Task 2: Schematic Capture of NMOS I-V/C-V Test Bench
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
The section describes the steps required to effectively capture the schematic for NMOS testing, including the placement of components, wiring, and the necessary tests for analyzing I-V and C-V characteristics. Additionally, it emphasizes the importance of configuration and proper simulation setup to achieve accurate results.
Detailed
Task 2: Schematic Capture of NMOS I-V/C-V Test Bench
In this section, the objective is to navigate the Electronic Design Automation (EDA) environment to create a schematic for NMOS I-V and C-V test benches. The steps begin with placing an NMOS transistor from the technology library, configuring it with appropriate dimensions and necessary voltage sources for gate, drain, and bulk connections. Wiring the test bench involves creating dedicated nets for each connection and ensuring ground is properly connected. After wiring, a crucial
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Placing the NMOS Transistor
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- Place NMOS Transistor:
β Locate the "Instance" placement tool (often an icon or from menu: Create > Instance).
β Browse the technology library (e.g., gpdk_180nm if using Cadence) to find the NMOS transistor (often named nmos or nfet).
β Place a single NMOS transistor on the schematic canvas.
β Set Parameters: Select the NMOS transistor. Modify its properties (often by pressing 'Q' or double-clicking). Set typical dimensions:
β Width (W): e.g., 500n (500 nanometers) or 0.5u (0.5 micrometers)
β Length (L): e.g., 180n (180 nanometers) or 0.18u (0.18 micrometers)
β Number of fingers (if applicable): 1
β Multiplier (if applicable): 1
Detailed Explanation
In this step, we start by placing the NMOS transistor into the design environment. First, you need to find the placement tool in the Electronic Design Automation (EDA) software you're using. This tool allows you to add components to your schematic. After locating the NMOS transistor within a designated technology library, you place it on the schematic canvas. You then set the parameters for the transistor: its width (W) and length (L), which affect its electrical characteristics. The width commonly ranges from 500 nanometers upwards, while the length typically is set to values like 180 nanometers.
Examples & Analogies
Think of placing the NMOS transistor like putting a block in a LEGO set. The NMOS transistor is one type of block, and by selecting different sizes, you can change how that block fits into your designβaffecting how the entire structure will behave.
Placing Voltage Sources
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- Place Voltage Sources:
β Place three independent DC voltage sources (vdc from analogLib or equivalent) for VGS, VDS, and VBS (Bulk voltage for NMOS).
β Place a Ground symbol (gnd from analogLib).
Detailed Explanation
In this step, you add three voltage sources to the schematic. Each voltage source serves a specific purpose: VGS (Gate-Source Voltage), VDS (Drain-Source Voltage), and VBS (Bulk-Source Voltage). These voltages are essential for controlling the NMOS transistor's operation. Additionally, you need to add a Ground symbol to ensure proper reference for the circuit layout. Ground acts as the common return point for electric current within the circuit.
Examples & Analogies
Placing voltage sources is akin to setting up the batteries in a toy circuit. Just like batteries supply power to your toy, these voltage sources provide the necessary electrical energy for the NMOS transistor to operate.
Wiring the Test Bench
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- Wire the Test Bench:
β Connect the Source terminal of the NMOS to a dedicated wire/net (e.g., "src").
β Connect the Drain terminal of the NMOS to a dedicated wire/net (e.g., "drn").
β Connect the Gate terminal of the NMOS to a dedicated wire/net (e.g., "gate").
β Connect the Bulk terminal of the NMOS directly to the Ground (gnd) symbol.
β Connect the positive terminal of one vdc to "gate" and its negative terminal to "src" (this is your VGS source).
β Connect the positive terminal of another vdc to "drn" and its negative terminal to "src" (this is your VDS source).
β Connect the third vdc to define the bulk voltage, connecting its positive terminal to "src" and negative to "bulk" (if not direct ground connection). For this lab, ensure bulk is at ground reference.
β Connect the gnd symbols from all voltage sources to a common ground net.
β Place output current probes if available (e.g., i_probe to measure ID from drain).
Detailed Explanation
Once you have placed the NMOS transistor and voltage sources, the next step is to make all the necessary connections to ensure that the circuit works correctly. Each terminal (Source, Drain, Gate, and Bulk) must be connected to specific parts of the circuit. This involves creating 'nets' or dedicated lines for each connection. The wiring enables the voltages to control the NMOS transistor properly, which influences its behavior in the circuit. Additionally, placing current probes allows measurement of the current flowing through the transistor, which is useful for analysis.
Examples & Analogies
Wiring the test bench is like organizing the plumbing in a house. Each pipe (wire) has to connect to specific points (terminals) to ensure water flows (current) correctly throughout the system and reaches all the necessary faucets (outputs).
Checking and Saving the Schematic
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- Check and Save:
β Perform a "Check and Save" (often a toolbar button or Design > Check and Save) to ensure your schematic is electrically valid. Resolve any warnings or errors.
Detailed Explanation
After completing the schematic wiring, it's crucial to verify that everything is connected correctly. The 'Check and Save' function in your EDA tool will examine the schematic for any errors or warningsβlike disconnected components or incorrect configurations. Resolving these issues is vital to ensure the simulation runs smoothly and provides accurate results.
Examples & Analogies
Performing a 'Check and Save' is like proofreading an essay before submitting it. You want to catch any mistakes or unclear sentences (errors in connections) to avoid problems during grading (simulation).