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연산증폭기(OP amp) 응용 – 비교기(Comparator) 기초

OP Amp 비교기(Comparator) 회로 – 공대생의 오아시스

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Summary of article content: Articles about OP Amp 비교기(Comparator) 회로 – 공대생의 오아시스 이름은 ‘비교기(Comparator)’라고 거창하게 붙여놨지만 사실 그 정체는 그냥 OP Amp 입니다.정확히는 ‘Negative Feedback 이 없는 OP Amp ‘라고 … …
Most searched keywords: Whether you are looking for OP Amp 비교기(Comparator) 회로 – 공대생의 오아시스 이름은 ‘비교기(Comparator)’라고 거창하게 붙여놨지만 사실 그 정체는 그냥 OP Amp 입니다.정확히는 ‘Negative Feedback 이 없는 OP Amp ‘라고 … 이름은 ‘비교기(Comparator)’라고 거창하게 붙여놨지만 사실 그 정체는 그냥 OP Amp 입니다.정확히는 ‘Negative Feedback 이 없는 OP Amp ‘라고 하는게 맞겠네요. OP Amp 의 Open Loop Gain 이라는 것은 크면 클수록 바람직하다고 했던 것을 기억하시나요?이 때문에 Ideal OP Amp 는 Open Loop Gain으로 ∞ 값을 가져야하며 실제 OP Amp들은 ∞ 까지는 아니더라도 수 만 ~ 수십 만의 충분히 큰 값을 가진다는 것도 배웠습니다.
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비교기(Comparator) 분석

비교기(Comparator) 응용

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OP Amp 비교기(Comparator) 회로 - 공대생의 오아시스 — OP Amp 비교기(Comparator) 회로 – 공대생의 오아시스

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Summary of article content: Articles about OP AMP 비교기 (Comparator)회로란? 비교기는 사실 Negative Feedback이 없는 그냥 OP Amp라고 생각하시면 됩니다. (feedback이 없는 유일한 연산증폭기 응용회로). OP AMP의 Open Loop … …
Most searched keywords: Whether you are looking for OP AMP 비교기 (Comparator)회로란? 비교기는 사실 Negative Feedback이 없는 그냥 OP Amp라고 생각하시면 됩니다. (feedback이 없는 유일한 연산증폭기 응용회로). OP AMP의 Open Loop … 안녕하세요 전자공학의 꽃, OP AMP! 오늘은 OP AMP의 비교기에 대해서 정리해보겠습니다 비교기는 사실 Negative Feedback이 없는 그냥 OP Amp라고 생각하시면 됩니다 (feedback이 없는 유일한 연산증폭기 응용회..
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OP Amp 비교기(Comparator) 회로

시작하기 전에…

안녕하세요 공대생의 오아시스입니다. ^^

이번 강의, 비교기(Comparator)를 마지막으로 길고도 길던 OP Amp 단원이 끝날 예정입니다.

간단한 내용이지만 OP Amp의 특징을 복습할 수 있는 기회가 되기 때문에 이렇게 따로 분리하였는데요.

마지막까지 열심히해서 얻어가는게 있는 강의가 되도록 노력하겠습니다.

비교기(Comparator) 분석

이름은 ‘비교기’라고 거창하게 붙여놨지만 사실 그 정체는 그냥 OP Amp입니다.

정확히는 ‘Negative Feedback이 없는 OP Amp’라고 하는게 맞겠네요.

OP Amp의 Open Loop Gain이라는 것은 크면 클수록 바람직하다고 했던 것을 기억하시나요?

이 때문에 Ideal OP Amp는 Open Loop Gain으로 ∞ 값을 가져야하며 실제 OP Amp들은 ∞ 까지는 아니더라도 수 만 ~ 수십 만의 충분히 큰 값을 가진다는 것도 배웠습니다.

(기억이 안나거나 무슨 소리인지 잘 모르겠다 하시는 분들은 이전 강의를 반드시 먼저 봐주세요…!)

