the op-amp is a dc amplifier. Gain bandwidth product is a simple formula that relates closed loop gain (Acl), bandwidth (cutoff frequency, fco), and unity-gain frequency, as such:
funity = (Acl)(fco)
Unity-gain frequency is the maximum frequency possible where the gain equals 1. Remember that a closed loop lowers the voltage gain, yet by lowering the voltage gain, higher operating frequencies are made available. So, depending on what is needed for the job, a certain degree of flexibility is available. A high gain, low frequency (or bandwidth) arrangement is possible, as is a low gain, high bandwidth configuration, as long as their product equals the unity-gain frequency. See the figure below to better understand op-amp frequency response.
Op-amp Circuits :
Derived Op-Amp Circuits
Summing amplifier
The summation amplifier is an easily understood op-amp circuit that sums each of the inputs according the inverting amplifier output formula. In fact, the summation amplifier is very closely derived from the inverting amplifier. Only, instead of having a single voltage source (Vs) and resistor (Ri), multiple sources converge on the inverting input (V-), each having its own resistor (Ri). See the circuit below.
Integrator
The subject of calculus involves exercises in differentiation and integration of which most students struggle in varying degrees to understand. Well, the amazing versatility and value of the op-amp can be seen in its ability to perform integration and differentiation. Integrators and differentiators, as they are called, are very similar and their circuits are simple to draw. The use of a capacitor is what makes the complex mathematical process possible.