Does collector resistor can limit collector current - Page 1
Forum section: electronics / beginners

author: khatus
author_rank: Regular Contributor
timestamp: January 03, 2024, 03:57:26 pm
content: As far as I know, the collector current depends on the base current by the equation IC = β × IB. However, the voltage drop across the collector resistor depends on the collector current. So how did the collector resistor limit the collector current?

author: wasedadoc
author_rank: Super Contributor
timestamp: January 03, 2024, 04:06:57 pm
content: Emitter follower. If the emitter resistor is shorted and the emitter goes to zero volts then the current that flows through the collector cannot exceed the supply voltage divided by the collector resistor. Ohm's Law, nothing to do with the base current and current gain of the transistor.

author: TimFox
author_rank: Super Contributor
timestamp: January 03, 2024, 05:51:12 pm
content: It’s important to remember that your equation for collector current proportional to base current is only useful when the collector-base junction is reverse-biased, i.e, the collector-emitter voltage is substantial (above saturation).

author: khatus
author_rank: Regular Contributor
timestamp: January 04, 2024, 08:11:15 am
content: What do you mean by saturation?Please explain this i am newbie

author: Gyro
author_rank: Super Contributor
timestamp: January 04, 2024, 10:16:13 am
content: Saturation is where the transistor is fully turned on - as hard as it can be. If you look in a transistor datasheet you will find the saturation voltage (VCE(sat)) listed for various collector currents. Most datasheets specify saturation when base current is 1/10 of collector current, but with modern high gain transistors and types specifically designed as saturated switches IBmight be specified as 1/20 or 1/30 of Ic.Saturated switching isn't usually used in high frequency applications because the transistor is slower to come out of saturation than its FTindicates (this is for non-saturated operation).

author: dmills
author_rank: Super Contributor
timestamp: January 04, 2024, 12:31:52 pm
content: Also note that beta is not to be relied upon, it has a huge spread, it drops precipitously as the device approaches saturation, and also drops at large collector currents (so called beta droop), there is a reason switching at modest currents is usually a Mosfet thing these days, they lack this behaviour (But have a few other traps).That saturation at a base current of 10 percent of the collector current implies a beta of just 10 at that point down from maybe a few hundred in the linear region, so if you want to switch an amp, you are wanting 100mA of base current, which usually means you need a driver transistor or Darlington or such, annoying.The advantage of the BJT is that in its linear region they have far higher transconductance then mosfets.

author: wasedadoc
author_rank: Super Contributor
timestamp: January 04, 2024, 12:52:36 pm
content: The collector current calculated by multiplying the base current by the transistor beta can never exceed the current calculated by applying Ohm's Law to the collector resistor and the supply voltage.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 04, 2024, 02:12:39 pm
content: The beta approximation is useful on seeing what is happening. You need to think of three things. Base current, collector current and collector voltage. Then imagine plotting these quantities as you increase base current. At zero, both currents will be zero and the collector voltage will equal the supply. As base current increases, collector current will follow the beta multiplication rule and collector voltage will equal the supply minus the ohms law drop across the collector resistor. At some point that ohms law drop will equal the supply voltage and collector voltage is zero. This is the point where the transistor is saturated. Further increases in base current will no longer result in more collector current.Once you understand this simple approximation you need to go back and look at some of the other things mentioned earlier. Actually doing the experiment with a few transistors with different collector resistors and perhaps a small emitter resistor will give you more insight on where the simple model is and isn't useful.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 04, 2024, 02:30:41 pm
content: It is indeed just Ohm's law. If you use a 1Meg resistor, then the collector will never have more then a few uA and the goal is to dimension it in such a way that it acts as a current limit without interfering with the transistor.

author: dmills
author_rank: Super Contributor
timestamp: January 04, 2024, 02:48:00 pm
content: Might want to add Vbe to the list of parameters there, most transistor design at least outside the saturation region is better done by considering Vbe as the driven thing and the base current as a side effect.Ic=Is * (e^Vbe/Vt -1) and all that (And if you think that looks a LOT like a diode you are not wrong (Id = Is * e(Vd/Vt -1), much the same thing with different value for Is).Or even just treating the thing as a transconductance gm = Ic/Vt.