결국 Open Loop Gain이라는 건 Negative Feedback이 없을 경우의 Gain인데, 비교기는 바로 그 Open Loop Gain이 엄청 큰 값을 가진다는 점을 이용한 회로라고 할 수 있습니다.

OP Amp의 본질은 두 입력단자에 들어오는 전압의 차이, 즉 \mathrm{\mathbf{V_+-V_-}}를 Open Loop Gain배만큼 증폭시켜 출력단자로 보내주는 것입니다.

Ideal OP Amp, 즉 Open Loop Gain이 ∞인 경우를 기준으로 생각해봅시다.

만약 \mathrm{V_+}가 \mathrm{V_-}보다 살짝이라도 크다면 어떻게 될까요?

\mathrm{V_+-V_-}(양수)는 ∞배 증폭되어 결국 출력값은 + Bias Voltage \mathrm{\mathbf{V_{CC}}}로 Saturation 될 것입니다.

반대로 \mathrm{V_-}가 \mathrm{V_+}보다 크다면 출력값은 – Bias Voltage \mathrm{\mathbf{V_{EE}}}로 Saturation 된다는 것도 이해 가시나요?

음… 말이 자꾸 어려워지네요. 결론만 말씀드리겠습니다.

\mathrm{V_+} 가 \mathrm{V_-} 보다 크면 \mathrm{V_{CC}} 를 출력

\mathrm{V_-} 가 \mathrm{V_+} 보다 크면 \mathrm{V_{EE}} 를 출력

이게 비교기, Comparator가 하는 일입니다.

더 단순하게 말씀드릴까요?

두 입력전압 중 어느 쪽이 큰 지 알려주는 역할을 합니다.

비교기(Comparator) 응용

좀 더 감을 잡으실 수 있도록 그림 하나를 가져와봤습니다.

왼쪽에 있는 회로가 복잡해보이신다면 신경쓰지 않으셔도 되지만… 자세히보면 크게 어려운 건 아니니 시도는 해보시기 바랍니다 ^^;

중요한 것은 오른쪽의 그래프입니다.

오른쪽 위의 그래프에서 초록색 선이 \mathrm{V_+}, 빨간색 선이 \mathrm{V_-}인데요.

\mathrm{V_+}가 \mathrm{V_-}보다 커질때 아래 그래프의 \mathrm{V_{out}}이 \mathrm{V_{CC}}로 Saturation 되는 것이 보이시나요?

바로 이걸 보여드리고 싶었습니다.

말로만 백번 듣는 것보다는 그래프로 한번 보는게 훨씬 나으니까요 ㅎㅎ.

시간이 되신다면 다른 자료들도 찾아보면서 비교기가 어떻게 쓰이는지 더 알아보시기 바랍니다. ^^

마치며…

일단 이렇게 OP Amp의 핵심적인 내용이 끝났는데요, OP Amp는 앞으로도 계속 전기회로를 공부하신다면 잊을만할 때마다 모습을 드러낼겁니다.

당장 Capacitor와 Inductor를 배울 때만 해도 미분기와 적분기라는 이름을 달고 다시 나올거구요… ㅠㅠ

아무튼 OP Amp는 그만큼 중요하다는 겁니다.

그래도 문제에 튀어나올때마다 무지막지하게 어려운 걸 요구하지는 않으니 부디 기본적인 내용은 꼭 숙지해두시길 바랍니다.

OP Amp 배우시느라 정말 고생 많으셨고, 긴 글 읽어주셔서 감사합니다.

앞으로는 더 좋은 글로 찾아뵙겠습니다. 지금까지 공대생의 오아시스였습니다. ^^

OP AMP 비교기 (Comparator)회로란?

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안녕하세요

전자공학의 꽃, OP AMP!

오늘은 OP AMP의 비교기에 대해서 정리해보겠습니다

비교기는 사실 Negative Feedback이 없는 그냥 OP Amp라고 생각하시면 됩니다

(feedback이 없는 유일한 연산증폭기 응용회로)

OP AMP의 Open Loop Gain은 클수록 좋은데 이 Open Loop Gain은 Negative Feedback이 없을 경우의 gain입니다

따라서 OP amp 비교기는 이 Open Loop Gain이 엄청 큰 값을 가진다는 점을 가진 회로입니다.

