Cascode Amplifier (BJT): DC Analysis & Multistage Amplifiers

Introduction to the Cascode Amplifier Calculator

This BJT Amplifier online calculator is designed to compute bias voltages, currents, voltage gain, and frequency response for a Cascode amplifier configuration.

The Cascode amplifier is renowned for its high gain and exceptionally wide bandwidth. By utilizing a second transistor as a common-base current buffer, the initial common-emitter stage overcomes the limitations caused by the Miller Effect. Consequently, this configuration allows for gain-bandwidth products orders of magnitude larger than a standard common-emitter amplifier.

Key Design Considerations:

• Voltage Supply: Since two transistors are stacked vertically, a higher voltage supply is often required to maintain decent operating margins for both devices.
• Gain Control: The gain is roughly determined by the ratio $R_C / R_{E2}$.
• Bandwidth: A higher sum of emitter resistances ($R_{E1} + R_{E2}$) generally contributes to greater bandwidth stability.
• Buffering: For real-world applications above 2 MHz, an output buffer (like an emitter follower) is recommended to prevent loading effects. Measuring high-frequency response directly with a standard 1 MHz scope probe without a buffer will artificially roll off the frequency response.

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Understanding the Cascode Amplifier

What is a Cascode Amplifier?

A Cascode Amplifier is a two-stage multistage amplifier consisting of a Common Emitter (CE) stage feeding into a Common Base (CB) stage.

• Stage 1 (Input): A Common Emitter amplifier that handles the input signal.
• Stage 2 (Output): A Common Base amplifier that acts as a current buffer, isolating the input from the output voltage swings.

(Note: In Field Effect Transistor (FET) terminology, this corresponds to a Common Source driving a Common Gate.)

Advantages & Disadvantages

Advantages

• Elimination of Miller Effect: The input transistor's collector voltage is held nearly constant by the common-base stage. This prevents the amplification of the base-collector capacitance (Miller capacitance), resulting in significantly higher bandwidth.
• High Output Impedance: The circuit offers very high output impedance, making it an excellent current source.
• High Isolation: There is excellent reverse isolation between the input and output.
• Simplicity: Despite the performance boost, the circuit topology remains relatively simple to manufacture.

Disadvantages

• High Voltage Requirement: Because the transistors are stacked in series, a higher supply voltage ($V_{CC}$) is needed to bias both transistors properly while allowing for sufficient signal swing.
• Reduced Voltage Swing: The signal headroom is reduced compared to a single-transistor amplifier.

Applications

• RF Tuners: Widely used in radio frequency front-ends due to low noise and high bandwidth.
• Modulation: Used in amplitude modulation circuits. Ideally, the oscillator signal is fed to the upper transistor (mixer), while the RF signal is fed to the lower transistor.
• Superheterodyne Receivers: Utilized as multiplying mixers.

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Introduction to Bipolar Junction Transistors (BJT)

A Bipolar Junction Transistor (BJT) is a solid-state device where the current flow between two terminals (the Collector and the Emitter) is controlled by the amount of current flowing through a third terminal (the Base).

BJTs are fundamental to analog circuits, especially in very-high-frequency applications such as Wi-Fi systems and radio transmitters. They can also be combined with MOSFETs in BiCMOS technology to create integrated circuits that leverage the high speed of BJTs and the low power consumption of FETs.

Historical Context: The BJT was invented in December 1947 at Bell Telephone Laboratories by John Bardeen, Walter Brattain, and William Shockley. This invention replaced fragile, power-hungry vacuum tubes with tiny, rugged silicon devices, revolutionizing the electronics industry.

BJT Structure and Layers

A BJT consists of a three-layer "sandwich" of doped semiconductor materials. The three layers are the Emitter, Base, and Collector.

The physical arrangement of these layers defines the two primary types of BJTs:

1. PNP: P-type / N-type / P-type
2. NPN: N-type / P-type / N-type

NPN vs. PNP Transistors

The practical difference lies in the polarity of the voltages and the direction of current flow.

• NPN: The majority charge carriers are electrons. The base is energized with a positive voltage relative to the emitter to turn it on.
• PNP: The majority charge carriers are holes. The base is pulled negative relative to the emitter to turn it on.

Comparison Table:

Feature	NPN Transistor	PNP Transistor
Structure	Two N-type layers sandwiching one P-type layer.	Two P-type layers sandwiching one N-type layer.
Majority Carriers	Electrons	Holes
Current Flow	Collector to Emitter	Emitter to Collector
Collector Voltage	Positive (relative to Emitter)	Negative (relative to Emitter)
Bias Configuration	Base-Emitter Forward, Base-Collector Reverse.	Base-Emitter Forward, Base-Collector Reverse.
Symbol Arrow	Points Out (Not Pointing In)	Points In (Pointing In)
Switching Speed	Faster (Electrons move faster than holes)	Slower
Symbol