Why won't transistor datasheets show Ic vs Vce curves?

Thread Starter

MathematicianKing

Joined Nov 10, 2017
34
Hi friends,

I am kinda new and I've been trying to get to grips making analogue circuits with real transistors rather than just simulating circuits.

Usually I simulate circuits rather than building them to learn electronics, but I've been trying to build them to see the real thing, but some just won't work because the transistors in the (crappy) simulatior have different parameters which I can't adjust.

Anyways, I thought that datasheets showed us some nice Ic vs Vce curves, but they don't!? All I can get are some Hfe tables at some values (Hybrid model) ?

Why won't they show us the Ic vs Vce curves? Seems to me they are quite important as thats the way textbooks teach about transistors.

Is it possible for me to draw my own curves from the parameters in the datasheets?
 

crutschow

Joined Mar 14, 2008
27,718
The reason they don't show Ic vs Vce is because that parameter is not normally needed in doing a transistor design.
Ic changes very little in the active region (Amplification Mode) with a change in Vce, as shown below.

Why do you think you need that info?
What textbook talks about Ic vs Vce as being important?
Are you referring to the transistor in the saturation region?

Hfe (Beta) is the current gain of the transistor (the collector current vs. the base-emitter current) which is one of the parameters used in a design.

I suggest you read up some more on how transistor work to get a better understanding of the important parameters.
 

GopherT

Joined Nov 23, 2012
8,012
Hi friends,

I am kinda new and I've been trying to get to grips making analogue circuits with real transistors rather than just simulating circuits.

Usually I simulate circuits rather than building them to learn electronics, but I've been trying to build them to see the real thing, but some just won't work because the transistors in the (crappy) simulatior have different parameters which I can't adjust.

Anyways, I thought that datasheets showed us some nice Ic vs Vce curves, but they don't!? All I can get are some Hfe tables at some values (Hybrid model) ?

Why won't they show us the Ic vs Vce curves? Seems to me they are quite important as thats the way textbooks teach about transistors.

Is it possible for me to draw my own curves from the parameters in the datasheets?
Bjt is a current gain device, not a voltage gain device. Calculate via ohms law, the current flowing into the base and that, multiplied by Hfe, will be the maximum current that can flow through the emitter collector (less smaller amount emitter to base) unless a current limiting transistor cuts the value to less. The devise performance is independent of base voltage except for the fact that higher voltage results in more current.
 

OBW0549

Joined Mar 2, 2015
3,566
Why won't they show us the Ic vs Vce curves?
Because, as @crutschow pointed out, collector current changes only slightly with changes in collector voltage except in saturation. Transistor data sheets often describe saturation behavior with a family of curves plotting Vce(sat) versus Ib for different values of Ic, such as this one from the ON Semi data sheet for the 2N3904:

Untitled.png

But they rarely show plots of Ic vs. Vce in the active region, because such plots would convey little or no useful information.

About the only circumstance I know of when Ic dependence on Vce is important is in the design of constant-current sources where the goal is achieving the highest collector output impedance possible; that's when clever circuit magic such as the Wilson Current Mirror comes into play.

Otherwise, changes in Ic with Vce are seldom important.

Seems to me they are quite important as thats the way textbooks teach about transistors.
I don't know of ANY textbook that does that.
 

tindel

Joined Sep 16, 2012
782
I don't know of ANY textbook that does that.
My undergraduate textbook "Microelectronic Circuits" used Ice/Vce curves almost extensively to describe transistor operation. I looked at my copy of "The Art of Electronics" and it does not show this graph at all that I could find. I really prefer "The Art of Electronics" way of teaching at an undergraduate level, but that's not what I was presented with in my studies. I very much feel the OP's frustration here.

When you're first learning about active mode transistors it is helpful to use these graphs to produce a load-line analysis. Of course, we all know you can do the same thing with the hFE value in the datasheet as well.

I learned about the ebers-moll model of the bjt transistor early effect, thermal voltage, output impedance, reverse saturation current, beta, etc. When someone was educated like I was, they look at a datasheet and freak out, because NONE of these are specified in a datasheet (beta is - kind of - as hFE (let's ignore the differences)). This information is helpful if you're designing integrated circuits, but is rarely a consideration when designing most transistor circuits - at least not initially. That's why I wish this information was saved for graduate level work where it matters much more - graduate level students are more likely to use this information if they aspire to work with an IC manufacturing company.

Anyway OP, I feel your pain. What is it that you want to know?
 

hobbyist

Joined Aug 10, 2008
889
All my course material, and every transistor circuit design book I read always use the IC / VCE curve to explain load line, they do put a lot of importance on it when teaching this, as if this curve was part of the manufacturer data, that is easily obtained for any transistor your using.

However the data they are using is from there own setups, in order to better explain how a change in base current, will move the collector current up and down the scale, so as to give amplification.

When designing a class A transistor amplifier, you are mainly trying to pick values of biasing resistors that will put your VCE around half the supply voltage, in order to have maximum voltage swing across your load.

