The “sweet spot” is called the Q-point and is taken at the center of the DC load line. You should be able to see that, when the signal oscillates about a bias corresponding to nearly IC = 0 or VCE = 0, the signal will become distorted due to saturation and cut-off. The image below shows the load line for an NPN transistor and what happens when an input AC signal oscillates about different bias levels.
#ALTIUM MISCELLANEOUS DEVICES INTLIB DRIVER#
Note that some prepackaged amplifiers for use with frequency modulated signals will already apply some bias to the driver and will quote the linear input range in terms of the peak-to-peak voltage for the input signal. This is particularly important when working with pure AC and modulated signals. When your circuit includes a transistor, you want your circuit to provide some gain, but you also need to know the linear range of input values such that saturation and cutoff are avoided.
In general, the load line tells you the range of input voltages within which there will be no cutoff or saturation in the nonlinear circuit while still providing an appreciable output. The goal in load line analysis is to determine the operating point in the circuit. More generally, diodes themselves and other nonlinear components are also candidates for load line analysis. A great example is a pulsed laser diode driver circuit for lidar, which typically uses a GaAs or GaN FET driver to amplify a PWM signal and produce high power output from a laser diode. This can include COTS amplifiers for different applications, custom amplifiers that you are designing yourself, or any other circuit that requires an amplifying driver. Load line analysis is one such test for any circuit that involves transistors. This comparison between theoretical predictions (i.e., circuit and post-layout simulations) and experimental measurements (i.e., from a test coupon) is the best way to qualify functionality before your first prototyping run, and it’s the best way to dictate redesigns should your test results not meet your standards. They also inform how your circuits should work once you test your prototype. Once you layout your board, your schematic-level simulations give you a comparative baseline for post-layout simulations. Building and simulating your circuits is a critical aspect of ensuring your board will function as you intended. Why Load Line Analysis for PCB Design?Įvery PCB begins its life as a schematic, and the circuits in your PCB must be properly defined at the schematic level. You can then apply these same techniques in more complex circuits. Let’s take a look at how you can use the MixedSim features in Altium Designer to perform a load line analysis for a transistor. Transistors and amplifiers are fundamental tools in the analog circuit designer’s toolbox. With nonlinear circuits, this can get more complicated, and it’s not always easy to see how the system operates unless you have some experience with similar systems. Linear systems are rather intuitive, even in the case where strong feedback becomes an important determinant of stability. If you’re in the business of analog circuit design, then you’ll likely need to run simulations of your system to determine its functionality.
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