The role of the preamplifier in the audio system is crucial. This article first explains how engineers should properly select components when designing a preamp for a home audio system or PDA. Subsequently, a detailed analysis of the source of the noise provides guidelines for designing a low-noise preamplifier.
Preamplifier refers to the circuit or electronic equipment placed between the source and the amplifier stage, such as the audio preamplifier placed between the optical disc player and the power amplifier of the advanced audio system. The preamplifier is designed to receive the weak voltage signal from the source. The received signal is first amplified with a small gain. Sometimes it is adjusted or modified before being transmitted to the power amplifier stage, such as audio preamp The amplifier can first equalize the signal and perform tone control. Whether designing a pre-amplifier for a home audio system or a PDA, one has to face a very headache problem, which components should be used properly?
Component selection principles
Due to the small size and excellent performance of the operational amplifier integrated circuit, many preamplifiers currently use this type of operational amplifier chip. When we design a preamp circuit for an audio system, we must know how to choose the appropriate technical specifications for the operational amplifier. During the design process, system design engineers often face the following problems.
1. Is it necessary to use a high-precision operational amplifier?
The input signal level amplitude may exceed the error tolerance of the op amp, which is not acceptable for the op amp. If the input signal or common-mode voltage is too weak, the designer should use a high-precision operational amplifier with a very low compensation voltage (Vos) and a high common-mode rejection ratio (CMRR). Whether high-precision operational amplifiers are used depends on how many times the amplification gain needs to be achieved in the system design. The greater the gain, the more accurate operational amplifiers need to be used.
2. What kind of power supply voltage does the operational amplifier need?
This problem depends on the dynamic voltage range of the input signal, the overall power supply voltage of the system, and the output requirements. However, the different power supply rejection ratio (PSRR) of different power supplies will affect the accuracy of the operational amplifier. Among them, the battery-powered system The most affected. In addition, the power consumption is also directly related to the quiescent current and power supply voltage of the internal circuit.
3. Does the output voltage need full swing?
Low-supply voltage designs usually require full-swing output to fully utilize the entire dynamic voltage range to expand the output signal swing. As for the problem of rail-to-rail input, the configuration of the op amp circuit will have its own solution. Since preamplifiers are generally configured with inverting or non-inverting amplifiers, the input does not need full swing, because the common-mode voltage (Vcm) is always less than the output range or equal to zero (with very few exceptions, such as a single with floating ground Supply voltage operational amplifier).
4. Is the issue of gain bandwidth more worrying?
Yes, especially for audio preamplifiers, this is a very worrying problem. Since human hearing can only perceive sounds in the frequency range of approximately 20 Hz to 20 kHz, some engineers ignore or underestimate this "narrower" 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.
Learn more about noise
Before designing a low-noise preamplifier, engineers must carefully examine the noise originating from the amplifier. Generally speaking, the noise of an operational amplifier mainly comes from four aspects:
1. Thermal noise (Johnson): The thermal noise with broadband characteristics due to the irregular fluctuation of the electronic energy of the current in the electrical conductor. The square of the voltage rms value is directly related to the bandwidth, electrical conductor resistance, and absolute temperature. For resistors and transistors (such as bipolar and field-effect transistors), since the resistance value is not zero, such noise effects cannot be ignored.
2. Flicker noise (low frequency): noise generated by the continuous generation or integration of carriers on the crystal surface. In the low-frequency range, this type of flicker appears in the form of low-frequency noise. Once it enters the high-frequency range, these noises will become "white noise". Most of the flicker noise is concentrated in the low frequency range, which will cause interference to the resistor and the semiconductor, and the interference to the bipolar chip is greater than that of the field effect transistor.
3. Shooting noise (Schottky): Schottky noise is generated by current carriers with particle characteristics in the semiconductor. The rms square of the current is directly related to the average bias current and bandwidth of the chip. This noise has broadband characteristics.
4. Popcorn frequency (popcorn frequency): If the surface of the semiconductor is contaminated, this noise will be generated, and its impact can be as long as a few milliseconds to a few seconds. The cause of the noise is still unknown. Under normal circumstances, there is no certain pattern. . The use of cleaner processes in the production of semiconductors will help reduce this type of noise.
In addition, because the input stages of different operational amplifiers use different structures, the difference in transistor structure makes the amount of noise of different amplifiers also very different. The following are two specific examples.
1. The noise of the bipolar input operational amplifier: the noise voltage is mainly caused by the thermal noise of the resistor and the shooting noise in the high frequency region of the input base current. The level of low frequency noise depends on the low frequency generated by the base current of the input transistor flowing into the resistor Noise; noise current is mainly caused by the shooting noise of the input base current and the low frequency noise of the resistor.
2. The noise of the CMOS input operational amplifier: the noise voltage is mainly caused by the thermal noise of the channel resistance in the high frequency region and the low frequency noise in the low frequency region. The corner frequency of the CMOS amplifier is higher than that of the bipolar amplifier, and the broadband noise is also far away. Higher than bipolar amplifier; the noise current is mainly caused by the shooting noise of the input gate leakage. The noise current of the CMOS amplifier is much lower than that of the bipolar amplifier, but the noise current will increase by about 40% for every 10 ° C increase in temperature .
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