http://es.wikipedia.org/wiki/Amplificador_operacional http://en.wikipedia.org/wiki/Class_A_amplifier Ejemplo del 741 Diagrama electrónico del operacional 741. En el diagrama se destaca en azul el amplificador diferencial. Éste es el responsable de que las corrientes de entrada no sean cero, pero si muy bajas respecto a las de los colectores (Nótese como a pesar de aproximar las corrientes de entrada a 0, si éstas realmente fueran 0 el circuito no funcionaría). La impedancia de entrada es de unos 2MΩ. Though designs vary between products and manufacturers, all op-amps have basically the same internal structure, which consists of three stages: 1. Differential amplifier o Input stage — provides low noise amplification, high input impedance, usually a differential output 2. Voltage amplifier o Provides high voltage gain, a single-pole frequency roll-off, usually single-ended output 3. Output amplifier o Output stage — provides high current driving capability, low output impedance, current limiting and short circuit protection circuitry Las etapas en rojo son espejos de corriente. El superior de la izquierda sirve para poder soportar grandes tensiones en modo común en la entrada. El superior de la derecha proporciona una corriente a la circuitería de salida para mantener la tensión. El inferior tiene una baja corriente de colector debido a las resistencias de 5kΩ. Se usa como conexión de gran impedancia a la alimentación negativa para poder tener una tensión de referencia sin que haya efecto de carga en el circuito de entrada. 1 Los pines llamados Offset null son usados para eliminar las tensiones de offset que pueda haber en el circuito. La etapa de ganancia en tensión es NPN. La sección verde es un desplazador de tensión. Esto proporciona una caída de tensión constante sin importar la alimentación. En el ejemplo 1V. Esto sirve para prevenir la distorsión. El condensador se usa como parte de un filtro paso bajo para reducir la frecuencia y prevenir que el A.O oscile. La salida en celeste es un amplificador PNP seguidor con emisor push-pull. El rango de la tensión de salida es de un voltio menos a la alimentación, la tensión colector-emisor de los transistores de salida nunca puede ser totalmente cero. Las resistencias de salida hacen que la corriente de salida esté limitada a unos 25mA. La resistencia de salida no es cero, pero con realimentación negativa se aproxima. Current mirrors The sections outlined in red are current mirrors. The primary current, from which other standing (bias) currents are generated, is determined by the chip's power supply and the 39 kΩ resistor acting (with the two transistor diode junctions) as a current source. The current generated is approximately (VS+ − VS− − 2Vbe)/39 kΩ. The input stage DC conditions are controlled by the two current mirrors on the left. The current mirror formed by Q8/Q9 allows for large common mode voltages on the inputs without exceeding the active range of any transistor in the circuit. The current mirror Q10/Q11 is used, indirectly, to set the input stage current. This current is set by the 5 kΩ resistor. The input stage bias control acts in the following manner. The outputs of current mirrors, Q8/Q9 and Q10/Q11 together form a high impedance current differencing circuit. If the input stage current tends to deviate (as detected by Q8) from that set by Q10, this is mirrored in Q9 and any change in this current is corrected by altering the voltage at the bases of Q3 and Q4. Thus the input stage DC conditions are stabilised by a high gain negative feedback system. The top-right current mirror Q12/Q13 provides a constant current load for the class A gain stage, via the collector of Q13, that is largely independent of the output voltage. Differential input stage The blue outlined section is a differential amplifier. Q1 and Q2 are input emitter followers and together with the common base pair Q3 and Q4 form the differential input stage. In addition, Q3 and Q4 also act as level shifters and provide voltage gain to drive the class A amplifier. They also help to increase the reverse Vbe rating on the input transistors. The differential amplifier formed by Q1 - Q4 drives a current mirror active load formed by transistors Q5 - Q7. Q7 increases the accuracy of the current mirror by decreasing the amount of signal current required from Q3 to drive the bases of Q5 and Q6. This current mirror provides differential to single ended conversion as follows: The signal current of Q3 is the input to the current mirror while the output of the mirror (the collector of Q6) is connected to the collector of Q4. Here, the signal currents of Q3 2 and Q4 are summed. For differential input signals, the signal currents of Q3 and Q4 are equal and opposite. Thus, the sum is twice the individual signal currents. This completes the differential to single ended conversion. The open circuit signal voltage appearing at this point is given by the product of the summed signal currents and the paralleled collector resistances of Q4 and Q6. Since the collectors of Q4 and Q6 appear as high resistances to the signal current, the open circuit voltage gain of this stage is very high. It should be noted that the base current at the inputs is not zero and the effective (differential) input impedance of a 741 is about 2 MΩ The offset null pins can be used in conjunction with a potentiometer to remove any offset voltage that would exist at the output of the op-amp when zero signal is applied between the inputs. Class A gain stage The section outlined in magenta is the class A gain stage. It consists of two NPN transistors in a Darlington configuration and uses the output side of a current mirror as its collector load to achieve high gain. The 30 pF capacitor provides frequency selective negative feedback around the