pp 228-236
An ideal operational amplifiers
At the end of this chapter the student should:
• understood the provisions which are called operational amplifiers and the principle of operation.
• Knows the principles of operation and use an operational amplifier
• Distinguishes the limits of the ideal operational amplifier from the real operational amplifier.
• They are able to decide when using the approach
the ideal operational amplifier.
• To read and understand the technical brochures of the manufacturers.
• Knows the basic principles of feedback to the basic operational amplifier circuits.
• Distinguishes the operation reversed from that of non-
inverted operational amplifier.
• Is able to design simple circuits with reversed
and non-inverting amplifier
Operational AMPLIFIERS
8.1 Characteristics of the operational amplifier.
8.1.1 General aspects of amplifiers.
O operational amplifier is ideal in principle an amplifier. This means that to understand the operation of the first should have understood the basic features and operating principles of amplifiers. So we begin with a brief presentation of these, even for a simple reminder.
O amplifier (amplifier) is one of the major steps or components used in analog electronics.
In its simplest form has two terminals at the entrance, at the ends of which applies the voltage (signal) input. It also has two outlet terminals at the ends of which shall be the voltage (signal) output.
In most cases, the amplifier is treated as a black box (black box), which is described by a limited set of parameters. One of them is the dependence of the output signal from the input signal.
In the simplest case we consider that the relationship is linear, ie that the output signal is proportional to the input signal.
Another response of amplifiers based on how their response to signals corresponding to dc or ac voltages. Indeed, some amps respond differently when applied at the entrance to a constant or an alternating voltage. Some other amps respond differently to different frequencies of the alternating voltage applied to the input.
Of all the parameters of an amplifier, special interest is the voltage gain or current, the input impedance, output impedance and frequency response.
The gain or voltage gain (voltage gain) is defined as the quotient of
y0 output voltage to input voltage u say
The input voltage can be simple or differential. The output voltages and input yr yi refer only to the corresponding input and output signals and do not include any compensation or trends polarizations.
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2 7
Entry 1 Positive supply voltage
Apart from the voltage gain there and enjoyed <current gain (current gain) and the gain of the power gain (power gain) which are defined respectively.
The input impedance (input impedance), the only input impedance is the resistance which shows the entrance of the amplifier. The input impedance load behaves as a resistance to any signal source connected to the input of the amplifier and affects the signal transmission from the source to the amplifier.
The output impedance (output impedance), only the output impedance is the resistance which manifests in leaving the amplifier when connected to a resistor load the next level. The output impedance affect the tunnel current in the output circuit.
The frequency response (frequency response) describes the voltage gain of the amplifier versus frequency of the input signal.
Beyond these four basic parameters, there are many others that are required when we want to specify a full amp. Moreover, beyond this simple amplifier and amplifiers are more entries, where the output signal is proportional to the sum or difference of input signals, or amplifiers with more than one exit.
Facing the amplifier as a black box, with the above parameters and basic functions and taking into account the principle of feedback, allow a first understanding and comprehensive study of the operational amplifier, as follows.
Input Output
Negative trend
Power
Figure 8.1.1 Circuit symbol operational amplifier,
mounted on the shell (package) that
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8.1.2 Operational Amplifier (TE)
A significant number of amplifiers based on one of the most basic units of modern electronics. This unit is called operational amplifier (operational amplifier or, abbreviated, op-amp). The "opamp» (operational) stems from the fact that this amp initially was used in analog computers to perform mathematical operations such as addition, subtraction, integration, differentiation, etc.
O operational amplifier has two inputs and one output. Also required two additional terminals to power. Supplying an operational amplifier can be done either by using a single voltage source or with the help of two sources, so the trends that feed into their respective terminals are symmetrical to the "land" of the circuit. In addition, there may be other terminals which allow accessing the internal circuit of the operational amplifier. So at least one form of the opamp requires five connectors and placed in a shell eight terminals, which are arranged in two rows (Dual In Line, DIL8), in top view shown in sch.8.1.1.
In electronic charts, to avoid complexity, usually not present all these details like power terminals and the shell which is mounted an operational amplifier.
