The presentation A hearing loop system in CMOS18 technology shows the application and the high level requirements of the complete hearing aid system.
The hearing loop sub system converts the magnetic field of a current-driven loop antenna into an audio signal. Hearing aid devices automatically switch from microphone reception to the hearing loop reception if correlation betweeen the signal at the output of the amplifier A1 and the output of the microphone is sufficently large.
Hence, the gain and the frequency characteristics of A1 should be such that its output signal corresponds with that of the microphone.
The ADC driver amplifier A2 drives the 10 pF input capacitance of the ADC to a level of 0.9 Vpp at 110 dB SPL audio input.
Amplifier A3 drives the loudspeaker that must produce 110 dBSPL when driven from a peak-peak voltage equal to the fully charged battery voltage.
For the amplifiers A1, A2, and A3:
Give their port configuration
Give the port impedance requirements
Give a negative-feedback configuration that satisfies above requirements
Amplifier A1 converts the voltage of the pick-up coil into a voltage equal to that at the output of the microphone. We can formulate the following requirements:
Type of amplifier: voltage amplifiers
Dersired transfer: \(\frac{68.2\times 10^3}{s}\).
Input can be differential or single-ended
Differential input resistance: \(10\) k:math:Omega
Ideal common-mode impedance in case of differential input: \(0\,\Omega\)
Single-ended output
From a signal processing point of view, a differential input is not required. Since it complicates the design of a feedback amplifier, a single-ended input is preferred. The suggested feedback topology is shown below.
The voltage transger of this amplifier can be obtained as:
With \(RC=\frac{1}{62.8 \times 10^3}=16\times 10^{-6}\) s. Above 10 kHz this circuit has unity voltage gain. This is well above the upper limit of the desired frequency range.
Amplifier A2 the voltage level at the microphone output to the full-scale ADC input voltage. The gain, however, is only 1.41. If the microphone can drive the input of the ADC, amplifier A2 can be omitted at the cost of a little loss of dynamic range with respect to that of the ADC. The maximum output impedance of the microphone is 5.5 k:math:Omega and the input capacitance of the ADC is 10 pF. When tight together they constitute a low-pass transfer with a bandwidth of 2.9 MHz. Although not specified (a specification of the minimum value of the load impedance is missing!), the drive capability of the microphone at low frequencies is assumed large enough and the microphone can simply be connected to the ADC.
Amplifier A3 converts the single-ended output voltage of the DAC into a differential voltage for driving the loudspeaker. The gain of the amplifier is 2. For a maximum drive capability, the common-mode output voltage must equal half the power supply voltage. Hence, the common-mode output impedance should be zero.
The figures below show a possible feedback structures for this amplifier.
The figure on the left (A) shows an amplifier with a high input resistance. It requires a controller with a rail-to-rail input voltage range. The figure on the left (B) shows an amplifier with an input resistance \(R\). It does not require a controller with a rail-to-rail input voltage range, but it needs a specification of the DAC output resistance and the drive capability, while, for accurate operation, the resistance \(R\) must be much larger than the output resistance of the DAC.
