domingo, 21 de marzo de 2010
Algo de Historia
Los primeros años del amplificador operacional no fueron los de un circuito integrado de 8 patitas. Este amplificador operacional era un tubo al vacío. El Sr. George Philbrick, que trabajaba en los Huntington Engeneering Labs, y a quien se le atribuye su invención, lo introdujo al mercado en el año 1948.
La idea principal de estos "operacionales" originales era la de ser utilizados en computadoras analógicas, para sumar, restar, multiplicar y realizar operaciones más complejas.
Fue la empresa Fairchild la que en los años 1964 y 1967 introdujo al mercado los conocidos Amplificadores operacionales 702, 709 y 741. Y la National Semicoductor hizo lo mismo con el 101/301.
Estos circuitos integrados son muy versátiles, de bajo precio, tamaño pequeño, con excelentes características y redujeron el diseño de un amplificador a la adición de unos resistores.
Con el paso de los años y la mejora en la tecnología de fabricación, Los amplificadores operacionales mejoraron notablemente. En su configuración interna se reemplazaron unos transistores bipolares por transistores de efecto de campo (JFET). Estos amplificadores JFET están a las entradas del amplificador operacional incrementándose así la impedancia de entrada de este. El operacional puede ahora amplificar señales que pueden tener la amplitud de la fuente que los alimenta y tomar muy poca corriente de la señal de entrada. Los transistores MOS (semiconductor de oxido metálico) se pusieron en los circuitos de salida.
El primer amplificador (BIFET) con transistores de efecto de campo fue en LF356. El amplificador operacional BIMOS como el CA3130 tiene entradas bipolares y salida MOS (de allí viene el nombre). Estos últimos amplificadores son mas rápidos y tiene unas respuesta mejor a las altas frecuencias que el conocido 741.
Hay versiones de varios operacionales en un solo integrado como el LM358 con 2 y el LM324 con 4 amplificadores operacionales juntos.
En algún momento se tuvo que especializar el amplificador de propósito general que hasta ahora se había utilizado y salieron al mercado una gran variedad del original:
- Capacidad de alta corriente, alto voltaje o ambos
- Amplificadores múltiples
- Amplificadores de ganancia programable
- Amplificadores de instrumentación y control automotriz
- Circuitos integrados para comunicaciones
- Circuitos integrados para radio / audio / video
Los amplificadores operacionales de propósito general no dejarán de usarse debido a su gran demanda e infinidad de posible aplicaciones, pero los amplificadores operacionales de propósito especifico como los de la lista anterior seguirán aumentando con el avance de la tecnología.
martes, 16 de marzo de 2010
Amplificadores
Los amplificadores son circuitos que se utilizan a aumentar (amplificar) el valor de la señal de entrada (generalmente muy pequeña) y así obtener una señal a la salida con una amplitud mucho mayor a la señal original.
ENTRADA
SALIDA
Algunas veces la amplificación puede causar que la señal a la salida del amplificador salga distorsionada causada por una amplificación muy grande.
Hay que tomar en cuenta que un amplificador no puede tener en su salida niveles de voltaje mayores a los que la fuente de alimentación, que lo alimenta, le puede dar.
Ejemplo: Si el amplificador es alimentado por 12 Voltios. La señal de salida no podrá tener un voltaje mayor a este.
Nota: Para aumentar la potencia de salida de un amplificador normalmente se aumenta la cantidad de corriente que éste puede suministrar. Acordarse que:
Hay que tomar en cuenta que un amplificador no puede tener en su salida niveles de voltaje mayores a los que la fuente de alimentación, que lo alimenta, le puede dar.
Ejemplo: Si el amplificador es alimentado por 12 Voltios. La señal de salida no podrá tener un voltaje mayor a este.
Nota: Para aumentar la potencia de salida de un amplificador normalmente se aumenta la cantidad de corriente que éste puede suministrar. Acordarse que:
P (potencia) = V (voltaje) x I (corriente)
Si no puede aumentar el voltaje hay que aumentar la corriente.
