LME49713 Datasheet, PDF (10/16 Pages) National Semiconductor (TI) – High Performance, High Fidelity Current Feedback Audio Operational Amplifier

LME49713
SNAS386F – SEPTEMBER 2007 – REVISED MARCH 2013
APPLICATION INFORMATION
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GENERAL AMPLIFIER FUNCTION
Voltage feedback amplifiers have a small-signal bandwidth that is a function of the closed-loop gain. Conversely,
the LME49713 current feedback amplifier features a small-signal bandwidth that is relatively independent of the
closed-loop gain. This is shown in Figure 31 where the LME49713’s gain is –1, –2, –5, and –10. Like all current
feedback amplifiers, the LME49713’s closed-loop bandwidth is a function of the feedback resistance value.
Therefore, R_s must be varied to select the desired closed-loop gain.
POWER SUPPLY BYPASSING AND LAYOUT CONSIDERATIONS
Properly placed and correctly valued supply bypassing is essential for optimized high-speed amplifier operation.
The supply bypassing must maintain a wideband, low-impedance capacitive connection between the amplifier’s
supply pin and ground. This helps preserve high speed signal and fast transient fidelity. The bypassing is easily
accomplished using a parallel combination of a 10μF tantalum and a 0.1μF ceramic capacitors for each power
supply pin. The bypass capacitors should be placed as close to the amplifier power supply pins as possible.
FEEDBACK RESISTOR SELECTION (R_f)
The value of the R_f, is also a dominant factor in compensating the LME49713. For general applications, the
LME49713 will maintain specified performance with an 1.2kΩ feedback resistor. Although this value will provide
good results for most applications, it may be advantageous to adjust this value slightly for best pulse response
optimized for the desired bandwidth. In addition to reducing bandwidth, increasing the feedback resistor value
also reduces overshoot in the time domain response.
21
19 _{65 MHz (Av) = -10}
17
15
13 _{84 MHz (Av) = -5}
11
9
7
5 _{111 MHz (Av) = -2}
3
1
-1 _{132 MHz (Av) = -1}
-3
1E+5
1E+6
1E+7
1E+8
FREQUENCY (Hz)
1E+9
Figure 31. Bandwidth as a Function of Gain
SLEW RATE CONSIDERATIONS
A current feedback amplifier’s slew rate characteristics are different than that of voltage feedback amplifiers. A
voltage feedback amplifier’s slew rate limiting or non-linear amplifier behavior is dominated by the finite
availability of the first stage tail current charging the second stage voltage amplifier’s compensation capacitor.
Conversely, a current feedback amplifier’s slew rate is not constant. Transient current at the inverting input
determines slew rate for both inverting and non-inverting gains. The non-inverting configuration slew rate is also
determined by input stage limitations. Accordingly, variations of slew rates occur for different circuit topologies.
DRIVING CAPACITIVE LOADS
The LME49713 can drive significantly higher capacitive loads than many current feedback amplifiers. Although
the LME49713 can directly drive as much as 100pF without oscillating, the resulting response will be a function
of the feedback resistor value.
10
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