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OP176GS データシート(PDF) 12 Page - Analog Devices |
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OP176GS データシート(HTML) 12 Page - Analog Devices |
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12 / 21 page  OP176 REV. 0 –12– A Low Noise Microphone Preamplifier with a Phantom Power Option Figure 39 is an example of a circuit that combines the strengths of the SSM2017 and the OP176 into a variable gain micro- phone preamplifier with an optional phantom power feature. The SSM2017’s strengths lie in its low noise and distortion, and gain flexibility/simplicity. However, rated only for 2 k Ω or higher loads, this makes driving 600 Ω loads somewhat limited with the SSM2017 alone. A pair of OP176s are used in the circuit as a high current output buffer (U2) and a DC servo stage (U3). The OP176’s high output current drive capability provides a high level drive into 600 Ω loads when operating from ±18 V supplies. For a complete treatment of the circuit design details, the interested reader should consult application note AN-242, available from Analog Devices. This amplifier’s performance is quite good over programmed gain ranges of 2 to 2000. For a typical audio load of 600 Ω, THD + N at various gains and an output level of 10 V rms is illustrated in Figure 40. For all but the very highest gain, the THD + N is consistent and well below 0.01%, while the gain of 2000 becomes more limited by noise. The noise performance of the circuit is exceptional with a referred-to-input noise voltage spectral density of 1 nV/ √Hz at a circuit gain of 1000. 1.0 0.1 0.010 0.001 20 100 1k 10k 20k G = 2000 G = 200 G = 20 G = 4 ±V S = ±18V 80kHz LPF Figure 40. Low Noise Microphone Preamplifier THD + N Performance at Various Gains (V OUT = 10 V rms and R L = 600 Ω) Figure 39. A Low Noise Microphone Preamplifier + + + + + ΩR10 100 Ω +48V R9 6.81k Ω R8 6.81k Ω C8 47µF/ 63V PHANTOM POWER SUPPLY CONNECTIONS, INTERLOCKED WITH +/–VS (SEE NOTE 5). Z1 Z2 TO MICROPHONE –IN +IN ΩR P1 49.9 Ω CIN1 47µF/ 63V ΩR P2 49.9 Ω CIN2 47µF/ 63V COMMON Z3 Z4 CG2 2200µF/ 10V CG1 2200µF/ 10V RG RB1 10k Ω RB2 10k Ω CN 4.7nF/ FILM 1) 3) +VS 3 8 1 2 6 5 4 7 U1 SSM2017P –VS CRF2 100pF CRF1 100pF R1 10k Ω R6 10k Ω R7 1k Ω C5 33pF OUTPUT 6 4 2 3 7 6 2 3 ΩR3 49.9 Ω R5 221k Ω R2 20k Ω C1 1µF FILM R4 221k Ω D1 D2 1N458 +VS –VS U2 OP176 U3 OP176 C2 1µF FILM OUT COMMON NOTES: 1) Z1–Z4 1N752 (SEE TEXT). 2) CINX, CGX LOW LEAKAGE ELECTROLYTIC TYPES (SEE TEXT). 3) GAIN = G = 2 x ((10k/RG ) +1) (SEE TEXT). 4) ALL RESISTORS 1% METAL FILM. 5) DOTTED PHANTOM POWER RELATED COMPONENTS OPTIONAL (SEE TEXT). +VS –VS +18V –18V C6 0.1µF C7 0.1µF C3 100µF/25V C4 100µF/25V + + 1N458 4 7 –VS +VS |
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