Un caso muy común de amplificador es el que usa transistores bipolares, hay otros que utilizan amplificadores operacionales, tubos o válvulas electrónicas, FETs, etc.
En el caso que se muestra en los diagramas, se ve que la señal de salida (diagrama inferior) es mayor que la de la entrada (diagrama posterior), pero adicionalmente está invertida.
Caso algunas veces se presenta en amplificadores y se llaman amplificadores inversores
Los Amplificadores Operacionales y otros circuitos analógicos, suelen basarse en:
1 - Los amplificadores diferenciales
2 - Etapas de ganancia implementados por amplificadores intermedios acoplados en corriente continua y
3 - Una etapa de salida tipo push-pull (etapa clase B en contrafase)
Ver el siguiente gráfico, donde se muesta el diagrama de bloques con la configuración interna de un amplificador operacional.
Principio de funcionamiento del Amplificador diferencial
El amplificador diferencial básico tiene 2 entradas V1 y V2.
Si la tensión de V1 aumenta, la corriente del emisor del transistor Q1 aumenta (acordarse que IE = BxIB), causando una caida de tensión en Re.
Si la tensión de V2 se mantiene constante, la tensión entre base y emisor del transistor Q2 disminuye, reduciéndose también la corriente de emisor del mismo transistor.
Esto causa que la tensión de colector de Q2 (Vout+) aumente.
La entrada V1 es la entrada no inversora de un amplificador operacional
Del mismo modo cuando la tensión en V2 aumenta, también aumenta la la corriente de colector del transistor Q2, causando que la tensión de colector del mismo transistor disminuya. (Vout+) disminuye.
La entrada V2 es la entrada inversora del amplificador operacional
Si el valor de la resistencia RE fuera muy grande, obligaría a la suma de las corrientes de emisor de los transistor Q1 y Q2, a mantenerse constante, comportándose como una fuente de corriente
Entonces, al aumentar la corriente de colector de un transistor, disminuirá la corriente de colector del otro transistor.
Por eso cuando la tensión V1 crece, la tensión en V2 decrece.
Amplificador Push-Pull. (amplificador contrafásico)
Se llama amplificador contrafásico, pues utilizan 2 grupos de transistores. Cada grupo se encarga de amplificar una sola fase de la onda de entrada.
Un grupo es de color amarillo y el otro es de color verde. (en este caso sólo hay un transistor por grupo). Cuando un grupo entra en funcionamiento el otro entra está en corte y viceversa.
Un amplificador emisor común se utiliza para amplificar señales pequeñas. En esta configuración la tensión de la señal de salida tiene prácticamente la misma amplitud que la de la señal de entrada (ganancia unitaria) y tienen la misma fase.
Cuando la señal de entrada es grande y lo que se desea es ampliar la capacidad de entrega de corriente, se utiliza un amplificador contrafásico o push-pull. (amplificador de potencia).
El amplificador que se muestra en el siguiente gráfico está constituido por dos transistores. Uno NPN y otro PNP de las mismas características
Si la tensión de V2 se mantiene constante, la tensión entre base y emisor del transistor Q2 disminuye, reduciéndose también la corriente de emisor del mismo transistor.
Esto causa que la tensión de colector de Q2 (Vout+) aumente.
La entrada V1 es la entrada no inversora de un amplificador operacional
Del mismo modo cuando la tensión en V2 aumenta, también aumenta la la corriente de colector del transistor Q2, causando que la tensión de colector del mismo transistor disminuya. (Vout+) disminuye.
La entrada V2 es la entrada inversora del amplificador operacional
Si el valor de la resistencia RE fuera muy grande, obligaría a la suma de las corrientes de emisor de los transistor Q1 y Q2, a mantenerse constante, comportándose como una fuente de corriente
Entonces, al aumentar la corriente de colector de un transistor, disminuirá la corriente de colector del otro transistor.
