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LTC6248 Datasheet(PDF) 12 Page - Linear Technology

Part No. LTC6248
Description  90MHz, 2200V/μs 30V Low Power Op Amps
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC6248 Datasheet(HTML) 12 Page - Linear Technology

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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.
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 I-to-V 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 first-order 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 open-loop
applications with large, sustained differential inputs.
Under normal, closed-loop operation, an increase of
power dissipation is only noticeable in applications with

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