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AD846BQ データシート(PDF) 9 Page - Analog Devices |
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AD846BQ データシート(HTML) 9 Page - Analog Devices |
9 / 12 page REV. C AD846 –9– Figure 39. AD846 Three-Terminal Model Figure 40. Op Amp Three-Terminal Model A more detailed examination of the closed-loop transfer func- tion of the AD846 results in the following equation: Closed-Loop Gain G(s) = −R F RS 1 + C COMP RF + 1+ RF RS RIN s Compare this to the equation for a conventional op amp: Closed-Loop Gain G(s) = −R F RS 1 + CCOMP gM 1 + RF RS s where: CCOMP is the internal compensation capacitor of the am- plifier; gM is the input stage transconductance of the amplifier. In the case of the voltage amplifier, the closed-loop bandwidth decreases directly with increasing values of (1 + RF/RS), the closed-loop gain. However, for the transimpedance amplifier, the situation is different. At low gains, where (1 + RF/RS) RIN is small compared to RF, the closed-loop bandwidth is controlled by the internal compensation capacitance of 7 pF and the value of RF, and not by the closed-loop gain. At higher gains, where (1 + RF/RS) RIN is much larger than RF, the behavior is that of a con- ventional operational amplifier in which the input stage transcon- ductance is equal to the inverting terminal input impedance of the transimpedance amplifier (RIN = 50 Ω). A simple equation can, therefore, be used to determine the band- width of an amplifier employing the AD846 in the inverting configuration. 3 dB Bandwidth = 23 R F + 0.05 1 + G () where: The 3 dB bandwidth is in MHz G is the closed-loop inverting gain of the AD846 RF is the feedback resistance in k Ω. NOTE: This equation applies only for values of RF between 10 k Ω and 100 kΩ, and for RLOAD greater than 500 Ω. For RF = 1 k Ω the bandwidth should be estimated from Figure 41. Figure 41 illustrates the closed-loop voltage gain vs. frequency of the AD846 for various values of feedback resistor. For com- parison purposes, the characteristic of a conventional amplifier having an 80 MHz unity gain bandwidth is also shown. Figure 41. Closed-Loop Voltage Gain vs. Bandwidth for Various Values of RF For the case where RF = 1 k Ω and RS = 100 Ω (closed-loop gain of –10), the closed-loop bandwidth is approximately 28 MHz. It should also be noted that the use of a capacitor to shunt RF, a normal practice for stabilizing conventional op amps, will cause this amplifier to become unstable because the closed-loop band- width will increase beyond the stable operating frequency. A similar approach can be taken to calculate the noise perfor- mance of the amplifier. A simplified noise model is shown in Figure 42. The equivalent mean-square output noise voltage spectral den- sity will equal: VON 2 = R F I NN ()2 + 1+ RF RS 2 [VN 2 + R P INP ()2 +4 kT R P ] + 4 kT R F RF RS +1 |
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同様の説明 - AD846BQ |
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