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I am currently learning about voltage controlled current sources and small signal models. Current sources in general have always been a source of confusion for me (no pun intended).

I drew a circuit diagram below to illustrate my problem. In this circuit, what happens to resistor Re? I thought that since there are current sources on both sides of it the current would come in from both ends and cancel eachother out. Ultimately, no net current would flow through it, making it "open".

If VCCS1 is greater than VCCS2, would that make current flow from left to right in Re? is it possible for the current coming from VCCS1 to "override" and oppose the current coming from VCCS2 and make current travel up as opposed to down that right branch?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Are those ground pins floating or connected to Re? \$\endgroup\$ – PDuarte Feb 29 '16 at 21:53
  • \$\begingroup\$ The ground pins are floating. There were current sources there that I set to 0. \$\endgroup\$ – TheSockCollector Feb 29 '16 at 22:14
  • \$\begingroup\$ I have the same question and tried to simulate and reverse the current in a current source with more external circuit potential/current but no matter what I do, it won't reverse. \$\endgroup\$ – paulplusx Dec 13 '18 at 20:06
  • \$\begingroup\$ @TheSockCollector Also, you can't even force extra current through a current source. In case, there are two parallel current sources you should draw an equivalent source to represent the new added current. You cannot force a current source to reverse it's current or add to it, unless the sources are in parallel to form a new equivalent source. \$\endgroup\$ – paulplusx Dec 14 '18 at 5:40
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This circuit only works if the current through VCCS1 is minus the current through VCCS2. So the input has to be purely differential. In every other case there would be no path for the current.

In a real circuit Re would be split into two 25 ohm resistors with a large resistor (resulting from a non-ideal current source) going from the middle tap to ground. This would create a voltage that counters the common-mode component (-> differential pair).

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  • \$\begingroup\$ So if VCCS1 had a small signal input of vi+ and VCCS2 had a small signal input of vi-, would that make it valid? current would then flow from VCCS1 to VCCS2 and continue up? EDIT: I'm familiar with differential amplifiers, but wondered how current and voltage would be affected in a circuit like this \$\endgroup\$ – TheSockCollector Mar 1 '16 at 0:40
  • \$\begingroup\$ Yes, exactly. You have two current sources in series, they have to have the same current. It's like putting two ideal voltage sources in parallel. It's only valid if they have the same voltage. \$\endgroup\$ – Mario Mar 1 '16 at 11:05
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When you say "cancel each other" you're thinking as they were voltage sources. That's where you are confusing yourself. As they are current sources you must realize that both produce current, not voltage. Hence, that net must have the current as applied by the source, it doesn't matter what's on its way. So, in this case, the first source's value must be equal to minus the second source value, so current can flow, otherwise this model is invalid.

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Putting two different value ideal current sources in series is invalid. Only one current can flow in a series circuit. But since these current sources are ideal each one will produce its specified current. Now you have two different currents flowing in a series circuit, which is impossible.

what happens to resistor Re?

Re has 1A flowing through it. No wait, Re has -1A flowing through it! Re detects that it is logically impossible, and promptly disappears...

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