Gain adjustment method of low-noise amplifier
There are many ways to adjust the gain of low-noise amplifiers (LNAs). The following are some common ways:
Change bias voltage
Principle: By adjusting the bias voltage of the transistor in the LNA, the operating point of the transistor can be changed, thereby affecting its amplification factor and achieving gain adjustment. Changes in bias voltage change the current-voltage characteristics of the transistor, causing the transistor to work in different operating regions, thereby changing the gain.
Application: Usually in the circuit design of LNA, a special bias circuit is set to provide an adjustable bias voltage. By changing the parameters of components such as resistors and capacitors in the bias circuit, or using an adjustable power supply to change the size of the bias voltage, the LNA gain can be fine-tuned. This method is simple and easy to implement, with low cost, but the adjustment range is relatively limited.
Using a variable gain amplifier (VGA)
Principle: VGA is a special amplifier whose gain can be adjusted by an external control signal. It usually consists of multiple cascaded amplifiers, and by controlling components such as switches or variable resistors, the transmission path or attenuation of the signal between different stages is changed to achieve gain adjustment.
Application: Combining VGA with LNA can flexibly adjust the gain of LNA within a certain range. For example, in some systems that need to automatically adjust the gain according to the input signal strength, by detecting the amplitude of the input signal and using the feedback circuit to control the gain of the VGA, the output signal of the LNA is kept within the appropriate level range to avoid signal overload or too small.
Adjusting the load resistance
Principle: The gain of the LNA is related to the load resistance. According to the basic principle of the amplifier, changing the size of the load resistance will affect the output voltage and current of the amplifier, thereby changing the gain.
Application: In the output stage circuit of the LNA, a variable resistor can be used as a load resistor. By changing the resistance value of the variable resistor, the gain of the LNA can be adjusted. This method can achieve a larger range of gain adjustment, but it may have a certain impact on the noise performance and output impedance of the LNA, which needs to be comprehensively considered and optimized during design.
Change bias voltage
Principle: By adjusting the bias voltage of the transistor in the LNA, the operating point of the transistor can be changed, thereby affecting its amplification factor and achieving gain adjustment. Changes in bias voltage change the current-voltage characteristics of the transistor, causing the transistor to work in different operating regions, thereby changing the gain.
Application: Usually in the circuit design of LNA, a special bias circuit is set to provide an adjustable bias voltage. By changing the parameters of components such as resistors and capacitors in the bias circuit, or using an adjustable power supply to change the size of the bias voltage, the LNA gain can be fine-tuned. This method is simple and easy to implement, with low cost, but the adjustment range is relatively limited.
Using a variable gain amplifier (VGA)
Principle: VGA is a special amplifier whose gain can be adjusted by an external control signal. It usually consists of multiple cascaded amplifiers, and by controlling components such as switches or variable resistors, the transmission path or attenuation of the signal between different stages is changed to achieve gain adjustment.
Application: Combining VGA with LNA can flexibly adjust the gain of LNA within a certain range. For example, in some systems that need to automatically adjust the gain according to the input signal strength, by detecting the amplitude of the input signal and using the feedback circuit to control the gain of the VGA, the output signal of the LNA is kept within the appropriate level range to avoid signal overload or too small.
Adjusting the load resistance
Principle: The gain of the LNA is related to the load resistance. According to the basic principle of the amplifier, changing the size of the load resistance will affect the output voltage and current of the amplifier, thereby changing the gain.
Application: In the output stage circuit of the LNA, a variable resistor can be used as a load resistor. By changing the resistance value of the variable resistor, the gain of the LNA can be adjusted. This method can achieve a larger range of gain adjustment, but it may have a certain impact on the noise performance and output impedance of the LNA, which needs to be comprehensively considered and optimized during design.