Before creating a layout for your nonlinear circuits, you should run a load line analysis so it doesn't end up like this
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. Linear systems are rather intuitive, even in the case where strong feedback becomes an important determinant of stability. 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.
Transistors and amplifiers are fundamental tools in the analog circuit designer’s toolbox. 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. You can then apply these same techniques in more complex circuits.
Why Load Line Analysis for PCB Design?
Every PCB begins its life as a schematic, and the circuits in your PCB must be properly defined at the schematic level. Building and simulating your circuits is a critical aspect of ensuring your board will function as you intended. Once you layout your board, your schematic-level simulations give you a comparative baseline for post-layout simulations. They also inform how your circuits should work once you test your prototype. 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.
Load line analysis is one such test for any circuit that involves transistors. This can include COTS amplifiers for different applications, custom amplifiers that you are designing yourself, or any other circuit that requires an amplifying driver. 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. More generally, diodes themselves and other nonlinear components are also candidates for load line analysis.
The goal in load line analysis is to determine the operating point in the circuit. 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. 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. This is particularly important when working with pure AC and modulated signals. Note that some prepackaged amplifier SoCs 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.
The image below shows the load line for an NPN transistor and what happens when an input AC signal oscillates about different bias levels. 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 “sweet spot” is called the Q-point and is taken at the center of the DC load line.
Load line analysis and clipping behavior in a transistor
Running a Load Line Analysis in Altium Designer
Getting started with load line analysis requires looking at a schematic for a nonlinear circuit. To show readers an example of how this is set up, I’ve created a schematic in Altium Designer® for an NPN transistor circuit, as shown in the image below. In this circuit, I’ve just used components from the Miscellaneous Devices.IntLib library and the Simulation Sources.IntLib library for simplicity. However, if you were designing a sophisticated signal chain or you were using specialty components, you could import these components and simulation models directly into Altium Designer (e.g., from the Manufacturer Part Search panel) and use them in your circuit.
Schematic used for this load line analysis simulation
In this schematic, I’ve placed some probes (differential voltage probes across the collector and emitter, and a current probe for the load current) around the schematic. Load line analysis requires generating several output curves (collector current vs. collector-emitter voltage) for different values of base bias voltage (V_BB).
To setup the simulation we need, just go to the Simulation menu and create a new simulation profile (I’ve named mine “Mixed Sim”). This is where you can define the DC sweep parameters you need to conduct a load line analysis. Here, you need to sweep the collector-emitter voltage up to a desired V_CC value by sweeping the base bias voltage (labeled V_BB above). Note that you can do this for various V_CC values as well. The parameters I’ve used are shown below:
Load line analysis requires running a DC sweep
Note that it is important to select the I_C probe in the simulation setup in order to create a V_CE vs. I_C curve for each V_BB value in a single plot. This will give you a set of output curves that is typical for an NPN transistor.
Once this is all set up, go to the Simulation menu and run your simulation, or you can press F9 on your keyboard. The image below shows output curves as a function of the collector-emitter voltage and for various values of the base bias voltage (1 V to 15 V). I’ve drawn the load line manually using the value for the desired V_CC value of 5 V. The green circle shows the typical midpoint (V_CE = 2.5 V, 2.25 mA collector current). In this result, you can see that the circuit can easily accommodate a reasonably large linear range. Note that, if you play around with the values of the resistors with a parameter sweep, you can get a much larger value for the linear range.
Load line for the circuit shown above and the Q-point (in green)
The great part about working in a unified design environment is that these load line analysis features and much more are available in a single program. Altium Designer includes a range of powerful simulation and analysis tools that you can access directly from your schematic. You won’t have to bring your schematics into a separate program to run these important analyses. Altium Designer also gives you access to a complete set of post-layout simulation tools for signal integrity analysis.
About the AuthorMore Content by Zachariah Peterson