Transistor VCE (collector-emitter voltage)

[LarryD]

Look at Vce(sat), it shows the ratio between Ic and Ib to achieve the collector to emitter saturation voltage ex: .5v (Ic=2Amps/Ib=.2Amps). 10:1

[Boffin]

@LarryD

Ic=2Amps/Ib=.2Amps

You have confirmed that the collector current is 10 times the base current, but not vice versa.

[LarryD]

For saturation, a collector current (Ic) of of 2A, you should have 200mA of base current (Ib).

Ic/Ib = 10:1, or turn it around         Ib/Ic = 1:10

Vce will be .5v at saturation in this case.

Examine the data sheet for value Vce(sat).

If you want to do a measure, increase the base current to the point where the collector voltage is less than the base voltage, i.e. base to collector become forward biased.

[Wawa]

@Boffin

Are you sure that the base current should be 10 times the collector current?

Haha, typo.
The other way round of course.
Leo..

[syahrul0122]

@Wawa

The datasheet shows a 1:10 Ib:Ic ratio for saturation.

Hmm actually I have been wondering about that. So does that mean I have to increase IB to a point its 1/10 of IC? Since the resistor limits the IC to not be greater than 0.8A, thats the only way i can think of to reduce the hFE.

[syahrul0122]

@Boffin

If you want to know more about how transistors work, read the book "Paul Horowitz, Winfield Hill - The Art Of Electronic" (chapter 2). This is my favorite book on electronics.

Thanks, I will look into it, all electronic books that I own doesn't go as far as this.

[Paul_B]

@syahrul0122

that's the only way i can think of to reduce the hFE.

The h_fe is the DC current gain of the transistor.

If you want it to saturate - that is, to minimise the collector-emitter voltage - the h_fe is not relevant.

[TomGeorge]

Hi,

@syahrul0122

thats the only way i can think of to reduce the hFE.

Why do you want to reduce hFE?

This circuit is a basic tutorial circuit.

If you want to make an amplifier, that is a linear amplifier that you want to adjust the gain.
Then you need a different configuration that works with some independence on the actual BJT Hfe.
Such as this AC amplifier;

And this is the tutorial;
https://www.electronics-tutorials.ws/amplifier/amp_2.html

I hope this is what you are looking for.

Tom..

[LarryD]

hfe is the AC gain.

HFE is the DC gain.

But, I may have forgotten some course material from 45-50 years past.

¯\(ツ)/¯

[TomGeorge]

Hi,

= =============================================

Transistor gain summary

The various notations for transistor gain can be summarised as below.

• Beta; β: This is the basic notation for the forward current gain of a transistor.
• hfe: This is the current gain for a transistor expressed as an h parameter (hybrid parameter). The letter f indicates that it is a forward transfer characteristic, and the letter e indicates it is for a common emitter configuration. The small letter h indicates it is a small signal gain. hfe and small signal Beta are the same.
• hFE: The Hfe parameter differs from hfe in that it is the h parameter for the DC or large signal steady state forward current gain.

The different abbreviation used for the transistor gain, Hfe, hfe & Beta are all widely used, although the parameters Hfe, hfe tend to be more widely used in datasheets.

= ==============================================
Tom...

[LarryD]

@TomGeorge

• hfe: This is the current gain for a transistor expressed as an h parameter (hybrid parameter). The letter f indicates that it is a forward transfer characteristic, and the letter e indicates it is for a common emitter configuration. The small letter h indicates it is a small signal gain. hfe and small signal Beta are the same.
• hFE: The Hfe parameter differs from hfe in that it is the h parameter for the DC or large signal steady state forward current gain.

Funny what sticks in ones head.

[syahrul0122]

@TomGeorge

Why do you want to reduce hFE?

Based on this

@syahrul0122

And this

@Wawa

The datasheet shows a 1:10 Ib:Ic ratio for saturation.

Since hFE for the transistor is 60-120, then I'm guessing there has to be a way to reduce it to reach that requirement for saturation.

@TomGeorge

Such as this AC amplifier;

I think that's more into analog, I'm just trying to understand the principles in digital for now.

[TomGeorge]

Hi,
Why?
What is the exercise?
There is a range of hFE for any BJT device, so trying to "program" a particular hFE seems futile.

