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LME49871MA Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LME49871MA Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 14 page Application Information GENERAL AMPLIFIER FUNCTION oltage feedback amplifiers have a small-signal bandwidth that is a function of the closed-loop gain. Conversely, the LME49871 current feedback amplifier features a small-signal bandwidth that is relatively independent of the closed-loop gain. This is shown in Figure 1 where the LME49871’s gain is –1,–2, –5 and –10. Like all current feedback amplifiers, the LME49871’s closed-loop bandwidth is a function of the feed- back 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 es- sential for optimized high-speed amplifier operation. The sup- ply 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 tran- sient fidelity. The bypassing is easily accomplished using a parallel combination of a 10 μF 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 LME49871. For general applications, the LME49871 will maintain specified performance with an 1.2k Ω feedback re- sistor. 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. 300426p0 FIGURE 1. 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 LME49871 can drive significantly higher capacitive loads than many current feedback amplifiers. Although the LME49871 can directly drive as much as 100pF without os- cillating, the resulting response will be a function of the feed- back resistor value. CAPACITIVE FEEDBACK It is quite common to place a small lead-compensation ca- pacitor in parallel with a voltage feedback amplifier’s feedback resistance, R f. This compensation reduces the amplifier’s peaking in the frequency domain and damps the transient re- sponse. Whereas this yields the expected results when used with voltage feedback amplifiers, this technique must not be used with current feedback amplifiers. The dynamic impedance of capacitors in the feedback loop reduces the amplifier’s stability. Instead, reduced peaking in the frequency response and bandwidth limiting can be accomplished by adding an RC circuit to the amplifier’s input. 11 www.national.com |
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