When an op amp is over-driven, sometimes by as little as a few millivolts, some of the internal stages may saturate. If this occurs the device will take a comparatively long time to come out of saturation and will therefore be much slower than if it always remained unsaturated see figure 4. The time to come out of saturation of an overdriven op amp is likely to be considerably longer than the normal group delay of the amplifier, and will often depend on the amount of overdrive.
Since few op amps have this saturation recovery time specified for various amounts of overdrive it will generally be necessary to determine, by experimental measurements in the lab, the behavior of the amplifier under the conditions of overdrive to be expected in a particular design. The results of such experimental measurements should be regarded with suspicion and the values of propagation delay through the op amp comparator which is chosen for worst-case design calculations should be at least twice the worst value seen in any experiment.
The output of a comparator is designed to drive a particular logic family or families, while the output of an op amp is designed to swing close to it's supply rails if not to the supply rails.
Frequently the logic being driven by the op amp comparator will not share the op amp's supplies and the op amp rail to rail swing may go outside the logic supply rails-this will probably damage the logic circuitry, and the resulting short circuit may damage the op amp as well. ECL is a very fast current steering logic family. It is unlikely that an op amp would be used as a comparator in applications where ECL's highest speed is involved, for reasons given above, so we shall usually be concerned only to drive ECL logic levels from an op amp's signal swing and some additional loss of speed due to stray capacities will be unimportant.
To do this we need only three resistors, as shown in figure 4. R 1 , R 2 and R 3 are chosen so that when the op amp output is positive the level at the gate is Using low resistance values for R 1 , R 2 and R 3 will minimize the effects of stray capacitance but at the same time will increase power consumption. A resistor between the op amp output and the MOS FET gate and the diode to ground are generally not needed left side of figure 4.
The speed of the transition depends on the value of R L and the stray capacity of the output node. The lower the value of R L the faster the response will be, but the higher the power consumption. Furthermore, it may be made inverting or non-inverting by simple positioning of components.
It does, however, have a large current surge during switching, when both devices are on at once, and unless MOS devices with high channel resistance are used a current limiting resistor may be necessary to reduce this effect. It is also important, in this application and the one in figure 4. There are a number of effects which must be considered regarding the inputs of op amps when used as comparators.
The first-level assumption engineers make about all op amps and comparators is that they have infinite input impedance and can be regarded as open circuits except for current feedback transimpedance op amps, which have a high impedance on their non-inverting input but a low impedance of a few tens of ohms on their inverting input.
But many op amps especially bias-compensated ones such as the OP and its many descendants contain protective circuitry to prevent large differential input voltages from damaging the input stage transistors. Protective circuitry such as current limiting resistors and clamp diodes, as shown in figure 4. Other op amp designs contain more complex input circuitry, which only has high impedance when the differential voltage applied to it is less than a few tens of mV , or which may actually be damaged by differential voltages of more than a few volts.
It is therefore necessary, when using an op amp as a comparator, to study the manufacturer's data sheet to determine how the input circuitry behaves when large differential voltages are applied to it. It is always necessary to study the data sheet when using an integrated circuit to ensure that its non-ideal behavior, and every integrated circuit ever made has some non-ideal behavior, is compatible with the proposed design - it is just more important than usual in the present case.
Of course some comparator applications never involve large differential voltages-or if they do the comparator input impedance when large differential voltages are present is comparatively unimportant. In such cases it may be appropriate to use as a comparator an op amp whose input circuitry behaves non-linearly-but the issues involved must be considered, not just ignored. As mentioned elsewhere in this text, nearly all BIFET op amps exhibit anomalous behavior when their inputs are close to one of their supplies usually the negative supply.
Their inverting and non-inverting inputs may become interchanged. If this should occur when the op amp is being used as a comparator the phase of the system involved will be inverted, which could well be inconvenient. The solution is, again, careful reading of the data sheet to determine just what common-mode range is acceptable. Also, the absence of negative feedback means that, unlike that of op amp circuits, the input impedance is not multiplied by the loop gain.
As a result, the input current varies as the comparator switches. Therefore the driving impedance, along with parasitic feedback paths, can play a key role in affecting circuit stability. While negative feedback tends to keep amplifiers within their linear region, positive feedback forces them into saturation.
Operational amplifiers are not designed to be used as comparators, so this section has been, intentionally, a little discouraging. Nevertheless there are some cases where the use of an op amp as a comparator is a useful engineering decision-what is important is to make it a considered decision, and ensure that the op amp chosen will perform as expected.
To do this it is necessary to read the manufacturer's data sheet carefully, to consider the effects of non-ideal op amp performance, and to calculate the effects of op amp parameters on the overall circuit. Since the op amp is being used in a non-standard manner some experimentation may also be necessary, since the amplifier used for the experiment will not necessarily be typical and the results of experiments should always be interpreted somewhat pessimistically.
Although the simple voltage comparator circuit using either an ordinary operational amplifier or a special comparator is often adequate, the input waveform may be slow or have noise superimposed on it. This can result in the possibility that the output will switch back and forth several times as the input transitions through the comparator threshold voltage. Voltage to Current converter.
In other words, input volt appears across R 1. Thus, we can conclude from the above equation that the current IL is related to the voltage, V IN and the resistor, R. Current to Voltage converter. World's No 1 Animated self learning Website with Informative tutorials explaining the code and the choices behind it all. Powered by Inplant Training in chennai Internship in chennai. We've detected that you are using AdBlock Plus or some other adblocking software which is preventing the page from fully loading.
We don't have any banner, Flash, animation, obnoxious sound, or popup ad. By keeping RF variable, it is possible to vary the sensitivity as per the requirements. In some applications, it is necessary to have matched diodes with equal voltage drops at a particular value of diode current.
The circuit can be used in finding matched diodes and is obtained from fig V-I converter with floating load by replacing RL with a diode. As long as V 0 and R 1 constant, I 0 will be constant. To avoid an error in output voltage the op-amp should be initially nulled. Thus the matched diodes can be found by connecting diodes one after another in the feedback path and measuring voltage across them.
When the switch position 2 when the switch is in position 2, the circuit becomes a Zener diode tester. The circuit can be used to find the breakdown voltage of Zener diodes.
The Zener current is set at a constant value by Vin and R1. The circuit becomes a LED when the switch is in position 3. LED current is set at a constant value by V in and R 1. LEDs can be tested for brightness one after another at this current. Matched LEDs with equal brightness at a specific value of current are useful as indicates and display devices in digital applications. One of the most common uses of the current to voltage converter is.
Digital to analog Converter DAC. Sensing current through Photo detector. Such as photo cell, photo diodes and photovoltaic cells. Let current I 1 and I 2 flows through the resistor R 1 and R 2 respectively. Ground load voltage to current converter circuit 2 as shown in figure 5.
The input V i is applied as shown in figure 5. The resistance R 3 connected to the non-inverting terminal of the op-amp is grounded. The analysis of the V-I converter circuit is shown in figure 6. This site uses Akismet to reduce spam. Learn how your comment data is processed. We've detected that you are using AdBlock Plus or some other adblocking software which is preventing the page from fully loading.
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