OP Amp는 두 입력단자에 들어오는 전압의 차이,

즉 V+−V− 를 Open Loop Gain의 배만큼 증폭시켜 출력단자로 보내주는데

Ideal OP Amp, 즉 Open Loop Gain이 무한대인 경우를 기준으로 생각해보겠습니다

연산증폭기의 출력은 연산증폭기의 Power supply 전압을 초과할 수 없기 때문에

만약 V+−V-보다 조금이라도 더 크다면 V+−V-는

무한대배만큼 증폭되어 결국 출력값은 +Bias Voltage Vcc로 Saturation상태가 됩니다

반대로 V+−V-보다 크다면 출력값은 -Bias Voltage VEE로 Saturation상태가 되겠죠

아래 그림을 보시면 V+가 V-보다 커질 때 아래 그래프의 Vout이 VCC로 Saturation되는 것을 확인할 수 있습니다.

아날로그 비교기의 주요 응용은

입력전압이 어느 일정 레벨을 넘는 것을 감지할 때 주로 레벨 검출로 사용되고

사인파 입력을 구형파로 만드는 구형파 발생회로에 주로 쓰입니다.

비교기 IC는 일반 OP Amp보다 두 출력 상태 간 비교 및 스위칭이 빠르고

잡음에도 강하게 제작 되는 것이 바로 OP Amp 비교기입니다

OP Amp는 전자공학 공부하실 때 정말 중요하기 때문에

기본적인 공부가 잘될 수 있도록 다음에도 더 알찬 포스팅으로 찾아뵙겠습니다

감사합니다 🙂

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Op-amp Comparator and the Op-amp Comparator Circuit

The Op-amp comparator compares one analogue voltage level with another analogue voltage level, or some preset reference voltage, V REF and produces an output signal based on this voltage comparison. In other words, the op-amp voltage comparator compares the magnitudes of two voltage inputs and determines which is the largest of the two.

We have seen in previous tutorials that the operational amplifier can be used with negative feedback to control the magnitude of its output signal in the linear region performing a variety of different functions. We have also seen that the standard operational amplifier is characterised by its open-loop gain A O and that its output voltage is given by the expression: V OUT = A O (V+ – V-) where V+ and V- correspond to the voltages at the non-inverting and the inverting terminals respectively.

Voltage comparators on the other hand, either use positive feedback or no feedback at all (open-loop mode) to switch its output between two saturated states, because in the open-loop mode the amplifiers voltage gain is basically equal to A VO . Then due to this high open loop gain, the output from the comparator swings either fully to its positive supply rail, +Vcc or fully to its negative supply rail, -Vcc on the application of varying input signal which passes some preset threshold value.

The open-loop op-amp comparator is an analogue circuit that operates in its non-linear region as changes in the two analogue inputs, V+ and V- causes it to behave like a digital bistable device as triggering causes it to have two possible output states, +Vcc or -Vcc. Then we can say that the voltage comparator is essentially a 1-bit analogue to digital converter, as the input signal is analogue but the output behaves digitally.

Consider the basic op-amp voltage comparator circuit below.

