Difference between input and output voltage
Operational amplifier (OP / OV / OPV / OpAmp)
The operational amplifier is abbreviated as OP, OV or OPV. The term OpAmp is the abbreviation for the English term Operational Amplifier.
The operational amplifier is far too complex to describe in simple words. But even with a lot of words and pictures you can't quite do it justice. It is one of the greatest inventions in electronics and goes back a long way to tube technology.
The term operational amplifier comes from the time when mathematical operations were still built using analog technology. Today this is done by digital components, right up to the microprocessor that is controlled by software.
The operational amplifier is a multi-stage, high-gain, galvanically coupled differential amplifier. It can amplify both direct and alternating voltages. The internal structure is designed in such a way that its mode of operation is influenced by the external negative feedback circuit. This highly complex semiconductor component is available in many variations, all of which have very different properties. They are optimized for a wide variety of applications.
The operational amplifier has an inverting (minus) input (N) and a non-inverting (plus) input (P). The plus symbol means that the gain factor must be multiplied with a positive sign. The minus symbol means that the gain factor must be multiplied with a negative sign. The difference between the two voltages is amplified on output (A). Via an appropriate wiring of the inputs and the output, one can use operational amplifiers in addition to the basic circuits such as adders, subtractors, amplifiers, active attenuators, filter circuits or complete regulator circuits, such as B. realize an electronically controlled power supply.
The operational amplifier is often operated symmetrically with two identical DC voltages. ± 5V, ± 12V and ± 15V are very common. But there are also applications in which the operational amplifier is operated with only one DC voltage. The negative connection is then connected to the GND of the operating voltage.
Depending on the type of operational amplifier, the supply voltage connections can be wired with a few to 100 V.
If the non-inverting input of the operational amplifier is controlled, the output voltage has the same polarity as the input voltage. If the inverting input of the operational amplifier is controlled, the output voltage has opposite polarity to the input voltage.
Many operational amplifiers cannot tolerate more voltage at the input than the operating voltage. For this reason, the input signals must first be removed for test purposes before the operating voltage is switched off. If external signal voltages are fed to the operational amplifier, protective circuits against overvoltages should be built in.
The operational amplifier always has a differential amplifier as its input stage. This is followed by a second amplifier stage, a short-circuit fuse and a push-pull amplifier at the output. The second amplifier stage always contains either an integrated frequency response compensation or one that can be connected externally to connection pins. Without this compensation circuit, the operational amplifier would be useless in its amplifying function. It would be unstable and would vibrate.
Circuit symbols according to DIN 40900 T.10 (outdated)
Circuit symbols according to DIN 40900 T.13 (current)
Ideal operational amplifier
An ideal operational amplifier has an infinitely large gain factor V, an infinitely large input resistance R.e, an output resistance Ra equal to zero and a frequency range from zero to infinity. In addition, the ideal operational amplifier is completely symmetrical. The same voltages at the two inputs result in an output voltage U.a from zero. The reason is the difference UPN between the input voltages that is zero. Assuming the amplitude and phase position are the same. One then speaks of common mode control. The gain is then called common mode gain. It is zero.
The ratio between the gain factor V and the common mode gain is called common mode rejection. It is infinitely large.
Distortion or noise, as well as the dependence on the ambient temperature, does not exist with the ideal operational amplifier. And there is a linear relationship between input and output voltage.
Real operational amplifier
When using operational amplifiers, ideal properties are desired. Unfortunately, such operational amplifiers cannot be made. Only the optimization of some properties towards the ideal value is possible. In many applications, ideal properties are not necessary at all. The deficient properties are ignored and ideal properties are assumed. Nevertheless, one should not completely lose sight of reality.
In the comparison table you can see which properties or parameters are present in the ideal operational amplifier and are possible in the real operational amplifier. Since the technical development does not stop, these values may no longer be up-to-date.
The parameters of the real operational amplifier are only achieved in particularly high-quality operating rooms. They are not needed in most applications.
|Parameter||Ideal operational amplifier||Real operational amplifier|
|Gain factor V||infinite||approx. 1,000,000|
|Input resistance Re||infinite Ω||1 ΜΩ to 1000 MΩ|
|Lower limit frequency fmin||0 Hz||0 Hz|
|Unitity gain frequency bandwidth||infinite Hz||> 100 MHz|
|Common mode gain VEq||0||approx. 0.2|
|Common mode rejection G||infinite||approx. 5,000,000|
|Noise output voltage Uintoxication||0 V||approx. 3 µV|
Characteristics of the operational amplifier
The following terms must be observed when operating an OP. However, this is only an excerpt of the most important terms. The respective values can be found in the data sheet of the Op to be used.
Offset voltage (English input offset voltage)
Differential voltage that must be applied on the input side in order to prevent a deflection from the rest position at the output.
Offset current (English input offset current)
Difference between the input currents that flow when the output is in a quiescent state.
Temperature coefficient (English temperature drift)
Influence of temperature on offset voltage and current.
Input Bias Current
Average value of the currents that flow in both inputs in the idle state.
Input Resistance / Impedance
Resistance of one input to zero when the other input is connected to zero.
Differential Input Voltage
Range of the admissible input differential voltage.
Open Loop Voltage Gain
The open loop gain (open loop voltage gain or simply open loop gain) of an OP is extremely high. In order to get a reasonable gain at a usable cut-off frequency, a part of the output input voltage, e.g. B. with a simple voltage divider, fed back to the inverting input. This feedback gain is called closed-loop voltage gain or simply closed-loop gain.
Output Resistance / Impedance
Resistance when the output is loaded. Only applies to low levels and is frequency-dependent.
There is hardly anything in the data sheets about the output resistance. This is because the output resistance is determined by the negative feedback. If the negative feedback gain is small in relation to the open loop voltage gain, then the OP regulates so that the output resistance is negligibly small within the permissible load current range and within the control limit. At higher frequencies, the open-loop voltage gain decreases and the output resistance increases with it.
Output Voltage Swing
Output-side modulation before the limitation occurs.
Common Mode Rejection Ratio
Attenuation that occurs before the signal is amplified.
The current that the supply voltage must deliver without an output load.
The DC power that the amplifier draws with no output load.
In the data sheets, the current and power consumption of opamps without load is always about the same. This has to do with the fact that the quiescent current in the output stages, in order to keep the distortion factor low, makes up a large proportion.
In connection with the inverting operation one reads again and again that the input signal is phase shifted by 180 ° to the output signal (inverting operational amplifier). It is a mistake. Though it is read over and over and is said over and over again. It is wrong. It is correct that the input signal is inverted to the output signal.
So the signals are of opposite polarity to each other. A positive signal at the input means a negative signal at the output. A negative signal at the input means a positive signal at the output. And this situation is independent of the signal frequency.
A phase shift of 180 °, on the other hand, would mean that the input and output signals would be offset in time by half a period. It is not so.
For this it is recommended to read "phase shift or inversion, that is the question ..." in amplifier / attenuator with symmetrical output.
There is no such thing as common-mode operation. It is only used to determine whether the common mode rejection is working properly. If so, then after applying an input signal to the output, there is no signal. Since every operational amplifier is unbalanced inside, there is still a signal at the output. This operation is only a test case to check the common mode rejection.
The operational amplifier is a very universal component. It can be used as an amplifier, oscillation generator, switch stage, in subtracting and adding circuits and active filters.
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