Por eso cuando la tensión V1 crece, la tensión en V2 decrece.
Amplificador Push-Pull. (amplificador contrafásico)
Se llama amplificador contrafásico, pues utilizan 2 grupos de transistores. Cada grupo se encarga de amplificar una sola fase de la onda de entrada.
Un grupo es de color amarillo y el otro es de color verde. (en este caso sólo hay un transistor por grupo). Cuando un grupo entra en funcionamiento el otro entra está en corte y viceversa.
Un amplificador emisor común se utiliza para amplificar señales pequeñas. En esta configuración la tensión de la señal de salida tiene prácticamente la misma amplitud que la de la señal de entrada (ganancia unitaria) y tienen la misma fase.
Cuando la señal de entrada es grande y lo que se desea es ampliar la capacidad de entrega de corriente, se utiliza un amplificador contrafásico o push-pull. (amplificador de potencia).
El amplificador que se muestra en el siguiente gráfico está constituido por dos transistores. Uno NPN y otro PNP de las mismas características
El transistor Q1 tendrá polarización directa en los semiciclos positivos (ver color amarillo) y a través de RL (resistor de carga) aparecerá una señal que sigue a la entrada (esto significa que la señal de entrada y salida están en fase).
En los ciclos negativos (color verde) el transistor Q1 se pone en corte y no aparecerá señal en la salida (se parece a la salida de un rectificador de media onda)
El transistor Q2 tendrá polarización directa en los semiciclos positivos (ver color verde) y a través de RL aparecerá una señal que sigue a la entrada (están en fase).
En los ciclos positivos (color amarillo) el transistor Q2 se pone en corte y no aparecerá señal en la salida (se parece a la salida de un rectificador de media onda)
Del tercer gráfico (el inmediato superior) se observa que la onda de salida se distorsiona (ver cerca del eje horizontal).
Esta distorsión se debe a la caída de tensión de 0.6 voltios que hay entre la base y el emisor de los transistores Q1 y Q2.
Nota: El valor máximo de la señal de salida siempre será inferior al valor máximo de la señal de entrada debido a la caída de voltaje base - emisor en los transistores. ( la ganancia es siempre ligeramente menor que 1). En otras palabras hay una ligera atenuación, pero una gran ganancia de corriente y como consecuencia una ganancia de potencia.
DAHIANA A. ROSALES H.
EES
Ideal Operational Amplifier
Ideal Operational Amplifier
As well as resistors and capacitors, Operational Amplifiers, or Op-amps as they are more commonly called, are one of the basic building blocks of Analogue Electronic Circuits. It is a linear device that has all the properties required for nearly ideal DC amplification and is used extensively in signal conditioning, filtering or to perform mathematical operations such as add, subtract, integration and differentiation. An ideal Operational Amplifier is basically a 3-terminal device that consists of two high impedance inputs, one an Inverting input marked with a negative sign, ("-") and the other a Non-inverting input marked with a positive plus sign ("+").
The amplified output signal of an Operational Amplifier is the difference between the two signals being applied to the two inputs. In other words the output signal is a differential signal between the two inputs and the input stage of an Operational Amplifier is in fact a differential amplifier as shown below.
Differential Amplifier
The circuit shows a generalized form of a differential amplifier with two inputs marked V1 and V2. The two identical transistors TR1 and TR2 are both biased at the same operating point with their emitters connected together and returned to the common rail, -Vee by way of resistor Re. The circuit operates from a dual supply +Vcc and -Vee which ensures a constant supply. The voltage that appears at the output, Vout of the amplifier is the difference between the two input signals as the two base inputs are in anti-phase with each other. So as the forward bias of transistor, TR1 is increased, the forward bias of transistor TR2 is reduced and vice versa. Then if the two transistors are perfectly matched, the current flowing through the common emitter resistor, Re will remain constant.