class A gain stage to stabilise the amplifier in feedback configurations. This technique is called Miller compensation and functions in a similar manner to an op-amp integrator circuit. It is also known as 'dominant pole compensation' because it introduces a dominant pole (one which masks the effects of other poles) into the open loop frequency response. This pole can be as low as 10 Hz in a 741 amplifier and it introduces a −3 dB loss into the open loop response at this frequency. This is done to achieve unconditional stability of the amplifier down to unity closed loop gain using non-reactive feedback networks and makes this type of internally compensated amplifier easier to use. Output bias circuitry The green outlined section (based around Q16) is a voltage level shifter or Vbe multiplier; a type of voltage source. In the circuit as shown, Q16 provides a constant voltage drop between its collector and emitter regardless of the current passing through the circuit. If the base current to the transistor is assumed to be zero, and the voltage between base and emitter (and across the 7.5 kΩ resistor) is 0.625 V (a typical value for a BJT in the active region), then the current flowing through the 4.5 kΩ resistor will be the same as that through the 7.5 kΩ, and will produce a voltage of 0.375 V across it. This keeps the voltage across the transistor, and the two resistors at 0.625 + 0.375 = 1 V. This serves to bias the two output transistors slightly into conduction reducing crossover distortion. In some discrete component amplifiers this function is achieved with (usually 2) silicon diodes. Output stage The output stage (outlined in cyan) is a Class AB push-pull emitter follower (Q14, Q20) amplifier with the bias set by the Vbe multiplier voltage source Q16 and its base resistors. This stage is effectively driven by the collectors of Q13 and Q19. The output range of the amplifier is about 1 volt less than the supply voltage, owing in part to Vce(sat) of the output transistors. The 25 Ω resistor in the output stage acts as a current sense to provide the output current limiting function which limits the current flow in the emitter follower Q14 to about 25 3 mA for the 741. Current limiting for the negative output is done by sensing the voltage across Q19's emitter resistor and using this to reduce the drive into Q15's base. Later versions of this amplifier schematic may show a slightly different method of output current limiting. The output resistance is not zero as it would be in an ideal op-amp but with negative feedback it approaches zero. Note: while the 741 was historically used in audio and other sensitive equipment, such use is now rare because of the improved noise performance of more modern op-amps. Apart from generating noticeable hiss, 741s and other older op-amps may have poor common-mode rejection ratios and so will often introduce cable-borne mains hum and other common-mode interference, such a switch 'clicks', into sensitive equipment. . Class A Class A amplifiers amplify over the whole of the input cycle such that the output signal is an exact scaled-up replica of the input with no clipping. Class A amplifiers are the usual means of implementing small-signal amplifiers. They are not very efficient — a theoretical maximum of 50% is obtainable, but for small signals, this waste of power is still extremely small, and can be easily tolerated. Only when we need to create output powers with appreciable levels of voltage and current does Class A become problematic. In a Class A circuit, the amplifying element is biased such that the device is always conducting to some extent, and is operated over the most linear portion of its characteristic curve (known as its transfer characteristic or transconductance curve). Because the device is always conducting, even if there is no input at all, power is wasted. This is the reason for its inefficiency. Class A Amplifier If high output powers are needed from a Class A circuit, the power wastage will become significant. For every watt delivered to the load, the amplifier itself will, at best, waste another watt. For large powers this will call for a large power supply and large heat sink to carry away the waste heat. Class A designs have largely been superseded for audio power amplifiers, though some audiophiles believe that Class A gives the best sound quality, due to it being operated in as linear a manner as possible. In addition, some aficionados prefer vacuum tube (or "valve") designs over transistors, for a number of reasons: Tubes are more commonly used in class A designs, which have an asymmetrical transfer function. This means that distortion of a sine wave creates both odd- and even-numbered harmonics. They claim that this sounds more "musical" than the purely odd harmonics produced by a symmetrical push-pull amplifier.[1][2] Though good amplifier design can avoid inducing any harmonic patterns in a sound 4 reproduction system, the differences in harmonic content are essential to the sound of intentional electric guitar distortion. Another is that valves use many more electrons at once than a transistor, and so statistical effects lead to a "smoother" approximation of the true waveform — see shot noise for more on this. Field-effect transistors have similar characteristics to valves, so these are found more often in high quality amplifiers than bipolar transistors. Historically, valve amplifiers often used a Class A power amplifier simply because valves are large and expensive; Many Class A design uses only a single device. Transistors are much cheaper, and so more elaborate designs that give greater efficiency but use more parts are still cost effective. A classic application for a pair of class A devices is the long-tailed pair, which is exceptionally linear, and