Input voltage difference
Polarity output voltage
Table 8.1.1. Polarity of the output voltage versus input voltage
O operational amplifier strengthens and elaborates the difference between the signals applied to two inputs and for this reason we say that the operational amplifier has a differential input (differential input), see sch.8.1.1. Because the operational amplifier processes the differential signal inputs the user must know which input corresponds to the abstract and what to be deducted, ie which signal
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subtracted from what. For this reason, the inputs to distinguish between the points (+), which represents the so-called non-inverting input (non-inverting input, NI) and (-), which represents the so-called per-turn input (inverting input, I). As the terms non-inverting and inverting input can cause confusion as to their name will be given immediately following the extensive explanation.
If the non-inverting input is more positive than the inverting then the operational amplifier output is positive. If the inverting input is more positive than the non-inverting output then the operational amplifier is negative. Using the symbol V (+) on the voltage applied to the non-inverting input and the V (-) for the voltage applied to the inverting input, these are summarized in Tab. 8.1.1.
The more accurately expressed by the notion that the output shows phase difference RoL = 0 °, ie is symfasiko,
branded non-inverting input phase difference and presents RoL = 180 °, the inverting input.
8.1.2.1 Ideal Operational Amplifier
Using an operational amplifier, even if it is treated as a black box, requires a minimum knowledge of a few basic parameters. To further facilitate the calculations of the designer or maintainer of electronic circuits, often resorting to using the simplest possible model, which is the ideal operational amplifier.
Characteristic parameter
Symbol
Size
Input
Output impedance
Voltage gain
AV
Frequency response (bandwidth)
BW
Perfect balance
VQ = 0 when V + = V_
These characteristics do not change with temperature
Table 8.1.2. Characteristic parameters of the ideal operational amplifier
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O characteristic of pi. 8.1.2 are defined as follows:
• Input impedance: Defined as the ratio of the voltage signal, which applies to the entrance to the current which it induces.
• Output impedance: Defined by Thevenin output circuit, which is the resistance which is connected in series with the "generator" output signal.
• voltage gain is defined as the ratio of output voltage to the difference of the input voltages.
• The Frequency bandwidth: Sets the gain dependence on the frequency and determines the frequency range (bandwidth) over which the operational amplifier is practically useful.
The ideal operational amplifier has the characteristic parameters are shown in fig. 8.1.2. Although in practice no such operational amplifiers, the current technology allows construction of amplifiers, including several important characteristics approaching those of pi. 8.1.2. This depends directly on the scope of applications for which the manufacturer has designed this operational amplifier. So there are operational amplifiers whose input impedance is 1TO ie 1012 (Code LF351), others who can provide the output currents of the order of 13A (code LM12), so the output impedance be ignored, and others who have a very large bandwidth, eg 1200MHz (Code CLC449).
Characteristic parameter
Symbol
Size
Input
Ri
2MO
Output impedance
Ro
~ 75W
Voltage gain
AV
200. 000
Frequency response (bandwidth)
BW
1,5 MHz
These features change slightly with temperature
Table 8.1.3. Characteristic parameters of a real operational amplifier (eg 741)
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8.1.2.2 Effective Operational Amplifier
O is a real operational amplifier is used in practice. It is usually an integrated circuit, which is also called monolithic because it is entirely constructed on a piece of silicon, which is not possible to separate the individual components.
The characteristics of the real operational amplifier vary considerably in type (ie code in code) where the emphasis has been given to some of them by the manufacturer. Generally, in a real operational amplifier, the gain is finite, as does the input impedance, bandwidth and all other elements, as shown in fig. 8.1.3, which refers to a standard operational amplifier which has the code 741.
For these reasons, when used in a real operational amplifier, the assumption that the ideal approach should be based on the requirements of the circuit which will be installed and running. Thus, in many cases requires the knowledge of other characteristic parameters of the operational amplifier, except those of fig. 8.1.3, and certain characteristics diagrams contained in the technical brochures of the manufacturers. The major characteristic parameters of a real operational amplifier, beyond those included in Table 8.1.3 are:
• The current bias input (input bias current) L. This equals imiathroisma polarization currents of the two input operands.
