Knowles RQU-35066-000

The BL series microphones require a bias voltage, typically supplied by the amplifier power source. In amplifier system design, the microphone power supply regulation requirements must be considered to avoid oscillation and other undesirable side effects. Stability issues with BL microphones are not new — they are analogous to the long-standing problem of biasing high-gain transistor amplifiers, where supply regulation variations can reach the amplifier input through the microphone.

When applying BL microphones to hearing aid amplifiers, the problems fall into two categories: (1) amplifiers containing an odd number of inverting stages; and (2) amplifiers containing an even number of inverting stages (emitter followers, Darlington pairs, and other non-inverting stages are excluded in each case).

Category 1: Odd-stage amplifiers represent the simplest application, because in this configuration, feedback through the microphone produces negative feedback at the center of the passband. Unless an excessive number of coupling capacitors are included or the amplifier is operated at very high gain, it will generally be stable. As battery life nears its end, the increasing internal resistance causes amplifier gain to drop. This effect can be minimized by using a decoupling network in the microphone supply lead. To prevent excessive loss of microphone supply voltage, the decoupling resistor should not exceed 2000 ohms. The value of the bypass capacitor determines the frequency below which negative feedback is eliminated. Consequently, as battery resistance increases, sensitivity below the frequency set by the decoupling network will decrease.

Category 2: Even-stage amplifiers present a more critical concern, because the feedback introduced by battery impedance is positive, making oscillation likely. To ensure stability, the product of amplifier transconductance (Ga), battery source resistance (Rb), and microphone attenuation (Am⁻) must be less than 1: Ga × Rb × Am < 1. When amplifier gain is high, it may be necessary to increase microphone attenuation within the amplifier passband to achieve stability. Since resistor-capacitor decoupling becomes ineffective at sufficiently low frequencies, it is necessary to intentionally reduce amplifier gain to a safe value at and below that low frequency. This can be accomplished by selecting a coupling capacitor to produce the desired low-frequency rolloff, while ensuring that all other capacitors are large enough not to introduce significant phase shift in the frequency region where stability remains in question.