If you assume the lowest hFE you can calculate the lowest base current needed, then you have all possible hFE values covered to get full saturation.

That way your circuit will not have to have "selected" hFE BJTs to work.
Saves time and money.

Tom...

[Paul_B]

Sorry, you have this completely backwards!

@syahrul0122

Since hFE for the transistor is 60-120, then I'm guessing there has to be a way to reduce it to reach that requirement for saturation.

There is no such thing as "reducing" the gain.

Whatever the symbol is, the DC/ AC current gain of the transistor is simply irrelevant. The gain is what it is - in fact, the gain is by definition actually the ratio of change in collector current to change in base current.

Saturation relates to when the collector-emitter voltage is almost zero so the collector current is essentially the maximum it is can possibly get - there is simply no more room for it to change because there is no more voltage available.

However that small remnant collector-emitter voltage - less that a Volt - can be reduced to some extent by driving the base with more current than was necessary to pull it down originally. Of course there is a limit to this, it will never reduce to zero and at a base current of somewhere between one twentieth and one tenth of the collector current, you will get no further reduction in the collector-emitter voltage. That point is saturation.

[Wawa]

Did you see the small-print in the right hand-bottom part of the graph?

β=10

That graph is drawn assuming you drive 1/10th of the collector current into the base.
Leo..

[syahrul0122]

@TomGeorge

If you assume the lowest hFE you can calculate the lowest base current needed, then you have all possible hFE values covered to get full saturation.

So that means what i said here is correct isn't it?

@syahrul0122

so given that we've taken the lowest hFE which makes it likely that the transistor will be in saturation. So I guess that its safe to assume RC = 3.7-0.18/0.8 = 4.38Ω - 4.4Ω based on your given datahsheet.

@TomGeorge

What is the exercise?

Lets say i want IC = 0.8A, and the transistor's hFE range is 60-120.
Using the lowest value of hFE = 60, assuming the transistor's hFE >= 60, then IB should be 0.8/60 = 13.3mA or greater. At the same time i want the transistor to be in saturation so that i can determine VCE using the VCE(sat) diagram. Since saturation can only be acheived when IC is limited by voltage or resistance, assuming the transistor is in saturation ( according to diagram, VCE(sat) at 0.8A is 0.18V), the limiting resistor should be 3.7-0.18/0.8 = 4.4 ohm. Since we have taken the lowest hFE, that should mean any IB value greater or equal to 13.3mA can probably be enough to put the transistor in saturation because now IC is calculated to be greater than 0.8A but limited by the resistance.

But based on other's replies, I don't know if this makes any sense anymore, If you have a better answer please suggest.

[syahrul0122]

@Wawa

Did you see the small-print in the right hand-bottom part of the graph?

β=10

That graph is drawn assuming you drive 1/10th of the collector current into the base.

Yes and that is why i was thinking, does that mean you're suggesting that i have to increase IB to to a point it's 1/10th of IC. I mean since

@syahrul0122

the limiting resistor would be 3.7-0.18/0.8 = 4.4 ohm.

IC shouldn't be able to be increased after reaching 0.8A, so are you saying IB should be 80mA?

[TomGeorge]

Hi,
If hFE range is 60 - 120.
If Vce sat current is at 800mA
then DC base current should be;

800/60 = 13.3mA
Then you calculate the base resistor;
Assume Vbb will be Vcc for the experiment.
Vcc - Vbe = 3.7-0.6 = 3.1V across the base resistor.
The base resistor = 3.1/0.0133 =233R or 220R is the closest, so just a little more Ib, which is okay.

Have you built it and looked at the measurement?

If not then this is where you need to get some hardware and start experimenting.

There are lots of cheap BJTs in quantity for you to let the smoke out off.
You can buy a pack of resistors of various values.
You can use a 9V battery as your supply to stay safe.
Have you got a DMM to help your experiments?

Now is the time to play with hardware, get yourself a protoboard.

Tom...

[DrDiettrich]

@LarryD

Funny what sticks in ones head.

In my head sticks that hfe and ß are AC gain, i.e. dIc/dIb, for a wide range of Ic.

[DrDiettrich]

@TomGeorge

You can use a 9V battery as your supply to stay safe.

Better a couple of, if 800mA is drawn for some time.