Op-amp Comparator Circuit

With reference to the op-amp comparator circuit above, lets first assume that V IN is less than the DC voltage level at V REF , ( V IN < V REF ). As the non-inverting (positive) input of the comparator is less than the inverting (negative) input, the output will be LOW and at the negative supply voltage, -Vcc resulting in a negative saturation of the output. If we now increase the input voltage, V IN so that its value is greater than the reference voltage V REF on the inverting input, the output voltage rapidly switches HIGH towards the positive supply voltage, +Vcc resulting in a positive saturation of the output. If we reduce again the input voltage V IN , so that it is slightly less than the reference voltage, the op-amp’s output switches back to its negative saturation voltage acting as a threshold detector. Then we can see that the op-amp voltage comparator is a device whose output is dependant on the value of the input voltage, V IN with respect to some DC voltage level as the output is HIGH when the voltage on the non-inverting input is greater than the voltage on the inverting input, and LOW when the non-inverting input is less than the inverting input voltage. This condition is true regardless of whether the input signal is connected to the inverting or the non-inverting input of the comparator. We can also see that the value of the output voltage is completely dependent on the op-amps power supply voltage. In theory due to the op-amps high open-loop gain the magnitude of its output voltage could be infinite in both directions, (±∞). However practically, and for obvious reasons it is limited by the op-amps supply rails giving V OUT = +Vcc or V OUT = -Vcc. We said before that the basic op-amp comparator produces a positive or negative voltage output by comparing its input voltage against some preset DC reference voltage. Generally, a resistive voltage divider is used to set the input reference voltage of a comparator, but a battery source, zener diode or potentiometer for a variable reference voltage can all be used as shown. Comparator Reference Voltages In theory the comparators reference voltage can be set to be anywhere between 0v and the supply voltage but there are practical limitations on the actual voltage range depending on the op-amp comparator being device used. Positive and Negative Voltage Comparators A basic op-amp comparator circuit can be used to detect either a positive or a negative going input voltage depending upon which input of the operational amplifier we connect the fixed reference voltage source and the input voltage too. In the examples above we have used the inverting input to set the reference voltage with the input voltage connected to the non-inverting input. But equally we could connect the inputs of the comparator the other way around inverting the output signal to that shown above. Then an op-amp comparator can be configured to operate in what is called an inverting or a non-inverting configuration. Positive Voltage Comparator The basic configuration for the positive voltage comparator, also known as a non-inverting comparator circuit detects when the input signal, V IN is ABOVE or more positive than the reference voltage, V REF producing an output at V OUT which is HIGH as shown. Non-inverting Comparator Circuit In this non-inverting configuration, the reference voltage is connected to the inverting input of the operational amplifier with the input signal connected to the non-inverting input. To keep things simple, we have assumed that the two resistors forming the potential divider network are equal and: R1 = R2 = R. This will produce a fixed reference voltage which is one half that of the supply voltage, that is Vcc/2, while the input voltage is variable from zero to the supply voltage. When V IN is greater than V REF , the op-amp comparators output will saturate towards the positive supply rail, Vcc. When V IN is less than V REF the op-amp comparators output will change state and saturate at the negative supply rail, 0v as shown. Negative Voltage Comparator The basic configuration for the negative voltage comparator, also known as an inverting comparator circuit detects when the input signal, V IN is BELOW or more negative than the reference voltage, V REF producing an output at V OUT which is HIGH as shown. Inverting Comparator Circuit In the inverting configuration, which is the opposite of the positive configuration above, the reference voltage is connected to the non-inverting input of the operational amplifier while the input signal is connected to the inverting input. Then when V IN is less than V REF the op-amp comparators output will saturate towards the positive supply rail, Vcc. Likewise the reverse is true, when V IN is greater than V REF , the op-amp comparators output will change state and saturate towards the negative supply rail, 0v. Then depending upon which op-amp inputs we use for the signal and the reference voltage, we can produce an inverting or non-inverting output. We can take this idea of detecting either a negative or positive going signal one step further by combining the two op-amp comparator circuits above to produce a window comparator circuit. Window Comparator A Window