Friday, 17 March 2017

VLSI - 3
So continuing on the previous post of two current sources in series, this example is important when you analyze lot of circuits.

Lets assume you simulate with ideal current sources

Case 1 : When top current source > lower current source

Since, they are ideal current sources, it happens that they will sink or source that much current ir respective of the voltage across the sources, so it implies that currents may be unequal in single wire and the node potential in between increases.

The reason it increases is that top current source pumps in current but the down source only sinks lesser current, hence forth the rest of the current will charge up the cap at the node and hence forth increases the node potential.

Case 2 : When top current source < lower current source

Since, they are ideal current sources, it happens that they will sink or source that much current ir respective of the voltage across the sources, so it implies that currents may be unequal in single wire and the node potential in between decreases.

The reason it increases is that top current source pumps in lesser current but the down source only sinks higher current, hence forth the rest of the current will be provided by the cap at the node and hence forth decreases the node potential.

Interested studentts should find time in simulating the circuit by replacing the current sources with NMOS and PMOS and understand what will happen.

This is an important step in understanding CMFB.


Wednesday, 1 March 2017

VLSI - 2 ( Analog)
For those of you who wanted to make a career in analog, be a devotee of IIT Madras faculty irrespective of which IIT or which college you end up joining for your Mtech.

1. Shanti Pavan - IIT Madras videos
2. Nagendra Krishnapura - IIT Madras videos
3. Razavi - Videos and Text book
4. Anirudhan  - IIT Madras videos

I would like to discuss few common blunders, that we end up doing, which henceforth make us loose confidence in analog


Mistake 1 :  Two current sources in series

In simulation if you put two current sources in series and simulate this, you observe some crazy things happen, kindly do this and learn on this.



Put up a current source with NMOS sinking the current with very low Vgs and observe

Repeat the experiment with PMOS too. Try increasing the sizing for the same Vgs and observe what happens.

This experiments are really the most powerful in analog. The deeper one digs into this, the faster he understands the analog concepts well.

Spend great time dealing with this circuit.

Feel free to discuss if any doubts.......

Will come back with the next post. and discuss the solution and concepts here.

Tuesday, 28 February 2017

VLSI - 1


Without doubt, VLSI is top branch of ECE which fascinates many graduates and also a hot cake, henceforth making it a very competitive branch to get into even in your Mtech.

Roughly one can divide VLSI into 3 specializations:

Analog VLSI
Digital VLSI
Device Electronics




Analog VLSI  --- is something that really fascinates many VLSI students but often, doesn't become a career option for many of those bright students. It is much more than the coding stuff. Analog needs your thinking cap to be on always. It needs understanding of device physics too to a reasonable extent.

Any chip that gets taped out has all blocks of analog, digital. Band gap ckts, Reference current sources, IO components, PLLs, Amplifiers, filters, transmitters, receivers and many blocks are all analog in nature.

Most of the digital stuff are coded and even the interface circuits that is A/D convertors, sensors, and D/A convertors are all analog blocks !!!

The real world is analog in nature and any advancements we talk about DSP, can be appreciated only if u have proper interface circuits to convert the analog signal to digital and do DSP and convert it back to analog again.