Write two features of low noise amplifier.
Answers
Low noise amplifiers are the building blocks of communication systems and instruments. The four important parameters in LNA design are: gain, noise figure, non-linearity and impedance matching. The required LNA design steps are:
Gain device
Amplifiers need a device to provide gain. In the 1940s, that device was a vacuum tube, but now it is usually a transistor or an integrated circuit. The transistor may be one of many varieties of bipolar transistors or field-effect transistors. Other gain devices, 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.
Large-signal models that consider non-linear mixing.
Circuit topology
Circuit topology covers issues such as gain and input impedance.
Gain is often a compromise. On one hand, having lots of gain is good because it takes weak signals above the noise floor. On the other hand, lots of gain means higher level signals and more problems with non-linear mixing.
The circuit topology also 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. There is another notion of a noise impedance match.
Another design issue is the noise introduced by biasing networks.
Noise
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.
High gain
With a low noise figure, an LNA must have high gain. An LNA without high-gain allows the signal to be affected by LNA circuit noise; the signal may become attenuated, so the LNA's high gain is an important parameter. Like noise figures, LNA gain also varies with operating frequency.