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MOSFET Circuit Design

Understanding Introduction to MOSFETs and Circuit Design

Designing effective circuits with MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) requires understanding their characteristics and how they interact with other components. This guide will simplify key considerations.

One crucial aspect is selecting an appropriate power supply voltage. Many MOSFETs have relatively low threshold voltages, which influences the design choices regarding the op-amp and other circuit elements.

Power Supply Optimization for MOSFET Circuits

When selecting your power supply, ensure it aligns with the MOSFET's threshold voltage (Vth). Many MOSFETs are compatible with 12V power supplies, as their threshold voltages (e.g., 2-4V) permit sufficient gate voltage application via an op-amp.

The LM358 op-amp, for example, can output a voltage up to approximately 2V below the supply voltage (e.g., ~10V with a 12V supply). This allows proper operation. Consider the 2N7000 MOSFET as a low-cost option, boasting a threshold voltage of 0.8-3V.

Alternatives Alternative Components: BJT Substitution

You can replace the MOSFET with a BJT (Bipolar Junction Transistor), such as the 2N3904. This necessitates a more critical consideration of the resistor (R4) value, although 100 ohms typically remains a suitable value.

Be mindful of base current (IB), which is typically negligible but contributes to emitter current (IE = IB + IC). In certain scenarios, there may be an error, potentially up to 10%.

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Understanding the relationship between power supply, MOSFET threshold voltages, and op-amp capabilities is key to successful circuit design.

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Resistor Usage and Dampening Oscillations

The resistor (R4) isn't actually always required when using a BJT. It's primarily used with MOSFETs to mitigate oscillations that could arise from capacitive loads.

Proper component selection, along with appropriate resistor values, will ensure a stable and efficiently functioning MOSFET circuit.