Design Method of Low Noise Preamplifier Circuit

　　Preamplifiers play a vital role in audio systems. This article first explains how engineers should select the right components when designing preamplifiers for home audio systems or PDAs. Then, the sources of noise are analyzed in detail to provide guidelines for designing low-noise preamplifiers. Finally, the design steps and related precautions are listed using the preamplifier of a PDA microphone as an example. A

　　preamplifier is a circuit or electronic device placed between the signal source and the amplifier stage, such as an audio preamplifier placed between a CD player and the power amplifier of a high-end audio system. The preamplifier is designed to receive weak voltage signals from the signal source, amplify the received signal with a small gain, and sometimes even adjust or correct it before it is sent to the power amplifier stage. For example, an audio preamplifier can equalize the signal and perform tone control. Whether designing a preamplifier for a home audio system or a PDA, you will face a very vexing problem, that is, which components should be used?

　　Component selection principles

　　Because operational amplifier integrated circuits are small in size and have excellent performance, many preamplifiers currently use such operational amplifier chips. When we design a preamplifier circuit for an audio system, we must clearly know how to select the appropriate technical specifications for the operational amplifier. During the design process, system design engineers often face the following questions.

　　Is it necessary to use a high-precision operational amplifier?

　　The input signal level amplitude may exceed the error tolerance of the operational amplifier, which is not acceptable to the operational amplifier. If the input signal or common-mode voltage is too weak, the designer should use a high-precision operational amplifier with a very low offset voltage (Vos) and a very high common-mode rejection ratio (CMRR). Whether to use a high-precision operational amplifier depends on how many times the amplification gain the system design needs to achieve. The greater the gain, the more accurate the operational amplifier needs to be.

　　What kind of supply voltage does the operational amplifier need?

　　This question depends on the dynamic voltage range of the input signal, the overall supply voltage of the system, and the output requirements. However, the different power supply rejection ratios (PSRR) of different power supplies will affect the accuracy of the operational amplifier, among which the battery-powered system is most affected. In addition, the power consumption is also directly related to the quiescent current and supply voltage of the internal circuit.

　　Does the output voltage need to be full swing?

　　Low supply voltage designs often require rail-to-rail outputs to fully utilize the entire dynamic voltage range to expand the output signal swing. As for the rail-to-rail input problem, the op amp circuit configuration has its own solution. Since preamplifiers are generally configured as inverting or non-inverting amplifiers, the input does not need to be rail-to-rail because the common-mode voltage (Vcm) is always less than the output range or equal to zero (with very few exceptions, such as single-supply voltage op amps with floating ground).

　　Is the gain-bandwidth issue more worrying?

　　Yes, it is a very worrying issue, especially for audio preamplifiers. Since human hearing can only detect sounds in the frequency range of approximately 20Hz to 20kHz, some engineers ignore or underestimate this "narrow range" bandwidth when designing audio systems. In fact, important technical parameters that reflect the performance of audio devices, such as low total harmonic distortion (THD), fast slew rate, and low noise, are all necessary conditions for high gain-bandwidth amplifiers.

Figure 1, recommended amplifier

　　A Deeper Understanding of Noise

　　Before designing a low-noise preamplifier, engineers must carefully examine the noise from the amplifier. Generally speaking, the noise of an operational amplifier comes from four main sources:

　　Thermal noise (Johnson): Thermal noise with broadband characteristics generated by irregular fluctuations in the energy of electrons in the current in the conductor. The square of its voltage root mean square value is directly related to the bandwidth, the resistance of the conductor, and the absolute temperature. For resistors and transistors (such as bipolar and field-effect transistors), since their resistance values ​​are not zero, the impact of this type of noise cannot be ignored. Flicker noise (low frequency): Noise generated by the continuous generation or integration of carriers on the surface of the crystal. In the low-frequency range, this type of flicker appears in the form of low-frequency noise. Once it enters the high-frequency range, this noise will become "white noise." Flicker noise is mostly concentrated in the low-frequency range, which will cause interference to resistors and semiconductors, and bipolar chips are more interfered than field-effect transistors. Shooting noise (Schottky): Schottky noise is generated by current carriers with particle characteristics in semiconductors. The square of its current root mean square value is directly related to the average bias current and bandwidth of the chip. This noise has broadband characteristics. Popcorn noise: If the surface of the semiconductor is contaminated, this noise will be generated. Its impact lasts for several milliseconds to several seconds. The cause of the noise is still unknown. Under normal circumstances, there is no definite pattern. If a cleaner process is used when producing semiconductors, it will help reduce this type of noise.

