Design microphone circuit with the industry's smallest operational amplifier
Voice commands are a popular feature in many applications and one of the advantages of giving products differentiated market competitiveness. Microphones are an integral part of any voice or speech-based system, and electret microphones are a common choice for such applications due to their small size, low cost and high performance.
This article focuses on the design of a very small, cost-optimized electret condenser microphone preamplifier in the topic of a series of high-performance, cost-sensitive circuit articles. The design uses the TLV9061, the industry's smallest op amp, with a 0.8mm x 0.8mm ultra-small leadless (X2SON) package. The circuit configuration of the electret microphone amplifier is shown in Figure 1.
Figure 1: In-phase electret microphone amplifier circuit
Most electret microphones are internally buffered with a junction field effect transistor (JFET) and the JFET is biased with a 2.2kΩ pullup resistor. The acoustic wave moves the microphone element, causing current to flow into the JFET drain inside the microphone. The JFET drain current produces a voltage drop across R2 that is ac-coupled, biased to the intermediate supply and connected to the IN+ pin of the op amp. The operational amplifier is configured as a bandpass filtered in-phase amplifier circuit. With the expected input signal level and the desired output amplitude and response, you can calculate the gain and frequency response of the circuit.
Let's look at an example design of a circuit for a +3.3V supply with an input of 7.93mVRMS and an output signal of 1VRMS. 7.93 mVRMS corresponds to a 0.63 Pa sound level input with a microphone and a -38 dB sound pressure level (SPL) sensitivity specification. The bandwidth goal is to pass the 300 Hz common voice frequency bandwidth to 3 kHz.
Equation 1 shows the transfer function that defines the relationship between VOUT and the AC input signal:
Equation 2 calculates the required gain based on the expected input signal level and the desired output level:
Select a standard 10kΩ feedback resistor and use Equation 3 to calculate R6:
To reduce the attenuation in the required passband from 300 Hz to 3 kHz, set the upper (fH) and lower (fL) cutoff frequencies outside the required bandwidth (Equation 4):
Select C7 to set the fL cutoff frequency (Equation 5):
Select C6 to set the fH cutoff frequency (Equation 6):
To set the input signal cutoff frequency low enough for low frequency sound waves to pass, select C2 to achieve a 30Hz cutoff frequency (fIN) (Equation 7):
Figure 2 shows the measured transfer function of the mic preamplifier circuit. Due to the narrow bandwidth and attenuation between the high pass filter and the low pass filter, the flat band gain is only 41.8 dB or 122.5 V / V, which is slightly below the target.
Figure 2: Transmission function of the microphone preamplifier
The circuit was mounted to the back of a 6mm diameter electret microphone using TI's X2SON packaging technology. Due to mounting size limitations, very small operational amplifiers are required: the TLV9061 has a footprint of only 0.8mm x 0.8mm. In addition, 0201 small size resistors and capacitors minimize printed circuit board (PCB) area, and you can use smaller resistors to further reduce this area. The printed circuit board layout is shown in Figures 3 and 4.
Figure 3: Microphone preamp layout on the back of a 6mm diameter electret microphone
Figure 4: 3D view of the PCB design showing different angles of the microphone and PCB
You can adjust the above design steps to meet different microphone sensitivity requirements. When designing with a small amplifier such as the TLV9061, please refer to "Layout Best Practices in Design and Manufacturing with TI X2SON Package"