Like the input signal, the output signal is also balanced and since the collector voltages either swing in opposite directions (anti-phase) or in the same direction (in-phase) the output voltage signal, taken from between the two collectors is, assuming a perfectly balanced circuit the zero difference between the two collector voltages. This is known as the Common Mode of Operation with the common mode gain of the amplifier being the output gain when the input is zero.
Ideal Operational Amplifiers also have one output (although there are ones with an additional differential output) of low impedance that is referenced to a common ground terminal and it should ignore any common mode signals that is, if an identical signal is applied to both the inverting and non-inverting inputs there should no change to the output. However, in real amplifiers there is always some variation and the ratio of the change to the output voltage with regards to the change in the common mode input voltage is called the Common Mode Rejection Ratio or CMRR.
Operational Amplifiers on their own have a very high open loop DC gain and by applying some form of Negative Feedback we can produce an operational amplifier circuit that has a very precise gain characteristic that is dependant only on the feedback used. An operational amplifier only responds to the difference between the voltages on its two input terminals, known commonly as the "Differential Input Voltage" and not to their common potential. Then if the same voltage potential is applied to both terminals the resultant output will be zero. An Operational Amplifiers gain is commonly known as the Open Loop Differential Gain, and is given the symbol (Ao).
Equivalent Circuit for Ideal Operational Amplifiers
Like the input signal, the output signal is also balanced and since the collector voltages either swing in opposite directions (anti-phase) or in the same direction (in-phase) the output voltage signal, taken from between the two collectors is, assuming a perfectly balanced circuit the zero difference between the two collector voltages. This is known as the Common Mode of Operation with the common mode gain of the amplifier being the output gain when the input is zero.
Ideal Operational Amplifiers also have one output (although there are ones with an additional differential output) of low impedance that is referenced to a common ground terminal and it should ignore any common mode signals that is, if an identical signal is applied to both the inverting and non-inverting inputs there should no change to the output. However, in real amplifiers there is always some variation and the ratio of the change to the output voltage with regards to the change in the common mode input voltage is called the Common Mode Rejection Ratio or CMRR.
Operational Amplifiers on their own have a very high open loop DC gain and by applying some form of Negative Feedback we can produce an operational amplifier circuit that has a very precise gain characteristic that is dependant only on the feedback used. An operational amplifier only responds to the difference between the voltages on its two input terminals, known commonly as the "Differential Input Voltage" and not to their common potential. Then if the same voltage potential is applied to both terminals the resultant output will be zero. An Operational Amplifiers gain is commonly known as the Open Loop Differential Gain, and is given the symbol (Ao).
Equivalent Circuit for Ideal Operational Amplifiers
Transistor Configurations
Transistors have three terminals connected to input and output circuit loops. One-transistor amplifiers are two-port networks; one of the three transistor terminals must be shared by both input and output ports as the common terminal. This results in three possibilities. The first is the common emitter (CE) amplifier. The emitter is common to both input and output, as shown below.
The emitter is part of both input and output loops. It is the common terminal of the transistor that is connected to both an input and output port terminal. With a series emitter resistor RE the emitter terminal is still common to both loops. The output loop current is shown flowing from the power supply (+VCC), through RL and the BJT, through RE to ground, which is connected to the negative terminal of the supply. The closure of the output loop from ground to +VCC implies flow through the voltage-source, +VCC.
The common-base (CB) configuration is shown below:
The common-collector (CE) configuration, also known as the emitter-follower, is shown below.
Common-Emitter Amplifier
The voltage gain was found by the transresistance approach: a ratio of output (load) resistance and transresistance, the resistance across which the input voltage develops the common (emitter) current. Not all of the emitter current gets to the collector. Some is lost to the base, and the a factor accounts for this in the voltage-gain equation:
Because a >>1, the voltage gain is a ratio of resistances. The input voltage vi is applied across rM, producing iE = vi/rM. Then iC ( = a x iE) gets through to the collector and develops a voltage of vo = - iC x RL at the output. By solving these equations for Av, the above gain equation results.