　　In addition, since the input stage of different operational amplifiers uses different structures, the difference in transistor structure makes the noise amount of different amplifiers vary greatly. The following are two specific examples.

　　Noise of bipolar input operational amplifier: The noise voltage is mainly caused by the thermal noise of the resistor and the high-frequency shooting noise of the input base current. The low-frequency noise level depends on the low-frequency noise generated by the input transistor base current flowing into the resistor; the noise current is mainly generated by the shooting noise of the input base current and the low-frequency noise of the resistor.

　　Noise of CMOS input operational amplifier: Noise voltage is mainly caused by thermal noise of channel resistance in high frequency region and low frequency noise in low frequency region. Corner frequency of CMOS amplifier is higher than that of bipolar amplifier, and broadband noise is also much higher than that of bipolar amplifier. Noise current is mainly caused by shooting noise of input gate leakage. Noise current of CMOS amplifier is much lower than that of bipolar amplifier, but its noise current will increase by about 40% for every 10(C increase in temperature.


　　Engineers must have a deep understanding of noise problems and perform a lot of calculations before they can accurately express these noises into numbers. In order to avoid complicating the problem, only the most critical parameters of audio technical specifications are selected here.

　　Input reference noise total ():

　　Where, refers to the source resistance; refers to the amplifier noise voltage; refers to the thermal noise of the source resistance; refers to the amplifier noise current

　　. The noise figure (NF) is the logarithm of the ratio between the input signal-to-noise ratio and the output signal-to-noise ratio, that is:

　　S and N in the above equations are both power.
　　PDA microphone preamplifier circuit

　　 Here we discuss how to design a microphone preamplifier suitable for use in PDAs. As mentioned above, we must understand that the source is the signal input to the preamplifier. First, we must know the following information:

　　Type of microphone to be used Microphone output signal level Microphone impedance and frequency gain specification for specified impedance, the relevant gain may be limited by the gain-bandwidth product of the operational amplifier Input signal frequency range Noise specification For example, the technical specifications of a ceramic microphone are as follows: Impedance: 2.2k((operates at a frequency of 1kHz) Output signal: 200(Vpp Audio input frequency range: 100Hz to 4kHz Thermal noise: 2nV/(Hz Preamplifier gain index: 500 (non-inverting), the first stage can reach a gain of 5 times, and the second stage can reach a gain of 100 times. We quote formula 1:

　　Equivalent input noise (EIN) = total input referred noise () × input frequency range

　　Output noise = equal input noise × gain = 545.81nV × 5 = 2.73uV (for 1st gain) or 545.81nV × 100 = 54.58uV (for 2nd gain).

　　The total output noise of the two amplifier stages

　　The signal-to-noise ratio level of 1V output voltage = 20×log(1V÷54.58uV)≈85.3dB.

　　The total output noise of the circuit is approximately the square root of the sum of the average RMS values ​​of each noise source. In addition, the output noise usually comes mostly from the signal source with the largest amount of noise. The actual circuit is shown in Figure 2.

Figure 2 MIC preamplifier circuit diagram

　　Please note that this circuit is only applicable to single-supply designs, where the input and output capacitors (C1 and C4) are only options that engineers can consider and select based on actual conditions. Whether it is applicable depends on how the input and output of the user's system are connected. If the microphone output has DC compensation, then the C1 input capacitor needs to be added to block the DC signal. The output capacitor can also play the same role.

　　Most of the microphones sold on the market are high-impedance microphones of about 2k( and low-impedance microphones of only a few hundred(. Both types of microphones can be designed with the above preamplifier. The high-impedance, high-output microphone preamplifier is relatively simple and can be configured with a non-inverting or inverting amplifier. Because its frequency response is relatively flat, it does not need to be specially balanced, and the input level is relatively large, and the amplifier has low noise requirements, but high-impedance microphones are extremely sensitive to unknown noise and magnetic fields. Low-impedance, low-output microphone preamplifiers can also use non-inverting or inverting amplifiers to amplify the input signal. The requirements for frequency response and equalization are roughly the same as those for high-impedance, high-output preamplifiers. If the output level of the microphone is low, engineers must pay attention to the selection of low-noise operational amplifiers. For example, a low-noise operational amplifier with good performance should produce low input-referred voltage noise, and the noise should not exceed 10nV/((Hz).