Intermediate Project Assignment: OTA simulation + Specifications
For your project progress meeting with the TAs in the week of Nov 28, 2022 , you are asked to prepare the following:
- Simulation and analysis of a pre-sized OTA;
- Table with the specifications of the OTA for your project design.
The transistors are laid out as fingered devices with m fingers with a size of Wf/Lf = 4um/0.5um; the total W/L of the transistor is then m*Wf/Lf.
Use the tsmc025.scs model file to analyze this amplifier when used in a unity-buffer configuration.
Analyze this amplifier
- DC operating point:
○ Based on the sizing of the devices and their currents, estimate the DC node voltages.
○ Run a .OP simulation and verify the DC node voltages and DC operating points of the transistors; note down the small signal parameters of the transistors.
- DC Transfer Characteristics:
○ Estimate the input and output voltage ranges.
○ Open loop: Simulate the open-loop DC transfer characteristic.
■ Verify if you get the correct gain. Also check what the output voltage is for a zero input.
○ Closed loop: Simulate the closed-loop DC transfer characteristic.
■ Verify if you get the correct gain and what the accuracy is both for positive and negative inputs. Make sure to also check what the output voltage is for a zero input.
- Build an equivalent, linear, AC model of this amplifier using voltage-controlled current sources, resistors and capacitors so that the response of the model matches the response of the transistor-level amplifier that you will simulate next.
- Open-loop gain and loop gain:
○ First calculate the DC gain, first pole, unity-gain frequency, second pole, phase margin, etc. using the simulated small signal parameters.
○ Then, run a .AC simulation (with the appropriate circuit configuration) to find the open-loop gain and loop gain and verify your calculations.
○ To determine the location of the poles and zeros it can be instructive to simulate the design with an Rc=0 and the specified Rc=720.
- Closed-loop gain
○ Calculate the closed-loop DC gain and bandwidth
○ Run a .AC simulation (with the appropriate circuit configuration) to find the closed-loop transfer function and verify your calculations.
- Step Response for the closed-loop configuration
○ Based on the system parameters, estimate the step response characteristics.
○ Run a .TRAN simulation with a small step input of 10mV. Does the step response behave as expected?
○ Increase the step input gradually in steps of 10mV. Does the step response change linearly with the input step amplitude?
○ Repeat for negative step inputs.
- Pulse Train Response for the closed-loop configuration
○ Run a .TRAN simulation with a small pulse train input at 10kHz with a positive magnitude of 10mV and a negative magnitude of -10mV.
○ Increase the step input gradually in steps of 10mV. Does the pulse-train response change linearly with the input step amplitude?
○ Increase the step input frequency gradually by 10 times. What’s the highest frequency that the circuits can handle?
- You do not need to write an extensive report on these simulations, but be ready to do a 5-min summary presentation on your results. The point of this assignment is to get you started with simulations so that we can find out challenges early and you can discuss them with the TA. Maybe bring a couple of key results, or if you have problems,point out where you got stuck and then we will consider setting up a recitation on those topics.
- Consider setting up a spreadsheet to do your calculations since you likely will have to do them a couple of times for different OTA sizes as you progress in your project.
- Put some thought into your simulation test benches so that you can run simulations efficiently. You will be able to reuse these test benches for your own design.
- Consider putting the OTA inside a symbol, then you can use it in different test benches in case you need different benches for different simulations.
- We deliberately sized this amplifier quite differently than the amplifiers you will need for your project specifications, including using a different load and closed-loop gain.
- You can reuse the topology or switch to a nMOS input pair and pMOS second stage if you think that gives better performance. You can also explore other topologies in your project if you like.
Specifications for your OTA
- Derive the specifications for the OTA you need for your project assignment. See below of a table with some suggested specifications. Feel free to add more if needed. Make sure you can justify your choices.
- Run a simulation using an equivalent model of your amplifier and make sure that the amplifier meets the overall specifications. Note that you can focus on the key specifications and don’t have to run an exhaustive characterization using the model.
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