Does collector resistor can limit collector current

[khatus]

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?

 

[D.A.(Tony)Stewart]

Limited by Ohm's Law. Yet when saturated hFE is only 10% of rated. so Ic then follows Vbe limited by Vcc/Rc.

 

[KlausST]

Hi,

Ohm´s law apply in either case.

So I_max = V_max / R

Example: If the supply voltage is 12V and the resistor is 6 Ohms ... no current bigger than 2A can flow.

Klaus

 

[D.A.(Tony)Stewart]

Limited by Ohm's Law. by Vcc/Rc. and or(Ic/Ib < 20 limited

and if Rc is very small < 20x Rb, pulled up to Vcc limited by bulk rCE which is determined by device specs for max power rCE ~ 0.5/Pmax [W] +/-50% and limited by max Temp rise from Vce*Ic=Pd

 

[khatus]

Hi,

Ohm´s law apply in either case.

So I_max = V_max / R

Example: If the supply voltage is 12V and the resistor is 6 Ohms ... no current bigger than 2A can flow.

Klaus

So, IC = β × IB this formula only works in the range 0 to 2A(max current set by collector resistor)

 

[D.A.(Tony)Stewart]

Browser simulation http://tinyurl.com/ymz56cuk

--- Updated Jan 3, 2024 ---

So, IC = β × IB this formula only works in the range 0 to 2A(max current set by collector resistor)

No . That only works for all transistors for Vce > 2V (most linear operation) then depending on current hFE drops to 10% of hFE ast Vce= Vce(sat) with Ic/Ib=10 when hFE max < 200. ( as long as you have unbinned parts where hFE varies over 2:1 range.)

For superbeta types 200 to > 1000 , Ic/Ib= 20 to 50 when saturated.

Here I simulated 12V with 2 Vpp noise and when R load = 100 Ohms @ 1/4W this linear regulator is only 34% efficient and Vout drops to 4.9V or -2% error.

--- Updated Jan 3, 2024 ---

You can use LED's as Zeners too ! 2.0V for R/Y and 3.0V for W/B at 15 mA and 2+3=5

--- Updated Jan 3, 2024 ---

Any questions?

 

[betwixt]

So, IC = β × IB this formula only works in the range 0 to 2A(max current set by collector resistor)

Yes, IC is not produced by beta x IB, it is controlled by it. You cannot increase IC beyond what Ohms Law and the collector resistor will allow.

Brian.

 

[D.A.(Tony)Stewart]

Using a current limiter (CC source) instead of a Zener bias resistor you can attenuate Vcc noise 70 dB (PSRR)

The RC filter will attenuate 6 dB / per octave or 20 dB/dec. but not DC and a CC (2 PNPs + 2 R's) will attenuate both DC & AC input variation.

But there will still be thermal drift from Vz and Vbe.

 

[khatus]

Limited by Ohm's Law. Yet when saturated hFE is only 10% of rated. so Ic then follows Vbe limited by Vcc/Rc.

Hello, i can't understand the meaning of the line

Yet when saturated hFE is only 10% of rated. so Ic then follows Vbe limited by Vcc/Rc.

Can you elaborate it.I am newbie

 

[D.A.(Tony)Stewart]

You may recognize this.

There is an excellent online Falstad simulator link below which follows this aequation above which does not model saturation. So I added diodes to resemble the effects of saturation. Other simulators like pspice, LTspice, and Micro-Cap x64 are excellent at using real models but poor at slow motion interactive real simulation. (unfortunately it is not like a real scope that has signal trigger Sync and more like a fast strip chart.

Imagine the NPN is two PN:NP junctions. When Vce = 0 collector PN junction is conducting and draining the current gain to nothing. It may start draining hFE at Vce=2V if you are near maximum current. But they only specify saturated in all datasheets as Vce(sat) = xxx mV.

You must memorize the diode exponential law of current vs voltage. look it up. This applies to transistors for Ic vs Vbe which is exponential. But when we add an emitter resistor we get Vbe+Ve which then becomes more linear because of this effective hFE property. There is also some internal rE which I show by lower case r. In fact, every diode or PN junction has series Rs and this matches the Vce(Sat)/Ic=Rce for any small or power transistor. The more power a bigger package can dissipate, the lower Rce will be.

Now to understand saturation, imagine the NPN transistor has two PN diodes across C-B and B-E that reduce the current gain hFE when V(BC) goes from -ve to + ve at Vce=Vbe now the current gain reduces rapidly as the C-B junction is forward biased. The collector in linear mode acts as a current source (more or less) but when Vce drops below 1V or so it it starts to act like a voltage source in the saturated state, so we call this now an inverting switch. Now none of this non-ideal saturation is shown in the Schockley Equation above.

If you can read a scope, I have put a power transistor with a triangular 0 to 5V linear sweep up and down from a 50 ohm signal generator so you can see the exponential base and collector current. I added the 2 diodes which sort of emulates what is inside the non-ideal (real) transistor.

You can learn a hundred things from these plots. Each plot shows the peak max min and avg. for each trace and the node is labelled like Vbe, Vce, Ib, Ic, Pd=Vce*Ic
See how many you can find in the relationships by usingyour mouse over Rc and thumbwheel to raise it to> 10k then drop it down to 100 ohms then towards 0.

Remember that you do not add these diodes, I only added them to simulate saturation. My Falstad SIM uses the Schockley equation with fixed hFE (you can change it) and does not model saturation which exists in all transistors.

This might be a 2N3055 but wasn't modelled exactly here for demo purposes.
Summary
he behavior of a bipolar junction transistor (BJT), possibly an NPN transistor like the 2N3055, and discussing the effects of saturation on its characteristics.

Let me break down the key points:

1. Simplified Model and Saturation:
   • The model presented is simplified and does not explicitly model saturation.
   • Saturation is briefly mentioned, and it is noted that datasheets typically specify the saturation voltage (Vce(sat)).
   • Saturation is characterized by the collector-emitter voltage (Vce) dropping below a certain threshold (around 1V).
2. Diode Exponential Law:
   • The diode exponential law is mentioned, and it's noted that this law applies to transistors when considering the collector current (Ic) versus base-emitter voltage (Vbe).
   • Adding an emitter resistor (Re) can make the relationship more linear due to the effective hFE property.
3. Emitter Resistance (re):
   • Internal emitter resistance (rere) is introduced, and it's noted that the addition of an emitter resistor (Re) makes the relationship more linear.
4. Collector-Emitter Resistance (Rce):
   • The collector-emitter resistance (Rce) is discussed, and it's mentioned that every diode or PN junction has series resistance (Rs).
   • Vce(sat)/Ic=Rce is highlighted, and it's noted that the lower Rce is, the more power a transistor can dissipate.
5. Behavior in Saturation:
   • In saturation, the transistor is described as acting like an inverting switch, and the current gain (hFE) reduces rapidly.
6. Simulation with Falstad:
   • A simulation using Falstad is mentioned, where a power transistor with diodes emulating non-ideal behavior is displayed.
   • Plots of various parameters like Vbe, Vce, Ib, Ic, and Pd are simulated.
7. Experimentation with Parameters:
   • Experimentation is encouraged by adjusting parameters such as Rc to observe the effects on the relationships.
8. Scope Readings:
   • Learning to read a scope is critical, and various relationships in the plots are highlighted, including peak, max, min, and average values for different traces.

It seems like you want to do hands-on, and simulation is a practical approach to understanding the behavior of transistors, especially in saturation, and how certain parameters and resistances influence their characteristics. This kind of experimentation and observation is crucial for understanding real-world transistor behavior beyond theoretical models