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HFA1105 데이터시트(PDF) 5 Page - Intersil Corporation |
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HFA1105 데이터시트(HTML) 5 Page - Intersil Corporation |
5 / 12 page 5 FN3395.8 June 6, 2006 Application Information Optimum Feedback Resistor Although a current feedback amplifier’s bandwidth dependency on closed loop gain isn’t as severe as that of a voltage feedback amplifier, there can be an appreciable decrease in bandwidth at higher gains. This decrease may be minimized by taking advantage of the current feedback amplifier’s unique relationship between bandwidth and RF. All current feedback amplifiers require a feedback resistor, even for unity gain applications, and RF, in conjunction with the internal compensation capacitor, sets the dominant pole of the frequency response. Thus, the amplifier’s bandwidth is inversely proportional to RF. The HFA1105 design is optimized for RF = 510Ω at a gain of +2. Decreasing RF decreases stability, resulting in excessive peaking and overshoot (Note: Capacitive feedback will cause the same problems due to the feedback impedance decrease at higher frequencies). At higher gains, however, the amplifier is more stable so RF can be decreased in a trade-off of stability for bandwidth. The table below lists recommended RF values for various gains, and the expected bandwidth. For a gain of +1, a resistor (+RS) in series with +IN is required to reduce gain peaking and increase stability. Non-Inverting Input Source Impedance For best operation, the DC source impedance seen by the non-inverting input should be ≥50Ω. This is especially important in inverting gain configurations where the non- inverting input would normally be connected directly to GND. Pulse Undershoot and Asymmetrical Slew Rates The HFA1105 utilizes a quasi-complementary output stage to achieve high output current while minimizing quiescent supply current. In this approach, a composite device replaces the traditional PNP pulldown transistor. The composite device switches modes after crossing 0V, resulting in added distortion for signals swinging below ground, and an increased undershoot on the negative portion of the output waveform (See Figures 5, 8, and 11). This undershoot isn’t present for small bipolar signals, or large positive signals. Another artifact of the composite device is asymmetrical slew rates for output signals with a negative voltage component. The slew rate degrades as the output signal crosses through 0V (See Figures 5, 8, and 11), resulting in a slower overall negative slew rate. Positive only signals have symmetrical slew rates as illustrated in the large signal positive pulse response graphs (See Figures 4, 7, and 10). PC Board Layout The amplifier’s frequency response depends greatly on the care taken in designing the PC board. The use of low inductance components such as chip resistors and chip capacitors is strongly recommended, while a solid ground plane is a must! Attention should be given to decoupling the power supplies. A large value (10 µF) tantalum in parallel with a small value (0.1 µF) chip capacitor works well in most cases. Terminated microstrip signal lines are recommended at the device’s input and output connections. Capacitance, parasitic or planned, connected to the output must be minimized, or isolated as discussed in the next section. Care must also be taken to minimize the capacitance to ground at the amplifier’s inverting input (-IN), as this capacitance causes gain peaking, pulse overshoot, and if large enough, instability. To reduce this capacitance, the designer should remove the ground plane under traces connected to -IN, and keep connections to -IN as short as possible. An example of a good high frequency layout is the Evaluation Board shown in Figure 2. Driving Capacitive Loads Capacitive loads, such as an A/D input, or an improperly terminated transmission line will degrade the amplifier’s phase margin resulting in frequency response peaking and possible oscillations. In most cases, the oscillation can be avoided by placing a resistor (RS) in series with the output prior to the capacitance. Figure 1 details starting points for the selection of this resistor. The points on the curve indicate the RS and CL combinations for the optimum bandwidth, stability, and settling time, but experimental fine tuning is recommended. Picking a point above or to the right of the curve yields an overdamped response, while points below or left of the curve indicate areas of underdamped performance. RS and CL form a low pass network at the output, thus limiting system bandwidth well below the amplifier bandwidth of 270MHz (for AV = +1). By decreasing RS as CL increases (as illustrated in the curves), the maximum bandwidth is obtained without sacrificing stability. In spite of this, the bandwidth decreases as the load capacitance increases. For example, at AV = +1, RS = 62Ω, CL = 40pF, the overall bandwidth is limited to 180MHz, and bandwidth drops to 75MHz at AV = +1, RS =8Ω, CL = 400pF. GAIN (ACL)RF (Ω) BANDWIDTH (MHz) -1 425 300 +1 510 (+RS = 510Ω) 270 +2 510 330 +5 200 300 +10 180 130 HFA1105 |
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