B What is the utility of grounding in this Transistor case?

[Wrichik Basu]

I am attending an online lecture course on semiconductor physics. While explaining the common emitter mode of transistors, the professor sketched this diagram on the board: (I added something more to explain better)

I understand that the emitter has to be at the same potential, and that is why its "common". but what is the utility of grounding the emitter and the negative terminals of the battery? Can't this circuit be constructed without the grounding? Why is the grounding necessary?

 

[phinds]

How would you have any return loop for anything with no ground? That is, how would you even have a complete circuit?

 

[Wrichik Basu]

How would you have any return loop for anything with no ground? That is, how would you even have a complete circuit?

The left circuit is complete by itself, as the current $I_B$ enters the circuit and again reaches the battery. The right circuit is also complete, as the current $I_C$ leaves and again enters the battery. Junction law of Kirchhoff says that $I_E = I_B + I_C$. So, why is grounding at all necessary?

 

[phinds]

If you mean why does the entire lower line show as grounded, that is just a convention. It doesn't mean "Earth ground", just a common return path. It is identical to NOT showing it grounded. As I said, it's just a convention.

 

[Wrichik Basu]

If you mean why does the entire lower line show as grounded, that is just a convention. It doesn't mean "Earth ground", just a common return path. It is identical to NOT showing it grounded. As I said, it's just a convention.

So I can simply skip that grounding symbol, right? I was confused in associating it with grounding as in case of earthing.

 

[cnh1995]

So I can simply skip that grounding symbol, right? I was confused in associating it with grounding as in case of earthing.

Yes you can skip it. Ground is just a reference point in a circuit and is not necessarily an Earth connection.

 

[phinds]

So I can simply skip that grounding symbol, right? I was confused in associating it with grounding as in case of earthing.

In practical terms, yes. In conventional notation, you should leave it in.

 

[litup]

The left circuit is complete by itself, as the current $I_B$ enters the circuit and again reaches the battery. The right circuit is also complete, as the current $I_C$ leaves and again enters the battery. Junction law of Kirchhoff says that $I_E = I_B + I_C$. So, why is grounding at all necessary?

It isn't needed at all unless you are planning to use that circuit in junction with more circuitry. There are cases where you DON"T want ground, where you want isolation from ground. Remember transistor radios or cell phones, there is no ground to the outside world, internal circuitry are all referenced together to some common point they call ground but the entire system is ungrounded and they work just fine.

So the ground would be for other circuits hooked to that transistor, inputs and outputs of amplification or oscillation for instance.

One reason for a ground though is to bleed off static electric charges but as long as a circuit is well insulated, static electricity would have to go to very high voltages to effect such circuits if they were insulated against ground, 20,000 volts could perhaps penetrate such insulation and it is not that difficult to generate that much voltage from say walking across a nylon rug picking up charges on a day with low humidity.

 

[CWatters]

+1

Sometimes people get lazy when explaining how a circuit works and they throw in an Earth symbol to indicate where any voltages or waveforms are referenced to.

Something to watch out for is that some transistors have their case connected to either the collector or emitter inside. So when using a heatsink there is a risk of accidentally connecting one to Earth via the heatsink and the case of the equipment. Insulating washers may have to be used to mount the transistor to the heatsink.