Opamp Type SSM2017 from PMI is eminently suitable for designing a very low-noise microphone nen According to the manufacturer’s don sheet• the input noise is only PV Hzi/2. The amplification (a 1 + 104/R1) may be set to a value of 1-1000 with a single resistor. R1.
The internal compensation of the IC is such that stability is retained even at unity amplification, yet, when the amplification is x1000, the bandwidth is 300 kHz.
The circuit in the diagram has a symmetrical input and a standard input impedance of 600 a The amplification is x1001 so that a microphone signal is converted directly into a line level. If the output of the amplifier is conveyed via a long cable (capacitive load), it is advisable to insert a 47-400 .C2 resistor in series with it to obviate any likelihood of oscillation. Also, at high amplification, the output may have an offset voltage, which, in certain applications may give rise to problems. If that is so, an output capacitor should be used.
The maximum output voltage of the IC is 9 V r.m.s. when the supply voltage is ±15 V. At a 1 kHz output at a level of 1 V r.m.s., the harmonic distortion does not exceed 0.005%. When the supply voltage is lower than +9 V, the distortion rises to 0.01%. The noise factor of the prototype ( including the noise contribution of R1) was measured at 1 nV Hz1/2. The current drawn by the amplifier from a ( + or -15 ) supply is about 11 mA.
