By Gibin Bose, Design Engineer at Farran Technology
Low noise amplifier (LNA) is a critical component of radio communication systems, electronic test equipment and RF receiver systems such as those found in mobile communication, GPS systems, satellites, radars and other applications.
Being the first active circuit in the receiver chain following the antenna (see Figure 1), figures of merit of LNA such as noise figure, gain, linearity and input VSWR play a crucial role in determining the noise performance, linearity and VSWR of the overall system and sensitivity of the receiver.
Figure 1. A conceptual diagram of RF front end
The main purpose of a low noise amplifier is to amplify the low power RF signals from the antenna to an appropriate power level required by subsequent stages while introducing minimal additive noise. It should be noted that any RF amplifier increases the power level of both the signal and noise present at its input by the same amount and also introduce some additional noise. Keeping the level of this added noise to a minimum is crucial in preserving the integrity of the received information and optimal retrieval of the desired signal in the later stages of the system.
With increasing demands, more stringent constraints on performance, size and cost, development of newer technologies and push towards higher operating frequencies, LNA designs are being rolled out with higher goals as well. For example, in cellular phone designs in order to reduce the battery consumption, current drain has to be reduced as much as possible so that the standby current of the overall receiver is low. At the same time, devices must be kept small, cost effective and high performing as well, rendering the LNA design process more complicated.
In addition to satisfying the demands of high gain and low noise figure, LNA’s exhibiting high dynamic range, linearity and good input and output matching are critical in modern communication systems. In communication systems using Code-Division Multiple Access (CDMA), LNA’s taking care of automatic gain control is important as well for the entire system to operate well. For instance, in mobile applications where the path loss from the base station to phone can alter, input signal strength at the receiver varies and automatic gain control becomes crucial in this kind of scenarios.
Critical Figures of Merit and Functions
Gain: The primary function of a low noise amplifier in the RF front end is to increase the amplitude of the low power signal at the input with minimal signal to noise ratio degradation and this increase in the signal amplitude is quantified by gain. Gain, usually measured as small signal gain is the ratio of the output power to input power expressed in dB in the linear region of operation and under fixed recommended operating conditions for the amplifier.
Noise Factor, Noise Figure, Noise Temperature: In the receiver system, noise added by the amplifier degrades the received signal by degrading the signal to noise ratio. Noise factor, noise figure and noise temperature are different ways of quantifying this degradation. Low noise amplifier in the receiver system plays the crucial role of amplifying the input signal with minimal signal to noise degradation and thereby preserving the integrity of the received information.
Noise Factor (F) is the ratio of signal to noise ratio at the input to the signal to noise ratio at the output, where Sin and Nin represent the input signal and noise powers respectively and Sout and Nout , output signal and noise powers respectively. The input termination here is assumed to be at standard noise temperature T0= 290 K and noise of input signal, Nin = kT0B, where k is the Boltzmann’s constant and B is the noise bandwidth.
Noise Factor expressed in dB provides the Noise Figure (NF), where F is the noise factor.
It should be noted that the output signal to noise ratio is never higher than the input signal to noise ratio and hence F>1 . For a noiseless system, we have, F=1 or NF=0 dB.
We can also use effective input Noise Temperature (Te) to describe the noise performance of a device. If the gain of the device is denoted by G, we have Sout=GSin and Nout=Gk(T0+Te)B and from the definition of Noise Factor
So, we have the effective input noise temperature (Te) related to the noise factor as below, where T0=290 K. Effective input noise temperature of an amplifier can be described as the equivalent noise temperature of a source when connected to a noise-free device that would produce the same output noise as of the amplifier when connected to a noise-free source.
Return Loss: Good input and output matching is crucial in the design of LNA’s as it minimize signal loss and helps in efficient power transfer between antenna and subsequent stages. This can be quantified by the return loss of LNA which is a measure of the ratio of the incident to reflected power at either port expressed in dB. Return Loss (RL(dB)) can be expressed as below, where Pi is the incident power and Pr is the reflected power.
Linearity and Dynamic Range: An amplifier provides a constant gain until a specific input power level and then there would be drop in gain resulting in compression effect i.e. the amplifier output power starts to saturate. The one dB compression point (P1dB) is the power level (input/output) where the gain is 1dB lower than the small signal gain. Beyond the P1dB point, amplifier becomes a non-linear device and can produce distortion, harmonics and intermodulation products. In the cases when a strong RF signal arrives at the antenna, in order to avoid overloading the circuit and leading to distortion, high linearity of LNA’s become crucial.
Dynamic range of a device is the ratio of the highest signal level it can handle to the lowest signal level it can handle. It should be noted that the highest signal level is usually limited by distortion which can be characterised by P1dB whereas on the other end it is limited by the output noise. LNA’s exhibiting high dynamic range possess the ability to handle wide range of signals that arrive at its input.
In addition to the critical figures of merit of low noise amplifier discussed here, bandwidth of operation, stability, reverse isolation, DC power requirements, power efficiency, cost, physical properties, performance in extreme conditions and other specific requirements based on the use case can play a key role in the selection of LNA’s. Farran offers a broad range of standard and custom-built low noise amplifiers over the frequency range of 10 MHz – 170 GHz.
Access our FLNA page for more details and to contact us https://farran.com/farran-products/amplifiers-low-noise-flna/