Low-Noise Microphone Preamplifier

054 LOW-NOISE MICROPHONE PREAMPLIFIER The microphone preamplifier described is based on the SSM2015 from Precision Monolithics Inc. (Pmi), which offers a very high gain and very low noise (1.3 nV /Alf ). It is intended for use with balanced input signals and is capable of providing an amplification of 10-2,000, depending on the value of R4. With R5 = R6 = 10 la the amplification, A, is calculated from A = (20,000 / R4) + 3.5 With values as shown in the diagram, the amplification is, therefore, about 1,000. Resistor R3 sets the operating point of the differential input amplifier and thus determines the bandwidth and the slew rate. A value of 331(0 gives near-optimum values of these characteristics, but results in a relatively high input bias current of 4.5 p.A (with R3 = 150 kL2 , the current is only 1 pA). Moreover, the input noise 200 100 0 50 0 27 10 100 BALANCED INPUT AUDIO & HI-FI 1k SOURCE RESISTANCE (OHMS) 2.5k 5k 894053.» 250 500 SINGLE ENDED INPUT ELEKTOR ELECTRONICS JULY...