LNA is the first gain block in the receive(Rx) chain of a transceiver. It has a significant impact on the noise performance of the Rx chain.
Why low noise amplifier?
In RF circuits noise is characterized by noise figure or noise factor.
Noise factor at the input of the Rx chain is given by
Noise factor of LNA
Cascaded noise factor of subsequent block of Rx except LNA
Power gain of LNA
Eq.(1) suggests higher gain of LNA is required to supress the input referred noise coming for subsequent stages after LNA in Rx chain. This improves NF and hence sensitivity of Rx front-end. Therefore high gain and low noise figure are very important LNA design specifications for best noise performance of any receiver.
LNA Design Specifications
- Noise figure – directly impacts the sensitivity of a receiver
- Gain – reduce the impact of noise of subsequent circuits after LNA, and thus the sensitivity of the receiver
- Linearity(IIP3/IIP2 and P1dB) – decides the blocking performance
- Input/Output impedance matching – minimize the reflections
- Power consumption – should be a low as possible to improve battery life.
- Resistive termination
- Common gate configuration
- Series-Shunt feedback
- LNA with source degeneration
In the following analysis, only thermal noise of MOS transistor and resistors are considered. In the real design, gate induced noise, and other noise sources need to be considered for accurate results. Flicker noise, most of times is shaped out by passive networks for narrow band circuits at radio frequencies and can be ignored.
LNA with resistive termination
Well suited topology for broadband LNA design. It provides input impedance matching Schematic of the LNA with resistive termination is shown in Figure 1a. The equivalent circuit of resistive termination LNA is shown in Figure 1c.
Gain of LNA,
Noise Factor of LNA is given by
noise power at the input of LNA due to the source
noise power added by the LNA referred to the output
where, . When , then noise factor is
Therefore the noise figure of Resistive terminated LNA is > 3dB.
Common gate configuration
A common gate configuration LNA is shown in Figure 2a. The equivalent circuit is shown if Figure 2c.
The gain of LNA is,
MOSFET channel noise only considered here for NF calculation.
Noise power added by the circuit referred to the output is ,
Noise power at input
Noise factor of the circuit is
When , noise factor is