BRUEL & KJAER 2204 - Condenser Microphone and Cathode Follower

3. Weighting networks.
4. Output amplifiers with output attenuators.
5. Meter rectifier and indicating meter.
6. Power supply.
Condenser Microphone and Cathode Follower.
The microphone supplied with the Sound Level Meter is a preclSlon measur-
ing condenser microphone designed for long term stability and high
accuracy. Particular care has been taken to make it insensitive to variations
in ambient conditions such as temperature, pressure and relative humidity.
The construction of the microphone can be inferred from the schematic
· diagram in Fig. 2.3. It consists essentially of a thin metallic diaphragm
mounted in close proximity to a rigid back plate. Diaphragm and back
plate are electrically insulated from each other and constitute the electrodes
of a capacitor. The capacitor is charged by a DC polarization voltage and
the charging time constant is made so high that for the frequency range
of ordinary acoustical measurements the charge on the capacitor will be
constant.
Diaphra
Insulator
Back plate
Housing
Hole for
static pressure equalization
Fig. 2.3. Schematic construction of a condenser microphone cartridge.
When the distance between the diaphragm and the back plate changes
because of variations in pressure on the diaphragm the capacity will also
change and so an alternating voltage appears across the capacitor. This
voltage component is proportional to the pressure fluctuations within U1e
linear range of the microphone.
The low internal capacitance of the microphone requires a high input im-
pedance in the succeeding amplifier stage in order to ensure a minimum
loss in sensitivity due to loading. A sour.ce-follower stage has therefore been
introduced between the microphone and the input amplifier. This source-
follower stage consists of a low noise silicon field-effect transistor (FET) and
two other silicon planar transistors. This stage has a very high input im-
pedance of approximately 2 Gohm (2 X 109 ohm) and a low output im-
pedance.
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