# Dependent voltage/current sources

I'm so sorry for that questions, but we need to understand what's going on behind the concept/theoretical of using device "dependent voltage/current source"!

My question: Yes, we are using dependent voltage/current source, but is there a device like this in real life? If so, let's assume that at branch k there's voltage k, my dependent voltage source is at branch k + 5 in the circuit, so how does my dependent voltage source knows the voltage at branch k? theoretically we say just assigning it once we find it .. but how it's going behind?!

• We can treat the BJT's or a MOSFET' as a VCCS. Or we can use an opamp to create any you like. – G36 Jun 10 at 20:29
• Look up Howland Current source. Notice the input voltage can regulate the current as long as the load resistance does not result in a voltage exceeding the supply rails. Due to high open loop voltage gain and feedback, it works over a very wide range. – Sunnyskyguy EE75 Jun 10 at 20:55
• The real power of dependent sources isn't that you can make them (you can), but that you can use them in small-signal circuit analysis in a way that makes all sorts of circuit elements (like transistors) easier to work with. – TimWescott Jun 10 at 21:18
• Typically dependent voltage/current sources in real life are probably things that contain various components. So if you have an dependent voltage source, for an example, of $5V_{in}$, it means that you have to design a circuit that will give an output that is five times large than your input voltage. – KingDuken Jun 10 at 21:21

A voltage/current depending source in real life needs to be built from actual electronic components. Usually made from operational amplifiers or transistors. They vary a bit from the ideal voltage/current sources in that they:

• Don't have infinite voltage/current and have finite power
• Usually have small offsets
• Have source impedance
• Have bandwidth or slew rate (can't source/sink current infinitely fast)

Here are some ways these circuits can be built in the real world:

B and C are voltage dependent current sources

An op amp in a voltage follower configuration is a voltage controlled voltage source.

• Low current opamps such as this are a great example of what can be designed. Note here that all the examples are voltage controlled current sources ….not just B and C. It's also worth noting that you can only implement this type of CC source where you have a single opamp in the package since you are using the +/- supplies as in circuit points. – Jack Creasey Jun 10 at 21:36

They exist but are circuits unto themselves so you abstract them away as singular components during circuit analysis so you don't get overwhelmed. Being circuits, they have to hook into other parts of the circuit you are dependent on to monitor things which isn't shown so you don't get overwhelmed.

It's no different from how you might draw an op-amp symbol, a current source symbol, or a voltage source symbol in your circuit when in reality that symbol represents an entire circuit (which might be more complicated than the circuit you just placed the symbol in).

Dependent sources are very useful for modeling behaviors of real life circuits.

In particular, you will see a lot of them in small signal analysis. Small signal analysis is a way of modeling non-linear devices (like transistors) which you only intend to operate in a mostly linear region (like when making an amplifier). When you look at transistors in this setting, they start to look like voltage controlled current sources. If you are prepared to think about dependent current sources like that, you can model the behavior of your circuit without too much trouble. If you try to work through the analysis without such simplifications, you find yourself trying to solve complicated differential equations just to get some traction.