Skip to content

Cascode Amplifier with BJTs

Cascode Amplifier with BJTs: Mitigation of Miller Effect and Reverse Signal Attenuation#

Cascode Amplifier:

  • A cascode amplifier is a configuration that consists of a common-emitter stage followed by a common-base stage. This arrangement is highly effective in high-frequency applications.

Miller Effect:

  • The Miller effect describes the phenomenon where the input capacitance of an amplifier stage is amplified by the gain, leading to a significant reduction in bandwidth.
  • In Bipolar Junction Transistors (BJTs), the parasitic capacitance between the collector and base can range from a few picofarads to fractional picofarads.
  • By employing a cascode configuration, the common-base stage isolates the input of the common-emitter stage from the output, thus mitigating the Miller effect and maintaining high gain without significant bandwidth reduction.

Reverse Signal Attenuation:

  • One significant advantage of the cascode amplifier is its ability to reject or attenuate signals that appear in the reverse direction.
  • In a typical common-emitter amplifier with high gain (10 to 100), a reverse signal such as a local oscillator signal inadvertently appearing at the output can pass through to the antenna, causing interference and other problems.
  • The cascode configuration effectively prevents this issue. The common-base stage offers high reverse isolation, preventing signals from the output from feeding back into the input stage.

Operational Mechanism:

  • The common-emitter stage provides the initial amplification.
  • The common-base stage acts as a buffer, offering high input impedance to the common-emitter stage and low output impedance to the load.
  • This combination ensures that signals do not easily pass from the output back to the input, which is crucial in applications like RF amplifiers where isolation from reverse signals is needed to maintain signal integrity and reduce interference.

Summary#

This description highlights the effectiveness of the cascode amplifier in mitigating the Miller effect and preventing reverse signal interference. This makes it an excellent choice for high-frequency applications where both forward gain and reverse signal isolation are crucial. It makes a splendid front-end amplifier for a direct conversion receiver.

References:#

  1. Analogue Devices — Understanding and Improving the Cascode Amplifier
  2. Electronics Tutorials — Cascode Amplifier
  3. All About Circuits — The Cascode Amplifier

PRACTICAL CASCODE AMPLIFIER CIRCUIT USING BJTs#

cascode circuit diagram

You can see the circuit that I have used in a direct conversion receiver. It has exceptional reverse signal attenuation while also providing good forward amplification characteristics.

You will note that there is a switch on the left-hand side of the circuit. This switches between GND and the RF signal, which is represented by a signal generator running at 7 MHz with an amplitude of 10 µV. When the switch is set to GND, we can assess the quiescent current, which is measured at 740 µA. Even if this winds up at 1 mA, it is still very low.

Moving further toward the right, the low voltage 10 µV signal is injected into this amplifier via a 100 nF capacitor. It is injected into the base of the lower transistor, which has a switchable bypass capacitor. When the bypass capacitor is not used, the gain of this amplifier is approximately 10. When the bypass capacitor is switched in circuit, the gain is approximately 100. This is a handy way for someone who uses a direct conversion receiver to choose the right level for the signal they want to hear.

The next transistor up from the one where the signal is injected has its base connected to GND via a 100 nF capacitor. For our signal, it means that this transistor has to look at the signal from the emitter against the AC GND provided by the 100 nF capacitor.

Both transistors have a 300 kΩ resistor from the bases to the collectors. There is a current limiting resistor from VCC to the collector of the top transistor. This sets the DC bias at the bases of these transistors, such that the voltages are fairly low.

KEY POINTS#

  • Cascode Configuration: Utilizes a common-emitter stage followed by a common-base stage for high-frequency performance and reverse signal attenuation.
  • Bypass Capacitor: Switchable bypass capacitor allows for gain adjustment between approximately 10 and 100.
  • Quiescent Current: The circuit's quiescent current is measured at 740 µA, and even at 1 mA, it remains low, making it efficient.
  • Signal Injection: The signal is injected via a 100 nF capacitor into the base of the lower transistor.
  • Base Connection: The upper transistor’s base is AC grounded through a 100 nF capacitor, improving signal handling.
  • Uses 2N3904 NPN transistors.

FINAL THOUGHTS#

I hope that this description provides a clear and detailed understanding of the cascode amplifier circuit using 2N3904 transistors, highlighting its key features and operational benefits.