Low noise amplifiers are the building blocks of communication systems and instruments. The most important LNA specifications or attributes are: • Gain • Noise figure • Linearity • Maximum RF input A good LNA has a low NF (e.g. ), enough gain to boost the signal (e.g. ) and a large enough inter-modulation and
compression point (IP3 and P1dB) to do the work required of it. Further specifications are the LNA's operating bandwidth, gain flatness, stability, input and output
voltage standing wave ratio (VSWR). For low noise, a high amplification is required for the amplifier in the first stage. Therefore, junction field-effect transistors
(JFETs) and
high-electron-mobility transistors (HEMTs) are often used. They are driven in a high-current regime, which is not energy-efficient but reduces the relative amount of
shot noise. It also requires input and output
impedance matching circuits for
narrow-band circuits to enhance the gain (
see Gain-bandwidth product).
Gain Amplifiers need a device to provide gain. In the 1940s, that device was a
vacuum tube, but now it is usually a transistor. The transistor may be one of many varieties of
bipolar transistors or
field-effect transistors. Other devices producing gain, such as
tunnel diodes, may be used. Broadly speaking, two categories of transistor models are used in LNA design: Small-signal models use quasi-linear models of noise and large-signal models consider non-linear mixing. The amount of gain applied is often a compromise. On the one hand, high gain makes weak signals strong. On the other hand, high gain means higher-level signals, and such high-level signals with high gain may exceed the amplifier's dynamic range or cause other types of noise such as harmonic distortion or nonlinear mixing.
Noise figure The
noise figure helps determine the efficiency of a particular LNA. LNA suitability for a particular application is typically based on its noise figure. In general, a low noise figure results in better signal reception.
Impedance The circuit topology affects input and output impedance. In general, the source impedance is matched to the input impedance because that will maximize the power transfer from the source to the device. If the source impedance is low, then a
common base or
common gate circuit topology may be appropriate. For a medium source impedance, a
common emitter or
common source topology may be used. With a high source resistance, a
common collector or
common drain topology may be appropriate. An input
impedance match may not produce the lowest noise figure.
Biasing Another design issue is the noise introduced by
biasing networks. In communication circuits, biasing networks play a critical role in establishing stable operating points for active components, but they also introduce noise. The primary types of noise introduced by these networks are thermal noise and flicker noise. Thermal noise arises from resistive elements in the network, which is inevitable as any resistive component generates noise due to the random motion of charge carriers. This type of noise is especially problematic at high frequencies. Flicker noise, also known as 1/f noise, is related to the current flow through devices like transistors and becomes more significant at lower frequencies. In an LNA, the biasing network must be carefully designed to minimize the impact of noise on the overall performance. Improper biasing can lead to increased noise figures, compromising the signal-to-noise ratio and degrading communication system performance. The design and selection of components within the bias network are therefore crucial to ensuring low-noise operation, particularly in systems that rely on amplifying weak signals. In practice, the bias point strongly affects an LNA’s noise figure, gain, linearity, and stability. A transistor’s key noise parameters, the minimum noise factor (F_min), the equivalent noise resistance (R_n), and the optimum source match (Γ_opt), all change with current and voltage. Designers therefore sweep the quiescent current and voltage and pick a bias that meets the noise figure target while leaving headroom for linear operation, rather than simply using the highest current available. Common bias schemes include simple
self bias, which uses a source or emitter resistor with a gate or base divider, and active bias loops that hold the quiescent current over temperature and process variation. Self bias is passive and tends to stabilize with temperature but slightly reduces available gain. Active bias improves repeatability, at the cost of extra circuitry and possible loop stability checks. Technology specific sequencing also matters. Many depletion mode LNAs need a negative gate bias and a positive drain supply. To prevent overcurrent during power up and power down, the gate bias is applied and removed before the drain supply. Designers often use bias controllers or simple sequencing circuits, sometimes with temperature compensation. By contrast, some LNAs, such as CMOS, usually run from a single positive supply and set current with emitter or source degeneration plus active bias. Because S parameters and stability factors change with bias and temperature, LNAs that may see arbitrary source impedances are typically checked for unconditional stability across bias and temperature corners. Only the minimum damping or feedback needed is added so that stability does not unduly raise the noise figure. == Applications ==