miércoles, 3 de febrero de 2010

Useof op-amps in electronics system design

 Use in electronics system design

The use of op-amps as circuit blocks is much easier and clearer than specifying all their individual circuit elements (transistors, resistors, etc.), whether the amplifiers used are integrated or discrete. In the first approximation op-amps can be used as if they were ideal differential gain blocks; at a later stage limits can be placed on the acceptable range of parameters for each op-amp.
Circuit design follows the same lines for all electronic circuits. A specification is drawn up governing what the circuit is required to do, with allowable limits. For example, the gain may be required to be 100 times, with a tolerance of 5% but drift of less than 1% in a specified temperature range; the input impedance not less than one megohm; etc.
A basic circuit is designed, often with the help of circuit modeling (on a computer). Specific commercially available op-amps and other components are then chosen that meet the design criteria within the specified tolerances at acceptable cost. If not all criteria can be met, the specification may need to be modified.
A prototype is then built and tested; changes to meet or improve the specification, alter functionality, or reduce the cost, may be made.

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The Ideal Operational Amplifier

In order to introduce operational amplifier circuitry, we will use an ideal model of the operational
amplifier to simplify the mathematics involved in deriving gain expressions, etc., for the circuits
presented. With this understanding as a basis, it will be convenient to describe the properties of
the real devices themselves in later sections, and finally to investigate circuits utilizing practical
operational amplifiers.
To begin the presentation of operational amplifier circuitry, then, it is necessary first of all to
define the properties of a mythical "perfect" operational amplifier. The model of an ideal
operational amplifier is shown in figure 9.

Figure 9. Equivalent Circuit of the Ideal Operational Amplifier

Defining the Ideal Operational Amplifier

  •  Gain: The primary function of an amplifier is to amplify, so the more gain the better. It can

always be reduced with external circuitry, so we assume gain to be infinite.

  • Input Impedance: Input impedance is assumed to be infinite. This is so the driving source

won't be affected by power being drawn by the ideal operational amplifier.

  • Output Impedance: The output impedance of the ideal operational amplifier is assumed to

be zero. It then can supply as much current as necessary to the load being driven.

  •  Response Time: The output must occur at the same time as the inverting input so the

response time is assumed to be zero. Phase shift will be 180°. Frequency response will be

flat and bandwidth infinite because AC will be simply a rapidly varying DC level to the ideal

  • Offset: The amplifier output will be zero when a zero signal appears between the inverting

and non-inverting inputs.

A Summing Point Restraint

An important by-product of these properties of the ideal operational amplifier is that the summing
point, the inverting input, will conduct no current to the amplifier. This property is to become an
important tool for circuit analysis and design, for it gives us an inherent restraint on our circuit - a
place to begin analysis. Later on, it will also be shown that both the inverting and non-inverting
inputs must remain at the same voltage, giving us a second powerful tool for analysis as we
progress into the circuits.



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