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LTC6248 Datasheet(PDF) 12 Page  Linear Technology 

LTC6248 Datasheet(HTML) 12 Page  Linear Technology 
12 / 20 page LT6274/LT6275 12 6275fa For more information www.linear.com/LT6275 Circuit Operation The LT6274/LT6275 circuit topology is a true voltage feedback amplifier that has the slewing behavior of a cur rent feedback amplifier. The operation of the circuit can be understood by referring to the simplified schematic. The inputs are buffered by complementary NPN and PNP emitter followers that drive a 1k resistor. The input voltage appears across the resistor generating currents that are mirrored into the high impedance node. Complementary followers form an output stage that buffers the gain node from the load. The bandwidth is set by the internal input resistor and the capacitance on the high impedance node. The slew rate is determined by the current available to charge the gain node capacitance. This current is the dif ferential input voltage divided by R1, so the slew rate is proportional to the input. This important characteristic gives the LT6274/LT6275 superior slew performance compared to conventional voltage feedback amplifiers in which the slew rate is constrained by a fixed current (bias ing the input transistors) available to charge the gain node capacitance (independent of the magnitude of the differen tial input voltage). Therefore, in the LT6274/LT6275, high est slew rates are seen in the lowest gain configurations. For example, a 10V output step in a gain of 10 has only a 1V input step, whereas the same output step in unity gain has a 10 times greater input step. The curve of Slew Rate vs Input Level illustrates this relationship. The LT6274/ LT6275 are tested in production for slew rate in a gain of –2 so higher slew rates can be expected in gains of 1 and –1, with lower slew rates in higher gain configurations. Special compensation across the output buffer allows the LT6274/LT6275 to be stable with any capacitive load. The RC network across the output stage is bootstrapped when the amplifier is driving a light or moderate load and has no effect under normal operation. When driving a capaci tive load (or a low value resistive load) the network is incompletely bootstrapped and adds to the compensa tion at the high impedance node. The added capacitance slows down the amplifier by lowering the dominant pole frequency, improving the phase margin. The zero created by the RC combination adds phase to ensure that even for very large load capacitances, the total phase lag does not exceed 180° (zero phase margin), and the amplifier remains stable. APPLICATIONS INFORMATION Comparison to Current Feedback Amplifiers The LT6274/LT6275 enjoy the high slew rates of Current Feedback Amplifiers (CFAs) while maintaining the char acteristics of a true voltage feedback amplifier. The pri mary differences are that the LT6274/LT6275 have two high impedance inputs, and the closed loop bandwidth decreases as the gain increases. CFAs have a low imped ance inverting input and maintain relatively constant bandwidth with increasing gain. The LT6274/LT6275 can be used in all traditional op amp configurations including integrators and applications such as photodiode ampli fiers and ItoV converters where there may be significant capacitance on the inverting input. The frequency com pensation is internal and does not depend on the value of the external feedback resistor. For CFAs, by contrast, the feedback resistance is fixed for a given bandwidth, and capacitance on the inverting input can cause peaking or oscillations. The slew rate of the LT6274/LT6275 in noninverting gain configurations is also superior to that of CFAs in most cases. Input Considerations Each of the LT6274/LT6275 inputs is the base of an NPN and a PNP transistor whose base currents are of opposite polarity and provide firstorder input bias current cancel lation. Because of differences between NPN and PNP beta, the polarity of the input bias current can be positive or negative. The offset current does not depend on NPN/PNP beta matching and is well controlled. The use of balanced source resistance at each input is therefore recommended for applications where DC accuracy must be maximized. The inputs can withstand transient differential input volt ages up to ±10V without damage and need no clamping or source resistance for protection. Differential inputs, however, generate large supply currents (tens of mA) as required for high slew rates. If the device is used with sustained differential inputs, the average supply current will increase, excessive power dissipation will result, and the part may be damaged. The part should not be used as a comparator, peak detector or in other openloop applications with large, sustained differential inputs. Under normal, closedloop operation, an increase of power dissipation is only noticeable in applications with 