The input resistance of the CE is vi/ii = vi/iB or
The resistance of the input loop is the base resistance in series with the resistance in the emitter-side of the circuit, referred to the base by the b transform.
At the output node, the BJT transistor model shows a current source (infinite resistance) in parallel with load resistance RL. The output resistance is therefore RL.
The CE amplifier has relatively high input resistance due to the b-transform effect at the base. It is better as a voltage-input port. Its output resistance is relatively low if the load resistor is not made too large.
The current gain of the CE is io/ii = iC/iB = b. Its input-loop transresistance used to calculate gain is rM, but the overall amplifier transresistance is Rm = vo/ii = Av x rin and its transconductance is the inverse of the transresistance, or Gm = 1/Rm.
At the output node, the BJT transistor model shows a current source (infinite resistance) in parallel with load resistance RL. The output resistance is therefore RL.
The CE amplifier has relatively high input resistance due to the b-transform effect at the base. It is better as a voltage-input port. Its output resistance is relatively low if the load resistor is not made too large.
The current gain of the CE is io/ii = iC/iB = b. Its input-loop transresistance used to calculate gain is rM, but the overall amplifier transresistance is Rm = vo/ii = Av x rin and its transconductance is the inverse of the transresistance, or Gm = 1/Rm.
Common-Base Amplifier
The CB amplifier input source is in the emitter loop so that emitter current flows through it. This current is b + 1 times larger than the base current. Consequently, the CB input resistance is relatively low and would make a better current-input than voltage-input port. Its input resistance is
Unlike the CE, it is non-inverting (no negative sign). The CB current gain is a, or slightly less than one.
Compared to the CE, the CB input resistance is lower by (b + 1) and is therefore better as a current-input port than the CE. According to the ideal-port table, the CB most closely approaches an ideal current amplifier, though its current gain is slightly less than one!
Compared to the CE, the CB input resistance is lower by (b + 1) and is therefore better as a current-input port than the CE. According to the ideal-port table, the CB most closely approaches an ideal current amplifier, though its current gain is slightly less than one!
Common-Collector Amplifier
The CC or emitter-follower has the same input resistance as the CE but its output resistance is
or typically about re, a relatively small resistance of around a few ohms. With high input resistance and low output resistance, the CC appears to approach the ideal voltage amplifier. Unfortunately, its voltage gain is only
or typically somewhat less than one. The port resistances approach the ideal but the voltage gain is not high enough to be useful. The current gain, however, is b + 1.
None of the three single-transistor configurations is ideal as any of the four amplifier types. Amplifiers can better approach the ideal by combining configurations into multi-transistor amplifiers.
None of the three single-transistor configurations is ideal as any of the four amplifier types. Amplifiers can better approach the ideal by combining configurations into multi-transistor amplifiers.
Cascade Amplifier
Amplifiers are cascaded when the output of the first is the input to the second. The combined gain is
where vi2 = vo1. The total gain is the product of the cascaded amplifier stages.
The complication in calculating the gain of cascaded stages is the non-ideal coupling between stages due to loading. Two cascaded CE stages are shown below.
Because the input resistance of the second stage forms a voltage divider with the output resistance of the first stage, the total gain is not the product of the individual (separated) stages.
The total voltage gain can be calculated in either of two ways. First way: the gain of the first stage is calculated including the loading of ri2. Then the second-stage gain is calculated from the output of the first stage. Because the loading (output divider) was accounted for in the first-stage gain, the second-stage gain input quantity is the Q2 base voltage, vB2 = vo1.
Second way: the first-stage gain is found by disconnecting the input of the second stage, thereby eliminating output loading. Then the Thevenin-equivalent output of the first stage is connected to the input of the second stage and its gain is calculated, including the input divider formed by the first-stage output resistance and second-stage input resistance. In this case, the first-stage gain output quantity is the Thevenin-equivalent voltage, not the actual collector voltage of the stage-connected amplifier. The second way includes interstage loading as an input divider in the gain of the second stage while the first way includes it as an output divider in the gain of the first stage.
