ssphysics

[ssphysics]

[ssphysics]

Ketchup on circuits!

To catch up on current affairs in the lab, I am currently working on troubleshooting a circuit I had built a year ago. This circuit is a voltage controlled current source.

"SSPhysics what is that?" 

A voltage controlled current source, is precisely what the name implies. With a VCCS we can send in a voltage and output a precise current. Many experiments require precise current to produce desired results and to achieve this we can only really apply a voltage easily, so thats where a VCCS comes into play.

In a VCCS there are several integrated chips called Operational Amplifiers (op-amps). These op-amps are what most circuits use to perform the task that is needed to be done by stringing together different configurations of op-amps. To achieve a VCCS we use 3 different configurations: an inverting amplifier, a summing amplifier, and a voltage follower.

The inverting amplifier is exactly how it sounds, the op-amp inverts the voltage and amplifies according the ratio of the resistances where: 

\[V_{\text{out}} = -\frac{R_{\text{2}}}{R_{\text{1}}} V_{\text{in}}\!\\]

This is referring to the first picture above. So we can tune the amplification of our voltage by adjusting the ratio of resistors, which is really easy to change.

The second configuration listed is the summing amplifier which takes several inputs, sums them, and then does another inverting amplification based upon the same equation, but remember that the sum of the input resistors are now in parallel.

The last but possibly most important is the voltage follower. This little opamp setup is meant match the impedance of two circuits to limit the load entering the second circuit. In our case we use a voltage follower in parallel to our sensing resistor (the resistor we are going to use to calculate our output current) to drive an op-amp reference pin.

The main point of the VCCS is to utilize Ohm's law and convert a voltage into a current. \(V = I\times R\). After we amplify, sum and adjust our voltage to our preferred value, we must pick a resistor value for the current we want to achieve and have that placed right before the output of the circuit.

One final thing to consider is if you want to control this via computer or DAQ board (I'll talk about what this is another time). If so, then adding an analog switch would help. With an analog switch we can send in a digital signal (1 or 0) and turn on or off a gate that will allow current to flow through. Its very hand to be able to stop the current when you are done with the measurement.

Credit to Northwestern for the diagrams!

[ssphysics]

WHERE WOULD YOU MOST LIKE TO VISIT ON YOUR PLANET?

Russia or southeast Asia

[ssphysics]

Adults follow paths. Children explore. Adults are content to walk the same way, hundreds of times, or thousands; perhaps it never occurs to adults to step off the paths, to creep beneath rhododendrons, to find the spaces between fences.

Neil Gaiman The Ocean at the End of the Lane 

[ssphysics]

Solid state Physics blog

I think making a blog about what I do on a daily basis as a solid state physics researcher would be a good thing. So as an introduction post I will talk a bit about myself, my research, and the best subject to study, Physics!

I am 24 year old graduate student studying nanoscale science and engineering, but technically the work and field that I am working in is solid state physics and quantum mechanics or more specifically Spintronics. I will get into what spintronics is exactly either in this post or a later one. I did my bachelors of science in Physics and got my master's degree in Applied Physics. The research I did for those two degrees was very different than what I am doing now. I was working on ceramics and doing electrical characterization of thermoelectric materials. I got into physics because of a friend that persuaded me that it was better than the current degrees I was pursuing of Math and Mechanical Engineering. Looking back I agree, those two subjects are a hell of a lot more boring.

My research now consists of mostly device physics, thin films, instrumentation, and quantum mechanics with a few other subjects sprinkled around. I am mainly interested in the area of spin Hall effect, STT-Ram and spin dependent scattering. I got into all these areas of research because I was able to attend and IEEE Magnetic Summer School. This school accepted 100 students and taunt them the fundamentals and advanced topics in Magnetism, spintronics being one of them. At the time I was performing Hall effect measurements (Hall effect is when a current is passed along a conductor / semiconductor

and a magnetic field is applied perpendicular to the velocity of the electrons and because of this a Lorentz force is created and the electrons are deflected orthogonal to the current and magnetic field) when I learned about the spin Hall effect and realized that there is a whole untouched degree of freedom in using electrons.

I guess this is starting to read like a bio for a dating website. I'll end here.