Comparator is basically the inverting and the non-inverting comparators above combined into a single comparator stage. The window comparator detects input voltage levels that are within a specific band or window of voltages, instead of indicating whether a voltage is greater or less than some preset or fixed voltage reference point. This time, instead of having just one reference voltage value, a window comparator will have two reference voltages implemented by a pair of voltage comparators. One which triggers an op-amp comparator on detection of some upper voltage threshold, V REF(UPPER) and one which triggers an op-amp comparator on detection of a lower voltage threshold level, V REF(LOWER) . Then the voltage levels between these two upper and lower reference voltages is called the “window”, hence its name. Using our idea above of a voltage divider network, if we now use three equal value resistors so that R1 = R2 = R3 = R we can create a very simple window comparator circuit as shown. Also as the resistive values are all equal, the voltage drops across each resistor will also be equal at one-third the supply voltage, 1/3Vcc. So for ease in this simple window comparator example, we can set the upper reference voltage to 2/3Vcc and the lower reference voltage to 1/3Vcc. Consider the window comparator circuit below. Window Comparator Circuit The inital switching condition of the circuit is the open-collector output of op-amp A 1 “OFF” with the open-collector output of op-amp A 2 , “ON” (sinking current) so V OUT is equal to 0V. When V IN is below the lower voltage level, V REF(LOWER) which equates to 1/3Vcc, V OUT will be LOW. When V IN exceeds this 1/3Vcc lower voltage level, the first op-amp comparator detects this and switches its open-collector output HIGH. This means that both op-amps have their outputs HIGH at the same time. No current flows through the pull-up resistor R L so V OUT is equal to Vcc. As V IN continues to increase it passes the upper voltage level, V REF(UPPER) at 2/3Vcc. At this point the second op-amp comparator detects this and switches its output LOW and V OUT becomes equal to 0V. Then the difference between V REF(UPPER) and V REF(LOWER) (which is 2/3Vccc – 1/3Vcc in this example) creates the switching window for the positive going signal. Lets now assume that V IN is at its maximum value and equal to Vcc. As V IN decreases it passes the upper voltage level V REF(UPPER) of the second op-amp comparator which switches the output HIGH. As V IN continues to decrease it passes the lower voltage level, V REF(LOWER) of the first op-amp comparator once again switching the output LOW. Then the difference between V REF(UPPER) and V REF(LOWER) creates the window for the negative going signal. So we can see that as V IN passes above or passes below the upper and lower reference levels set by the two op-amp comparators, the output signal V OUT will be HIGH or LOW. In this simple example we have set the upper trip level at 2/3Vcc and the lower trip level at 1/3Vcc (because we used three equal value resistors), but can be any values we choose by adjusting the input thresholds. As a result, the window width can be customized for a given application. If we used a dual power supply and set the upper and lower trip levels to say ±10 volts and V IN was a sinusoidal waveform, then we could use this window comparator circuit as a zero crossing detector of the sine wave which would produce an output, HIGH or LOW every time the sine wave crossed the zero volts line from positive to negative or negative to positive. We can take this idea of detecting voltage levels further by connecting a number of different op-amp comparators together with them all using a common input signal, but with each comparator using a different reference voltage set by our now familiar voltage divider network across the supply. Consider the voltage level detector circuit below. Comparator Voltage Level Detector As above, the voltage divider network provides a set of reference voltages for the individual op-amp comparator circuits. To produce the four reference voltages will require five resistors. The junction at the bottom pair of resistors will produce a reference voltage that is one-fifth the supply voltage, 1/5Vcc using equal value resistors. The second pair 2/5Vcc, a third pair 3/5Vcc and so on, with these reference voltages increasing by a fixed amount of one-fifth (1/5) towards 5/5Vcc which is actually Vcc. As the common input voltage increases, the output of each op-amp comparator circuit switches in turn thereby turning OFF the connected LED starting with the lower comparator, A 4 and upwards towards A 1 as the input voltage increases. So by setting the values of the resistors in the voltage divider network, the comparators can be configured to detect any voltage level. One good example of the use of voltage level detection and indication would be for a battery condition monitor by reversing the LED’s and connecting them to 0V (ground) instead of V CC . Also by increasing the number of op-amp comparators in the set, more trigger points can be created. So for example, if we had eight op-amp comparators in the chain and fed the output of each comparator to an 8-to-3 line Digital Encoder, we