By cascading a CE stage followed by an emitter-follower (CC) stage, a good voltage amplifier results. The CE input resistance is high and CC output resistance is low. The CC contributes no increase in voltage gain but provides a near voltage-source (low resistance) output so that the gain is nearly independent of load resistance. The high input resistance of the CE stage makes the input voltage nearly independent of input-source resistance. Multiple CE stages can be cascaded and CC stages inserted between them to reduce attenuation due to inter-stage loading.
The total voltage gain can be calculated in either of two ways. First way: the gain of the first stage is calculated including the loading of ri2. Then the second-stage gain is calculated from the output of the first stage. Because the loading (output divider) was accounted for in the first-stage gain, the second-stage gain input quantity is the Q2 base voltage, vB2 = vo1.
Second way: the first-stage gain is found by disconnecting the input of the second stage, thereby eliminating output loading. Then the Thevenin-equivalent output of the first stage is connected to the input of the second stage and its gain is calculated, including the input divider formed by the first-stage output resistance and second-stage input resistance. In this case, the first-stage gain output quantity is the Thevenin-equivalent voltage, not the actual collector voltage of the stage-connected amplifier. The second way includes interstage loading as an input divider in the gain of the second stage while the first way includes it as an output divider in the gain of the first stage.
By cascading a CE stage followed by an emitter-follower (CC) stage, a good voltage amplifier results. The CE input resistance is high and CC output resistance is low. The CC contributes no increase in voltage gain but provides a near voltage-source (low resistance) output so that the gain is nearly independent of load resistance. The high input resistance of the CE stage makes the input voltage nearly independent of input-source resistance. Multiple CE stages can be cascaded and CC stages inserted between them to reduce attenuation due to inter-stage loading.
Darlington Amplifier
A CC stage followed by another CC stage has an input resistance of about (b + 1)2 times the emitter resistance of the second stage. More precisely, using the b transform, it is
If RE1 is removed, the second term is about b2 times RE2. Furthermore, if the collectors are connected together, the result is a Darlington stage, as shown below
This stage can be viewed as a "Darlington transistor" because it has three terminals and an equivalent b of about b2. Darlington BJTs can be used in any of the three BJT configurations.
Differential Amplifier
A differential or emitter-coupled BJT pair is formed, as shown below, by a CC/CE stage driving a CB stage. The first stage is a CE to the first output, vo- and is a CC to the second stage.
Differential Amplifier
A differential or emitter-coupled BJT pair is formed, as shown below, by a CC/CE stage driving a CB stage. The first stage is a CE to the first output, vo- and is a CC to the second stage.
A differential-input amplifier has an input port for which the negative (- ) terminal is not necessarily connected to the common node (usually ground). A differential amplifier (or diff-amp) amplifies the difference between its input terminals:
Amplifiers with differential outputs have two output terminals, neither of which is necessarily common with an input terminal or ground. The output is
xo = xo+ - xo-
The 2-transistor diff-amp has differential inputs and outputs. The voltage gain is found by calculating the gain from each input to each output (4 gains). The differential gain is the ratio of the difference of the outputs over the difference of the inputs. If the gain magnitude (absolute value, neglecting sign) to the output is different for the two inputs, the amplifier is not differential.
The above amplifier gain can be calculated using the transresistance method. The current-source resistor REE forms a divider between stages. Ideally, REE is a current source. The diff-amp circuit is also symmetrical if corresponding components have equal values:
xo = xo+ - xo-
The 2-transistor diff-amp has differential inputs and outputs. The voltage gain is found by calculating the gain from each input to each output (4 gains). The differential gain is the ratio of the difference of the outputs over the difference of the inputs. If the gain magnitude (absolute value, neglecting sign) to the output is different for the two inputs, the amplifier is not differential.
