Hello all. I just wanted to make a forum where we can exchange design ideas from basic to more advanced techniques in Analog integrated circuit. While i realize we do not have IC design companies e.g INTEL, AMD etc in Nigeria, this does not stop us from sharing knowledge and becoming guru's. Circuit design is a very interesting art. It is very synonymous to the game of LEGO. It is very important that one understands basic building blocks to form more advanced circuits. With that said, to become a guru also requires extensive knowledge and experience, like every other field. So this forum is for Electrical Engineers who are interested in circuit design (Analog) and people who want to pick it up assuming you have the fundamentals. So if you have any design projects/questions or ideas related to Analog IC design, this is a good forum for us to share knowledge and become masters of our trade!

Between, i can direct you to where you can get free spice simulator to simulate your circuit e.g Linear Technology spice simulator. I can also provide anyone with circuit models for your circuit simulation is TSMC 180nm technology node (MOSFET devices). So you can simulate your circuits and verify its behavior. All you need is a PC or MAC (with crossover PC emulator) to download the free LTspice simulator.

Download link for LTspice circuit simulator

http://tech.groups.yahoo.com/group/LTspice/

Just click on "download now" first line under title "Description", its pretty much self explanatory, but if anyone runs into problems, just let me know.

Wideband Transimpedance Amplifier Design
Introduction
Fiber-optic communication is one of the most common methods of high speed information transfer largely due to the extensive bandwidth available at optical frequencies.
In fiber-optic transmitters, the output of a light source, often a laser, is turned on/off to represent 0/1 bits of a digital stream of data. Encoding the digital data by changing the intensity of light belongs to a special class of modulations referred to as Amplitude Shift Keying (ASK).
In fiber-optic communication, the frequency of laser is much higher than the data rate (x 104-105). Therefore, there will be many more cycles of the sinusoid for each bit compared to what is shown here.time Output of a continuous laser Digital data stream 1 1 1 1 0 0 0 1 10 0 1 00 0 Modulated data stream (transmitted waveform) Photodiode
Optical fibers are the medium in which the light modulated data streams are transferred1. In fiber-optic receivers, a photo-detector is used to extract the information from the modulated light. A photodiode is a p-n junction that is sensitive to light. The light produces electron-hole pairs in the depletion region of the reverse biased photodiode, causing a current, proportional to the intensity of light, to flow in the external circuit . The simplified electrical model of a photodiode is given is figure 1, where current source Ii models the data sequence extracted by the photodiode and capacitor Ci is the reversed bias junction capacitance of the photodiode.
The first electrical block in fiber-optic receivers is a wideband transimpedance amplifier (TIA) that converts the photodiode generated current, containing the data stream, into voltage with a large gain . The TIA is followed by data recovery circuitry to extract the digital data from the bit stream.
In that sense, optical fibers play the same role in fiber-optic communication as twisted pairs do in phone communications or air does in wireless communication.

Please Refer to attached file for block diagram of project.


PROJECT DEFINITION
In this project you are to design a wideband transimpedance amplifier in a 0.25m CMOS process technology for fiber-optic standard SONET OC48 with a data-rate of about 2.488-Gbit/s.
Design Specifications and Constraints
- Power Supply: Vdd=2.5V
- Photodiode Capacitance: Ci=0.5pF
- Transimpedance > 1kΩ
- Bandwidth (ω-3dB): from DC to 2.5GHz
- Total Equivalent Input Noise Current (from DC to 2.5GHz) < 1μA
- Total Power Dissipation < 50mW
- Group Delay Flatness < ±10% (from DC to 2.5GHz)
- Output load: RL=50Ω
- The output could be either single-ended or differential. In the latter case, the transimpedance gain should be satisfied for single-ended output.
- The output could be either DC or AC coupled to RL=50Ω.

PROCESS PARAMETERS
Transistors
The process technology used for this design is a 0.25m CMOS process. The HSPICE model files for the NMOS and PMOS transistors are available upon request.
Capacitors
You are allowed to use a maximum total of 20pF of capacitance in your design. You may use ideal capacitors in your design.
Other Passives (Inductors, Transmission Lines, etc.)
You are strongly encouraged NOT to use any inductors, transmission lines, etc.

Hello People, the above is a good project for a final year EE student in college! Anyone can also attempt it if you just want a fun design project. The time limit is 2 weeks!

Attached is the 0.25um transistor models. Do not modify this text file. All you need is to include this file in ur simulation and make sure you use the transistor model NAME
CMOSN : for NMOS and CMOSP : for PMOS.

I really hope someone has the courage to take on this design problem, like i said a FINAL year EE student can use this as their final year thesis (Off-course you have to be able to analyze the chosen circuit topology thoroughly in terms of theory and equations). This project is really fun and it can give you a complete experience in problems encountered at different stages of circuit design.

Below is the way i approach a design problem which in my opinion is impeccable, its just a RECIPE, not absolute:

1.) Read Project specification carefully: This enables you to decide what topology to use to solve your problem. This will force you to do research on different circuit topologies available and the design trade-offs associated. NOTE: You might not be able to find a perfect topology at first, but pick one that is close and i promise you, there will be circuit techniques to improve the other specs that needs to be met.

2.) Circuit Analysis : I cannot stress how important this step is. After you have decided what circuit topology to use, you need to be able to derive design equations that govern the specifications mentioned. Example: Lets say we have a power requirement 2mW and the available power supply is 2v. Simple I=P/V tells us that the TOTAL current that can be drawn by your design is 1mA. After you have derived all your design equations, if you are a seasoned designer, you can always use matlab and or excel to optimize these equations using design goal values etc. This is not necessary, but can save time. Remember: Less time spent on design means lower product cost thus more profit margin that can be transfered to your company and your boss will be well pleased with u !

3.) Circuit design: After getting all your transistor, Resistor and capacitor values, you have to enter the schematic. You obviously have to know what bias voltages and currents needed for different stages of your circuit. As a first design iteration, i usually just use IDEAL current sources or voltage sources to bias my transistors and replace them with built circuits at the end.

3.) SImulation: Now after step 2 above, you would expect that once you simulate, you should get all the expected results right WRONG! more than likely not.While deriving your design equations, some circuit assumptions must be made to simplify the equation so we can have an intuition to the first order of our circuit behavior. So what you need to do is check your circuit assumptions and make sure it holds in your simulation. So if these assumptions wont affect your circuit in a negative way, change the variable e.g W or L of transistor that affects these circuit to make the assumption holds or you take into consideration the effect of that assumption in your derivation.
This is not easy and requires some iterations, but that's why we are engineers, if you know what you are doing, you will get the sh*t done!

VOILA! u got your circuit , Now you can make your layout etc .

Attached are some references, that might be helpful, i won't post anything on this anymore cause it seems there is no interest. So if u are interested in the design question or u have any design question, u can post!

sorry, attached is a reference.

files are too large to attach, so contact me if u need the references!

Hi EngineerD - are you interested in working on an an open source IC Design project?
please email me - kassemmkk@gmail.com