Choosing the right low-noise amplifier (LNA)

This application note examines the key parameters that affect amplifier noise and illustrates the impact of different amplifier designs (bipolar, JFET input, or CMOS input design) on noise. This article also explains how to select a low-noise amplifier suitable for low-frequency analog applications such as data converter buffering, strain gauge signal amplification, and microphone preamplifiers. Based on a CMOS input amplifier, the MAX4475, illustrates the design advantages of this new CMOS amplifier in most low-frequency analog applications.

Currently, discussions about low-noise amplifiers often focus on RF/wireless applications, but in actual applications, noise also has a great impact on low-frequency analog products such as data converter buffers, strain gauge signal amplification, and microphone preamplifiers. important considerations. In order to select an appropriate amplifier, the design engineer must first understand whether the amplifier has low noise characteristics and the associated noise parameters. Also, understand the differences in noise parameters of different types of amplifiers (bipolar, JFET input, or CMOS input).

Noise parameters

Although there are many parameters that affect the noise performance of an amplifier, the two most important parameters are: voltage noise and current noise. Voltage noise refers to the voltage fluctuation that appears at the input terminal when the amplifier input is short-circuited without its noise interference. Current noise refers to the current fluctuation that appears at the input end of the amplifier when the input is open circuit in the absence of other noise interference.

A typical metric used to describe amplifier noise is noise density, also known as point noise. The unit of voltage noise density is nV/√Hz, and the current noise density is usually expressed as pA/√Hz. These parameters can be found in low-noise amplifier data sheets, and values ​​are generally given at two frequencies: one for flicker noise below 200Hz; the other for noise in the 1kHz passband. For simplicity, these measurements are referenced to the amplifier input and do not require amplifier gain to be considered.

Figure 1 shows the corresponding relationship curve between voltage noise density and frequency. The noise curve is related to two main noise components: flicker noise and shot noise. Flicker noise is random noise inherent in all linear devices and is also called 1/f noise because the noise amplitude is inversely proportional to frequency. Flicker noise is usually the dominant noise source at frequencies below 200Hz, as shown in Figure 1. The 1/f corner frequency refers to the starting frequency where the noise size is basically the same and is not affected by frequency changes. Shot noise is white noise caused by current fluctuations flowing through a forward-biased pn junction and also appears in this frequency band. It is worth noting that the 1/f corner frequency of voltage noise may be different from the 1/f corner frequency of current noise.

Figure 1. Voltage noise density versus frequency curve, mainly affected by two noise sources: flicker noise and shot noise. Flicker noise or 1/f noise is inversely proportional to frequency and is the dominant noise source at frequencies below 200Hz. The total noise of an amplifier circuit depends on parameters such as the amplifier itself, external circuit impedance, gain, circuit bandwidth, and ambient temperature. Thermal noise generated by the circuit's external resistance is also part of the total noise. Figure 2 shows an example of an amplifier and associated noise components.

Figure 2. The source impedance of the amplifier circuit determines the type of noise that dominates. As the source impedance increases, current noise is the dominant source.

Calculate total noise

The standard expression for the total input noise of an op amp at a specific frequency is:

Among them: R _n = inverting input equivalent series resistance
R _p = non-inverting input equivalent series resistance
e _n = input voltage noise density at a specific frequency
i _n = input current noise density at a specific frequency
T = in Kelvin (°K) The absolute temperature of the unit
is k=1.38 x 10 ^-23 J/°K (Boltzmann’s constant).

Formula 1 is the corresponding relationship between noise and bandwidth at a specified frequency. To calculate the total noise, multiply et (in nV/√Hz) by the square root of the bandwidth. For example, if the bandwidth of the amplifier ranges from 100Hz to 1kHz, then the following equation is the total noise over the entire bandwidth:

The above example gives the calculation formula of the total noise when the voltage noise and current noise are fixed over the entire bandwidth range (applicable to the case where the lower frequency value of the amplifier circuit bandwidth is greater than the voltage noise and circuit noise 1/f frequency of the op amp) . If the voltage noise and current noise vary across the bandwidth, the total noise calculation becomes more complex.

The effect of circuit source impedance on noise can be easily seen based on Equation 1 and Figure 2. In systems with low source impedance, voltage noise is the main source of noise; when the source impedance increases, resistor noise dominates, and the voltage noise of the amplifier can even be ignored. As the source impedance continues to increase, current noise becomes the dominant contributor to the noise.

Impact of Amplifier Design on Noise Performance

Noise performance is a consideration in amplifier design, and the three common types of low-noise amplifiers are: bipolar, JFET input, and CMOS input. Although each design offers low noise characteristics, its performance differs.

bipolar amplifier

Bipolar amplifiers are the most common choice among low-noise amplifiers. Low-noise, bipolar amplifiers such as the MAX410 offer extremely low input voltage noise density (1.8nV/√Hz) and relatively high input current noise density (1.2pA/√Hz). The typical unity gain bandwidth of this type of amplifier is less than 30MHz.

To ensure low voltage noise from a bipolar op amp, IC designers set a higher collector current in the input stage. This is because voltage noise is inversely proportional to the square root of the input stage collector current; however, op amp current noise is directly proportional to the square root of the input stage collector current. Therefore, external feedback and source impedance must be as low as possible to obtain good noise performance. The input bias current is proportional to the input collector current, so the source impedance must be kept as low as possible to reduce the offset voltage produced by the bias current.

A bipolar amplifier's voltage noise usually dominates when its equivalent source impedance is less than 200Ω. The large input bias current and relatively large current noise make bipolar amplifiers well suited for applications with low source impedance.

JFET input amplifier

Compared with bipolar designs, JFET input low-noise amplifiers have ultra-low input current noise density (0.5fA/√Hz), but the input voltage noise density is relatively large (greater than 10nV/√Hz). The JFET design allows single-supply operation . The 1pA input bias current makes the JFET amplifier ideal for high-impedance signal source applications. However, due to the large voltage noise of JFET amplifiers, it is usually not the first choice for design engineers in applications with low source impedance.

CMOS input amplifier

New CMOS input low-noise amplifiers provide voltage noise specifications comparable to bipolar designs. The current noise of CMOS input amplifiers is comparable to or better than the best JFET input designs. For example, the MAX4475 has low input voltage noise density (4.5nV/) and low input current noise density (0.5fA/), and delivers ultra-low distortion (0.0002% THD+N) from a single supply. These characteristics make CMOS input amplifiers an excellent choice for low-distortion, low-noise applications such as audio preamplifiers. In addition, CMOS input amplifiers allow very low input bias current, low offset voltage and very high input impedance, which can meet signal conditioning with high source impedance, such as the photodiode preamplifier circuit shown in Figure 3. Figure 4 shows the buffer used for the 16-bit DAC output.

Figure 3. A low-noise amplifier with CMOS inputs has very low bias current and offset voltage, and very high input impedance. These devices are ideal for signal conditioning with higher source impedances (such as photodiode preamplifiers).

Figure 4. Low noise performance and low input bias current make CMOS input amplifiers ideal for 16-bit DAC output buffers.

in conclusion

No single amplifier is suitable for all applications. Table 1 summarizes typical noise parameters for three common amplifier designs.

Comparing all noise sources, it can be seen that new CMOS input amplifiers (such as the MAX4475) provide the best noise specifications for lower frequency analog applications and most front-end applications, especially high-source impedance, wideband designs.