The above amplifier gain can be calculated using the transresistance method. The current-source resistor REE forms a divider between stages. Ideally, REE is a current source. The diff-amp circuit is also symmetrical if corresponding components have equal values:
RL1 = RL2 = RL
RE1 = RE2 = RE
RB1 = RB2 = RB
and
REE >> RE
then the voltage gain is
RE1 = RE2 = RE
RB1 = RB2 = RB
and
REE >> RE
then the voltage gain is
For non-negligible REE, a divider is formed between stages consisting of the source-transistor RE and REE. Apply Thevenin’s theorem for a Thevenin equivalent source driving RE of the other stage.
Ideal Amplifiers
Ideal Amplifiers
A port is a pair of terminals of a network (circuit). Across the port is a voltage, v, and through it flows a current, i, as shown below.
Amplifiers have two ports, input and output. An electrical waveform is a voltage or current as a function of time. A waveform to be amplified is applied to the input port and another waveform appears at the output port that is larger than the input waveform. Input and output quantities can be either voltages or currents, resulting in four basic kinds of amplifiers:
In the table under amplifier type is the expression for amplification or gain (or transfer function), which is the output quantity divided by the input quantity. In general, A = xo/xi, where x is either a voltage or current.
An ideal input port is not affected by input source resistance nor is an ideal output port affected by output load resistance. The general amplifier is shown below:
The combination source/resistance symbol is a generalized source: either a Thevenin or Norton equivalent circuit. The amplifier has an input resistance Rin and output resistance, Rout. The input source, xi (where xi is vi or ii), has resistance Ri. It forms a divider (voltage or current) with Rin so that xi ¹ xin. Similarly, output resistance Rout forms a divider with output port load resistance RL so that the output xout = K× xin ¹ xo. The amplification of xin by K results in xout that is K times larger. K is the gain, and it scales xin. (Gain less than 1 is called attenuation.) If source or load resistance is unknown or varies with K, then error in the overall amount of gain results. An accurate (or at least unchanging) gain is required for calibrated sensor circuits, so that the transducer output is multiplied by a known (and constant) amount.
An example of a voltage amplifier is shown below:
The first factor is the input voltage divider attenuation, the second is the amplifier voltage gain and the third is the output voltage divider attenuation. For the ideal voltage amplifier, Av = K. This is achieved when Rin approaches infinity (open-circuit input) and Ro = 0. The ideal port resistances are given in the following table:
In practice, good amplifier design approaches the ideal so that input and output loading does not affect the overall amplifier gain accuracy.
Operational amplifier comparator circuit
Operational amplifier comparator circuit
Comparator circuits find a number of applications in electronics. As the name implies they are used to compare two voltages. When one is higher than the other the comparator circuit output is in one state, and when the input conditions are reversed, then the comparator output switches.
These circuits find many uses as detectors. They are often used to sense voltages. For example they could have a reference voltage on one input, and a voltage that is being detected on another. While the detected voltage is above the reference the output of the comparator will be in one state. If the detected voltage falls below the reference then it will change the state of the comparator, and this could be used to flag the condition. This is but one example of many for which comparators can be used.
In operation the op amp goes into positive or negative saturation dependent upon the input voltages. As the gain of the operational amplifier will generally exceed 100 000 the output will run into saturation when the inputs are only fractions of a millivolt apart.
Although op amps are widely used as comparator, special comparator chips are often used. These integrated circuits offer very fast switching times, well above those offered by most op-amps that are intended for more linear applications. Typical slew rates are in the region of several thousand volts per microsecond, although more often figures of propagation delay are quoted.
A typical comparator circuit will have one of the inputs held at a given voltage. This may often be a potential divider from a supply or reference source. The other input is taken to the point to be sensed.
Circuit for a basic operational amplifier comparator
There are a number of points to remember when using comparator circuits. As there is no feedback the two inputs to the circuit will be at different voltages. Accordingly it is necessary to ensure that the maximum differential input is not exceeded. Again as a result of the lack of feedback the load will change. Particularly as the circuit changes there will be a small increase in the input current. For most circuits this will not be a problem, but if the source impedance is high it may lead to a few unusual responses.