• The current apostathmisis inputs (input offset current) IO is equal to the difference of the polarization currents of the two inputs.
• The trend apostathmisis input (input offset voltage) VO, is expressed by the measure of the voltage to be applied between the input to reset the output voltage.
• The common mode rejection ratio (CMRR), (Common Mode Rejection Ratio) Measure the voltage gain that occurs when a signal is applied simultaneously to both inputs. That is, indirectly reflects the amount is in the operational amplifier from the treaty when Vo = 0 V + = V_.
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• The rate of change of output voltage (slew rate). Indicates the maximum-
the speed with which it can vary the output voltage.
A similar comparison between the characteristic parameters
a real and an ideal operational amplifier is presented
in fig. 8.1.4
Characteristic parameter
Ideal
Real
Units
Input
2MO
Output impedance
75W
Voltage gain
200,000
Frequency response (bandwidth)
1.5
MHz
Input bias current
80iA
Current input apostathmisis
20iA
Voltage input apostathmisis
0
2
mV
Common mode rejection ratio
90
dB
Rate of change of output voltage
0.5
V ^ s
Table 8.1.4. Comparison of characteristic parameters of ideal and real operational amplifier (eg 741)
The full description of a real operational amplifier includes other characteristic parameters. These can be exploited by an experienced technician when required detailed scheme to exploit the full potential of the operational amplifier.
We close with the observation that the voltage gain AV a real operational amplifier is a differential voltage gain. So, based on ex.8.1.1, the output signal is proportional to the difference of input signals, ie
Vo = Av (V + - V-) 8.1.2
where the indices (+) and (-) refer to non-inverting and inverting input, respectively.
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Example 8.1.1
In non-inverting operational amplifier input an applied-
Tai 2 memory voltage and inverting the 1 Mem. If the (differential) gain
voltage is 80 000 to calculate the voltage at the amplifier output.
Solution
We use Eq. (8.1.2) where now we
Therefore
V0 = A x V = 80000 x 1 m V = 8 x 104 x 10-6 V = 8 x 10-2 V = 80 mV
Example 8.1.2
Between the two input operational amplifier applies a TA-
Vi = 20 significant memory and output voltage of 4 V. appears To computers
HE the (differential) voltage gain of the operational amplifier.
Solution
Starting from Eq. (8.1.2), we
n0 _ 4V 4V
If = ~ = '<br /> Vi 20mn 2 · 10-5V <br /> V = V + - V = 1 + memory
SUMMARY 8.1
• The operational amplifier has a differential input and the output signal is proportional to the difference of two input signals.
• The ideal operational amplifier used as a generally good approach for designing and constructing simple circuits.
• The use of real operational amplifiers requires in understanding the difference from the ideal.
• The full utilization of real operational amplifiers requires knowledge of all characteristics parameters.
QUESTIONS
Exercises 8.1 8.1.1
How many species are found in a gain amplifier,
c 10 mV simple or operands?
d 1 V
8.1.2 What is the difference between them
Mark the correct answer inputs an operational amplifier?
8.1.8 O operational amplifier 741
8.1.3 Why use the concept is the ideal aid operand> a voltage gain 100 000 schyti?
b impedance eiso
8.1.4 What are the key characteristics of an ideal operator
c output impedance amplifier ment?
75 O
8.1.5 What are the key differences
D. All of the above hours between ideals and note the correct reply on the real important op. amplifiers?
8.1.9 An operational amplifier
8.1.6 It is possible to use voltage gain is 200,000. If thei any opamp at the entrance of the amplifier is applied to trade a trend stei memory 12 to implement any calculation? gistei the output voltage.
8.1.7 An operational amplifier
8.1.10 O operational amplifier 741 has a voltage gain of 100 000. voltage gain is 100,000. If the voltage at the output is What are the input voltages 1 V, the input voltage if the output is measured a 10 nl tions 0,1 V, 1 V and 10 V; 236
