Contemporary high-quality condenser microphones offer noise performance close to the theoretical limit. Because large-diaphragm capsules and transformerless electronics have a high sensitivity, only modest gain from the preamplifier is required. Yet, designers of microphone preamplifiers have traditionally focused on the noise performance at highest gain, where the equivalent input noise (EIN) is at a minimum. At lower gain settings this figure worsens by an amount that depends on the detailed implementation, and may dominate the noise of the microphone. The EIN at highest gain is an insufficient and possibly even misleading criterion. The author presents a detailed noise analysis of the classic current-feedback instrumentation amplifier topology. The collector load resistors and the voltage noise of the operational amplifiers were identified as significant noise sources at low-gain settings. Connecting the collector load resistors to a higher supply rail is an effective technique for reducing the magnitude of the noise sources. Practical verification was found to be in excellent agreement with the predicted performance.
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