Back-to-back MOSFETs, also known as complementary MOSFET pairs, are a fundamental building block in analog and digital circuit design. These pairs, formed by connecting the drain of one MOSFET to the source of another, exhibit distinct characteristics depending on the configuration of their sources and drains. This article delves into the two most common configurations: common source and common drain, comparing their properties and applications.
Understanding the Basics: Back-to-Back MOSFETs
Before diving into the specific configurations, let's establish a foundational understanding of back-to-back MOSFETs. Essentially, these pairs consist of two MOSFETs of opposite types: an N-channel MOSFET (NMOS) and a P-channel MOSFET (PMOS), connected such that the drain of one MOSFET is directly connected to the source of the other. This arrangement allows for the creation of more complex circuits with enhanced functionalities.
Common Source Configuration: Amplification and Switching
In the common source configuration, the source terminal of both MOSFETs is connected to a common point, typically ground. This configuration is widely used in amplifiers and switching applications. The common source arrangement provides a high input impedance and a low output impedance, ideal for amplifying signals while minimizing loading effects.
Advantages:
- High Input Impedance: The high input impedance of the common source configuration stems from the gate terminal of the NMOS being directly connected to the input. This prevents the circuit from being loaded by the input signal source.
- Low Output Impedance: The output impedance is determined by the drain-source resistance (RDS) of the PMOS. Since the PMOS is typically biased in the saturation region, its RDS is low, resulting in a low output impedance.
- Amplification: The configuration effectively amplifies the input signal due to the voltage gain provided by the NMOS and PMOS pair. The gain is typically high, especially in the saturation region.
- Switching: The common source configuration can also be used as a switch. By applying a high voltage to the gate terminal of the NMOS, the NMOS turns on, allowing current to flow through the circuit. Conversely, applying a low voltage to the gate terminal of the NMOS turns it off, blocking the current flow.
Applications:
- Linear Amplifiers: The high gain and low output impedance make the common source configuration suitable for linear amplifiers, where the amplification is proportional to the input signal.
- Switching Circuits: The ability to turn the MOSFETs on and off with a high voltage allows the use of this configuration in various switching applications, such as digital logic circuits.
- Voltage Buffers: The low output impedance makes the common source configuration ideal for voltage buffers, which provide a high-current output with minimal voltage drop.
Common Drain Configuration: Voltage Followers and Level Shifting
The common drain configuration, also known as a source follower, connects the drain terminal of both MOSFETs to a common point. This arrangement is primarily used for voltage following, level shifting, and impedance matching.
Advantages:
- High Input Impedance: Similar to the common source configuration, the common drain configuration boasts a high input impedance due to the gate terminal of the NMOS being connected to the input.
- Low Output Impedance: The output impedance is determined by the RDS of the NMOS, which is low in the saturation region. This characteristic allows the circuit to provide a low output impedance and drive loads effectively.
- Voltage Following: The output voltage closely follows the input voltage, resulting in a unity voltage gain.
- Level Shifting: The common drain configuration can be used to shift the voltage level of a signal, by adjusting the gate-source voltage of the NMOS.
Applications:
- Voltage Followers: The unity gain and low output impedance make the common drain configuration ideal for voltage followers, which can provide a high-current output with minimal voltage drop.
- Level Shifters: The ability to shift voltage levels is crucial in circuits requiring different voltage levels, such as interfacing between different voltage domains.
- Input Impedance Matching: The high input impedance makes the common drain configuration suitable for matching the input impedance of a circuit to the output impedance of a signal source.
Comparing the Configurations: A Summary
Here's a table summarizing the key characteristics and applications of both common source and common drain configurations:
| Feature | Common Source | Common Drain |
|---|---|---|
| Configuration | Source terminal common | Drain terminal common |
| Gain | High | Unity (close to 1) |
| Input Impedance | High | High |
| Output Impedance | Low | Low |
| Applications | Amplifiers, Switching circuits, Voltage buffers | Voltage followers, Level shifters, Impedance matching |
Selecting the Appropriate Configuration
The choice between a common source and common drain configuration depends on the specific circuit requirements. If amplification and high gain are desired, the common source configuration is suitable. If voltage following, level shifting, or low output impedance are the primary goals, the common drain configuration is preferred.
Conclusion
Back-to-back MOSFETs, particularly in the common source and common drain configurations, are fundamental building blocks in analog and digital circuits. Understanding their properties and applications is essential for designing efficient and robust circuits.