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!
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