forms the basis of many more complex circuits, including many audio amplifiers and almost all op-amps. Class B and AB Class B amplifiers only amplify half of the input wave cycle. As such they create a large amount of distortion, but their efficiency is greatly improved and is much better than Class A. Class B has a maximum theoretical efficiency of 78.5%. This is because the amplifying element is switched off altogether half of the time, and so cannot dissipate power. A single Class B element is rarely found in practice, though it can be used in RF power amplifiers where the distortion is unimportant. However Class C is more commonly used for this. Class B Amplifier A practical circuit using Class B elements is the complementary pair or "push-pull" arrangement. Here, complementary devices are used to each amplify the opposite halves of the input signal, which is then recombined at the output. This arrangement gives excellent efficiency, but can suffer from the drawback that there is a small glitch at the "joins" between the two halves of the signal. This is called crossover distortion. A solution to this is to bias the devices just on, rather than off altogether when they are not in use. This is called Class AB operation. Each device is operated in a non-linear region which is only linear over half the waveform, but still conducts a small amount on the other half. Such a circuit behaves as a class A amplifier in the region where both devices are in the linear region, however the circuit cannot strictly be called class A if the signal passes outside this region, since beyond that point only one device will remain in its linear region and the transients typical of class B operation will occur. The result is that when the two halves are combined, the crossover is greatly minimised or eliminated altogether. However, it is important to note that while the efficiency of Class AB is greater than Class A, it is less than Class B. 5 Class B Push-Pull Amplifier Class B or AB push-pull circuits are the most common form of design found in audio power amplifiers. Class AB is widely considered a good compromise for audio amplifiers, since much of the time the music is quiet enough that the signal stays in the "class A" region, where it is reproduced with good fidelity, and by definition if passing out of this region, is large enough that the distortion products typical of class B are relatively small. Class B and AB amplifiers are sometimes used for RF linear amplifiers as well. Negative feedback Feedback feeds the difference of the input and part of the output back to the input in a way that cancels out part of the input. The main effect is to reduce the overall gain of the system. However the unwanted signals introduced by the amplifier are also fed back. Since they are not part of the original input, they are added to the input in opposite phase, subtracting them from the input. Careful design of each stage of an open loop (non-feedback) amplifier can achieve about 1% distortion. With negative feedback, 0.001% is typical. Noise, even crossover distortion can be practically eliminated. Feedback was originally invented so that replacing a burnt-out vacuum tube would not change an amplifier's performance (manufacturing realities require that tubes and transistors with the same part number will have close but not identical gain). Negative feedback also compensates for changing temperatures, and degrading or non-linear components. While amplifying devices can be treated as linear over some portion of their characteristic curve, they are inherently non-linear; their physics dictates that they operate using a square law. The result of non-linearity is distortion. The application dictates how much distortion a design can tolerate. For hi-fi audio applications, instrumentation amplifiers and the like, distortion must be minimal, often better than 1%. While feedback seems like a universal fix for all the problems of an amplifier, many believe that negative feedback is a bad thing. Since it uses a loop, it takes a finite time to react to an input signal, and for this short period the amplifier is "out of control." A musical transient whose timing is of the same order as this period will be grossly distorted, even though the amplifier will show incredibly good distortion performance on steady-state signals. This, essentially, is the rationale for the existence of "transient intermodulation distortion" in amplifiers which was exhaustively discussed and debated 6 from the late 1970s through much of the 1980s [3]. Proponents of feedback refute this, saying that the feedback "delay" is of such a short order that it represents a frequency vastly outside the bandwidth of the system, and such effects are not only inaudible, but not even present, as the amplifier will not respond to such high frequency signals. The argument has caused controversy for many years, and has led to all sorts of interesting designs — such as feedforward amplifiers (e.g. digital signals on many cellsite base-station transmitters are precompensated for the radio amplifier's distortion). The fact remains that the majority of modern amplifiers use considerable amounts of feedback, though many of the high-end audiophile designs seek to minimise this. Whatever the merits of these arguments about its effect on waveform distortion, feedback also affects the output impedance of the amplifier and therefore its damping factor. Roughly speaking, the damping factor is a measure of the ability of the amplifier to control the speaker. All other things being equal, the greater the amount of feedback, the lower its output impedance and the higher its damping factor. This has an effect on the low frequency performance of many speaker systems where low damping factors lead to irregular