could make a very simple analogue-to-digital converter, (ADC) that would convert the analogue input signal into a 3-bit binary code (0-to-7). Op-amp Comparator with Positive Feedback We have seen here that operational amplifiers can be configured to operate as comparators in their open-loop mode, and this is fine if the input signal varies rapidly or is not too noisy. However if the input signal, V IN is slow to change or electrical noise is present, then the op-amp comparator may oscillate switching its output back and forth between the two saturation states, +Vcc and -Vcc as the input signal hovers around the reference voltage, V REF level. One way to overcome this problem and to avoid the op-amp from oscillating is to provide positive feedback around the comparator. As its name implies, positive feedback is a technique for feeding back a part or fraction of the output signal that is in phase to the non-inverting input of the op-amp via a potential divider set up by two resistors with the amount of feedback being proportional to their ratio. The use of positive feedback around an op-amp comparator means that once the output is triggered into saturation at either level, there must be a significant change to the input signal V IN before the output switches back to the original saturation point. This difference between the two switching points is called hysteresis producing what is commonly called a Schmitt trigger circuit. Consider the inverting comparator circuit below. Inverting Op-amp Comparator with Hysteresis For the inverting comparator circuit above, V IN is applied to the inverting input of the op-amp. Resistors R 1 and R 2 form a voltage divider network across the comparator providing the positive feedback with part of the output voltage appearing at the non-inverting input. The amount of feedback to the non-inverting input is determined by the resistive ratio of the two resistors used and which is given as: Voltage Divider Equation Where: β (beta) can be used to indicate the feedback fraction. When the input signal is less than the reference voltage, V IN < V REF , the output voltage will be HIGH, V OH and equal to the positive saturation voltage. As the output is HIGH and positive, the value of the reference voltage on the non-inverting input will be approximately equal to: +β*V called the Upper Trip Point or UTP. As the input signal, V IN increases it becomes equal too this upper trip point voltage, V UTP level at the non-inverting input. This causes the comparators output to change state becoming LOW, V OL and equal to the negative saturation voltage as before. But the difference this time is that a second trip point voltage value is created because a negative voltage now appears at the non-inverting input which is equal to: -β*V as a result of the negative saturation voltage at the output. Then the input signal must now fall below this second voltage level, called the Lower Trip Point or LTP for the voltage comparators output to change or switch back to its original positive state. So we can see that when the output changes state, the reference voltage at the non-inverting input also changes creating two different reference voltage values and two different switching points. One point being called the Upper Trip Point (UTP), while the other is called the Lower Trip Point (LTP). The difference between these two trip points is known as Hysteresis. The amount of hysteresis is determined by the feedback fraction, β of the output voltage fed back to the non-inverting input. The advantage of positive feedback is that the resulting comparator Schmitt trigger circuit is immune to erratic triggering caused by noise or slowly changing input signals within the hysteresis band producing a cleaner output signal as the op-amp comparators output is only triggered once. So for positive output voltages, V REF = +β*Vcc, but for negative output voltages, V REF = -β*Vcc. Then we can say that the amount of voltage hysteresis will be given as: We can also produce a non-inverting op-amp comparator circuit with built in hysteresis by changing the input and reference terminals as shown: Non-inverting Op-amp Comparator with Hysteresis Note that the arrows on the hysteresis graph indicate the direction of switching at the upper and lower trip points. Op-amp Comparator Example No1 An operational amplifier is to be used with positive feedback to produce a Schmitt trigger circuit. If resistor, R 1 = 10kΩ and resistor, R 2 = 90kΩ, what will be the values of the upper and lower switching points of the reference voltage and the width of the hysteresis if the op-amp is connected to a dual ±10v power supply. Given: R 1 = 10kΩ, R 2 = 90kΩ. Power supply +Vcc = 10v and -Vcc = 10v. Feedback Fraction Upper Voltage Trip Point, V UTP Lower Voltage Trip Point, V LTP Hysteresis width: Then the reference voltage V REF , switches between +1V and -1V as the output saturates from one level to the other. Hopefully we can see from this simple example that the width of this hysteresis, 2 volts in total, can be made larger or smaller simply by adjusting the voltage divider ratio of the feedback resistors R 1 and R 2 . The Voltage Op-amp Comparator Although we can use operational amplifiers such as the 741 as a