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A Better Cascode Amplifier with BJTs

A HIGHER CURRENT VERSION OF THE RF CASCODE AMPLIFIER#

Main Objective: The primary goal of this RF cascode amplifier is to serve as the front end for a direct conversion receiver. This circuit achieves low noise amplification and exceptionally high levels of reverse signal rejection.

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Circuit Description: This version of the cascode amplifier is designed to operate at higher current levels for improved performance.

  • Quiescent Current: 3 mA at 12 Volts.
  • Gain: With an input signal of 10 µV, the output is 1.22 mV.
  • It is possible to cut the gain by switching out the bypass capacitor; this results in an output of just over 500 µV with an input signal of 10 µV.
  • Uses 2N3904 NPN transistors.

INTRODUCTION#

An RF cascode amplifier is a critical circuit-block in direct conversion receivers, where it provides the necessary front-end amplification with low noise and high reverse signal rejection.

CIRCUIT DESCRIPTION#

  • Quiescent Current: 3 mA at 12 Volts ensures robust operation.
  • Gain Performance: Achieves an output of 1.22 mV from an input of 10 µV, demonstrating its high gain capability.

IMPORTANCE OF CURRENT IN CASCODE AMPLIFIERS#

  • Noise Considerations: Operating the circuit at higher currents helps mitigate the amplification of unwanted noise. This is particularly important in RF applications where signal purity is paramount.
  • Stability and Linearity: Higher current operation improves the linearity and stability of the transistors, leading to better overall performance of the amplifier.

PRACTICAL INSIGHTS#

When designing or optimising RF cascode amplifiers, we need to consider the balance between current levels and performance. Although there is no definitive guideline regarding the precise current to be used, enhancing the current can frequently result in significant enhancements in noise reduction and signal integrity. From a different perspective, since we are referring to the front end of a receiver, it would be preferable to refrain from introducing unnecessary noise at this point. From this point forward, the desired signal and any noise will be amplified until we arrive at the audio amplifier, which itself is often a source of noise.

CONCLUSION#

The higher current RF cascode amplifier described here offers a practical solution for enhancing the front end of direct conversion receivers. By carefully managing the quiescent current and understanding its impact on noise and performance, designers can achieve superior results in their RF applications.

This content is the basis of an informative article aimed at electronics enthusiasts and professionals looking to improve their understanding and implementation of RF cascode amplifiers, within the context described above.