bass response. The concept of feedback is used in operational amplifiers to precisely define gain, bandwidth and other parameters. A practical circuit For the purposes of illustration, this practical amplifier circuit is described. It could be the basis for a moderate-power audio amplifier. It features a typical (though substantially simplified) design as found in modern amplifiers, with a class AB pushpull output stage, and uses some overall negative feedback. Bipolar transistors are shown, but this design would also be realisable with FETs or valves. A practical amplifier circuit The input signal is coupled through capacitor C1 to the base of transistor Q1. The capacitor allows the AC signal to pass, but blocks the DC bias voltage established by resistors R1 and R2 so that any preceding circuit is not affected by it. Q1 and Q2 form a differential amplifier (an amplifier that multiplies the difference between two inputs by some constant), in an arrangement known as a long-tailed pair. This arrangement is used to conveniently allow the use of negative feedback, which is fed from the output to Q2 via R7 and R8. The negative feedback into the difference amplifier allows the amplifier 7 to compare the input to the actual output. The amplified signal from Q1 is directly fed to the second stage, Q3, which provides further amplification of the signal, and the DC bias for the output stages, Q4 and Q5. R6 provides the load for Q3 (A better design would probably use some form of active load here, such as a constant-current sink). So far, all of the amplifier is operating in Class A. The output pair are arranged in Class AB push-pull, also called a complementary pair. They provide the majority of the current amplification and directly drive the load, connected via d.c. blocking capacitor C2. The diodes D1 and D2 provide a small amount of constant voltage bias for the output pair, just biasing them into the conducting state so that crossover distortion is minimised. This design is simple, but a good basis for a practical design because it automatically stabilises its operating point, since feedback internally operates from DC up through the audio range and beyond. Further circuit elements would probably be found in a real design that would roll off the frequency response above the needed range to prevent the possibility of unwanted oscillation. Also, the use of fixed diode bias as shown here can cause problems if the diodes are not both electrically and thermally matched to the output transistors — if the output transistors turn on too much, they can easily overheat and destroy themselves, as the full current from the power supply is not limited at this stage. A common solution to help stabilise the output devices is to include some emitter resistors, typically an ohm or so. Calculating the values of the circuit's resistors and capacitors is done based on the components employed and the intended use of the amp. For the basics of radio frequency amplifers using valves, see Valved RF amplifiers. Class C Class C amplifiers conduct less than 50% of the input signal and the distortion at the output is high, but efficiencies of up to 90% can be reached. Some applications can tolerate the distortion, such as audio bullhorns. A much more common application for Class C amplifiers is in RF transmitters, where the distortion can be vastly reduced by using tuned loads on the amplifier stage. The input signal is used to roughly switch the amplifying device on and off, which causes pulses of current to flow through a tuned circuit. The tuned circuit will only resonate at particular frequencies, and so the unwanted frequencies are dramatically suppressed, and the wanted full signal (sine wave) will be abstracted by the tuned load. Provided the transmitter is not required to operate over a very wide band of frequencies, this arrangement works extremely well. Other residual harmonics can be removed using a filter. Class C Amplifier 8 Crossover distortion The term crossover signifies the "crossing over" of the signal from the upper transistor to the lower and vice-versa. Crossover distortion can be suppressed by using a slight forward bias in the base circuit such that the transistors are idling at a small output current and also by the use of negative feedback. The forward bias causes the circuit to operate in class-AB mode. Most modern power amplifiers (including those used in hi-fi) employ class-AB in their output stages as it offers good to very good efficiency and distortion figures. The next image shows a typical class-B emitter-follower complementary output stage. Most of the accompanying circuitry has been omitted for clarity. Under no signal conditions, the output is exactly mid-way between the supplies (i.e., at 0V). When this is the case, the base-emitter bias (voltage) available both transistors is zero and so they are in the cut-off region, i.e., the transistors are not conducting. Consider a positive going swing: As long as the input is less than the required forward VBE drop (≈ 0.65V) of the upper NPN transistor, it will remain off or conduct very little - this is the same as a diode operation as far as the base circuit is concerned, and output voltage does not follow the input (the lower PNP transistor is still off because its baseemitter diode is being reverse biased by the positive going input). The same applies for the lower transistor, but for a negative going input. Thus, between about ±0.65V of input, the output voltage is not a true replica or amplified version of the input and we can see that as a "kink" in the output waveform near 0V (or where one transistor stops conducting and the other starts). This kink is known as crossover distortion and it becomes more evident and intrusive when the output voltage swing is reduced. 9