The main problem with this circuit is that new the changeover point, even small amounts of noise will cause the output to switch back and forth. Thus near the changeover point there may be several transitions at the output and this may give rise to problems elsewhere in the overall circuit. The solution to this is to use a Schmitt Trigger.
In electronics, a Schmitt trigger is a comparator circuit that incorporates positive feedback.
In the non-inverting configuration, when the input is higher than a certain chosen threshold, the output is high; when the input is below a different (lower) chosen threshold, the output is low; when the input is between the two, the output retains its value. The trigger is so named because the output retains its value until the input changes sufficiently to trigger a change. This dual threshold action is called hysteresis, and implies that the Schmitt trigger has some memory. In fact, the Schmitt trigger is a bistable multivibrator.
Schmitt trigger devices are typically used in open loop configurations for noise immunity and closed loop positive feedback configurations to implement multivibrators.
The Schmitt trigger was invented by US scientist Otto H. Schmitt in 1934 while he was still a graduate student, later described in his doctoral dissertation (1937) as a "thermionic trigger". It was a direct result of Schmitt's study of the neural impulse propagation in squid nerves.
The symbol for Schmitt triggers in circuit diagrams is a triangle with an inverting or non-inverting hysteresis symbol. The symbol depicts the corresponding ideal hysteresis curve.
A Schmitt trigger can be implemented with a simple tunnel diode, a diode with an "N"-shaped current–voltage characteristic in the first quadrant. An oscillating input will cause the diode to move from one rising leg of the "N" to the other and back again as the input crosses the rising and falling switching thresholds. However, the performance of this Schmitt trigger can be improved with transistor-based devices that make explicit use of positive feedback to implement the switching.
Schmitt trigger with two transistors
In the positive-feedback configuration used in the implementation of a Schmitt trigger, most of the complexity of the comparator's own implementation is unused. Hence, it can be replaced with two cross-coupled transistors (the transistors that would otherwise implement the input stage of the comparator). An example of such a 2-transistor-based configuration is shown below. The chain RK1 R1 R2 sets the base voltage for transistor T2. This divider, however, is affected by transistor T1, providing higher voltage if T1 is open. Hence the threshold voltage for switching between the states depends on the present state of the trigger.
For NPN transistors as shown, when the input voltage is well below the shared emitter voltage, T1 does not conduct. The base voltage of transistor T2 is determined by the mentioned divider. Due to negative feedback, the voltage at the shared emitters must be almost as high as that set by the divider so that T2 is conducting, and the trigger output is in the low state. T1 will conduct when the input voltage (T1 base voltage) rises slightly above the voltage across resistor RE (emitter voltage). When T1 begins to conduct, T2 ceases to conduct, because the voltage divider now provides lower T2 base voltage while the emitter voltage does not drop because T1 is now drawing current across RE. With T2 now not conducting the trigger has transitioned to the high state.
With the trigger now in the high state, if the input voltage lowers enough, the current through T1 reduces, lowering the shared emitter voltage and raising the base voltage for T2. As T2 begins to conduct, the voltage across RE rises, further reducing the T1 base-emitter potential and T1 ceases to conduct.
In the high state, the output voltage is close to V+, but in the low state it is still well above V−. This may not be low enough to be a "logical zero " for digital circuits. This may require additional amplifiers following the trigger circuit.
The circuit can be simplified: R1 can be omitted, connecting the T2 base directly to the T1 collector, and the connection of the T2 base to V- via R2 can be completely omitted. When T1 conducts, it connects the T2 base to the T2 emitter so that T2 does not conduct. When T1 does not conduct, RK1 pulls up the T2 base and T2 conducts.
DAHIANA A. ROSALES H.
EES
http://www.radio-electronics.com/info/circuits/opamp_comparator/op_amp_comparator.php
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