basic comparator circuit, the problem with this is that op-amps are only optimised for linear operation. That is where the input terminals are at virtually the same voltage level and its output stage is designed to produce a linear output voltage that is not saturated for long periods of time. Also standard operational amplifiers are designed to be used in closed-loop applications with negative feedback from its output to its inverting input. A dedicated voltage comparator on the other hand is a non-linear device that allows for heavy saturation, due to its very high gain, when the input signals differs by a relatively small amount. The difference between an op-amp comparator and a voltage comparator is in the output stage as a standard op-amp has an output stage that is optimized for linear operation, while the output stage of a voltage comparator is optimized for continuous saturated operation as it is always intended to be close to one supply rail or the other and not in between. Commercial comparators such as the LM311 single comparator, the LM339 quad comparator or the LM393 dual differential comparator, are voltage comparators which come in a standard IC package operating from a single or dual supply. These dedicated voltage comparators are designed for the sole purpose of switching the output very quickly from one saturated state the another as the transistors used for a voltage comparators output stage are generally switching transistors. Since voltage comparators convert a linear input signal into a digital output signal, they are commonly used to connect two dissimilar electrical signals with different supply or reference voltages. As a result the output stage of the voltage comparator is generally configured as a single open collector (or Drain) transistor switch with open or closed states rather than actual output voltages as shown. Voltage Op-amp Comparator Circuit Here, the open collector output from the voltage comparator is connected to a voltage source via a single pull-up resistor (and an LED for indication) which pulls the single output high to the power supply. When the output switch is HIGH it creates a high impedance path, therefore no current flows as V OUT = Vcc. When the comparator changes state and the output switch is LOW, it creates a low impedance path to ground and current flows through the pull-up resistor (and LED) causing a voltage drop across itself with the output being pulled to the lower supply level, ground in this case. Then we can see that there is very little difference between the schematic symbol of an op-amp comparator and a voltage comparator or their internal circuits. The main difference is in the output stage with the open collector or drain configuration is useful for driving relays, lamps, etc. By driving a transistor from the output allows for a greater switching current capacity than that of the comparators output alone. Op-amp Comparator Summary In this tutorial about the Op-amp Comparator we have seen that a comparator circuit is basically an operational amplifier without feedback, that is, the op-amp is used in its open-loop configuration, and when the input voltage, V IN exceeds a preset reference voltage, V REF , the output changes state. Due to the very high open-loop gain of the operational amplifier, using it with positive feedback or even with no feedback at all causes the output to saturate to its supply rail producing one of two distinct output voltages depending on the relative values of its two inputs. This bistable behaviour is non-linear and forms the basis of op-amp comparator and Schmitt trigger circuits. The output stages of dedicated comparators, such as the single LM311, the dual LM393 or the quad LM339 are designed to operate in their saturation regions allowing these voltage comparator circuits to be widely used in analogue-to-digital converter applications and for various types of voltage level detection circuits. The erratic switching behaviour of an open-loop comparator can be easily overcome by adding positive feedback between the output and input of the comparator. With positive feedback, the circuit has hysteresis with the output switching occurring between two different switching points, UTP and LTP. Op-amp window comparators are a type of voltage comparator circuit which uses two op-amp comparators to produce a two-state output that indicates whether or not the input voltage is within a particular range or window of values by using two reference voltages. An upper reference voltage and a lower reference voltage. While operational amplifiers and comparators may look similar, they are very different and designed to be used in different applications as an op-amp may be used as a comparator, a voltage comparator can not be used as an op-amp due to its non-linear output stage. We know from previous tutorials that an operational amplifier is an analogue device with a differential analogue input and an analogue output and if operated in its open-loop configuration its output acts like a comparator output. But dedicated voltage comparators (LM311, LM393, LM339) are widely available which will perform much better than a standard op-amp comparator.

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