The biasing resistors are calculated using ohms law. If your interested in learning how to choose bias resistors, then let us know, we can give you examples of how to choose resistor values based on specific parameters you would like to design for.
 

Thread Starter

MathematicianKing

Joined Nov 10, 2017
34
Hi there,

Well, I have two books which I treasure, the first is Boylestad (Devices & Circuit Theory), and the other is Microelectronic Circuits by Sedra & Smith. Here in the UK Sedra & Smith seems to be the undergraduate book of choice, but I find Boylestad a breeze of fresh air compared to it. However Boylestad doesn't go into as much depth as Sedra does.

Anyways, both of these great books use Ic vs Vce curves extensively and throughout when teaching transistors. Why ? Because of load line analysis, output impedance, early effect, and breakdown voltages.

I find these curves very important because they give me a birdseye view of the operation. I am sure this is because I am a newbie, so I will have to find another way.

I guess as you all say I just didn't understand what's important here. The main reason I am looking for the curves is because I want to set my load line so that the transistor doesn't go either into saturation or into breakdown.

So what is the standard procedure to get the amplifier's Q point? Choose the Q point to be about half the supply and that's it? While avoiding it getting close to breakdown Vce, and also close to saturation?

I think my problem is I don't know what is the optimal place for the Q point given a supply voltage V. Would it be half the supply then?

If I put the Q point at half the supply, and keep the input voltage swing small enough so it doesn't go either into breakdown or saturation, will that be ok ? Is that all I need to do ? How do I find the appropriate quiescent current ?

These textbooks are getting those who read it used to these characteristic curves, and when we go to the real world they aren't really used. I think this is a bit of a fail by the authors. No offense meant because these books are great anyhow. I am completely self taught so anything I learn wrong I can't see right away.
 
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Thread Starter

MathematicianKing

Joined Nov 10, 2017
34
The datasheet for the BC300 transistor shows parameters: Vceo, Vebo and Vcbo under the heading Absolute Maximum ratings. What do these mean ?

Take Vceo, is it Vce with base open ? What does it mean to have the base open? Does open mean floating or does it mean zero volts?

Anyhow, if Vceo means Vce with base open, or at zero volts, does this mean the maximum Vce voltage ? So that when I am designing my Q point, I should stay away from Vceo Max?

What about Vebo, I guess this is Veb with collector open.

Then finally Vcbo, I guess this is Vcb with emitter open.

Why do they measure these with one end open though ? I remember reading about this in Boylestad. I will have to reread that session.

Anyhow when designing the amplifier, what should I pay attention to ? Maximum Vce ratings, Max power ratings, and VceSat ?

I really want to know what is the best quiescent current to choose. Should it be half of the maximum ? If I choose IcQ to be half of the max, and VceQ to be half of my supply, and if IcQ * VceQ is within the bounds of the maximum power ratings, and if my input swing keeps it in the bounds, is that all I have to worry about ?

Here's the datasheet:

 

Thread Starter

MathematicianKing

Joined Nov 10, 2017
34
Because, as @crutschow pointed out, collector current changes only slightly with changes in collector voltage except in saturation. Transistor data sheets often describe saturation behavior with a family of curves plotting Vce(sat) versus Ib for different values of Ic, such as this one from the ON Semi data sheet for the 2N3904:

View attachment 139860

But they rarely show plots of Ic vs. Vce in the active region, because such plots would convey little or no useful information.

About the only circumstance I know of when Ic dependence on Vce is important is in the design of constant-current sources where the goal is achieving the highest collector output impedance possible; that's when clever circuit magic such as the Wilson Current Mirror comes into play.

Otherwise, changes in Ic with Vce are seldom important.


I don't know of ANY textbook that does that.

So you don't hate me anymore ? :)
 

Thread Starter

MathematicianKing

Joined Nov 10, 2017
34
My undergraduate textbook "Microelectronic Circuits" used Ice/Vce curves almost extensively to describe transistor operation. I looked at my copy of "The Art of Electronics" and it does not show this graph at all that I could find. I really prefer "The Art of Electronics" way of teaching at an undergraduate level, but that's not what I was presented with in my studies. I very much feel the OP's frustration here.

When you're first learning about active mode transistors it is helpful to use these graphs to produce a load-line analysis. Of course, we all know you can do the same thing with the hFE value in the datasheet as well.

I learned about the ebers-moll model of the bjt transistor early effect, thermal voltage, output impedance, reverse saturation current, beta, etc. When someone was educated like I was, they look at a datasheet and freak out, because NONE of these are specified in a datasheet (beta is - kind of - as hFE (let's ignore the differences)). This information is helpful if you're designing integrated circuits, but is rarely a consideration when designing most transistor circuits - at least not initially. That's why I wish this information was saved for graduate level work where it matters much more - graduate level students are more likely to use this information if they aspire to work with an IC manufacturing company.

Anyway OP, I feel your pain. What is it that you want to know?

It's all good my man :)
 

tindel

Joined Sep 16, 2012
782
So what is the standard procedure to get the amplifier's Q point?
To set your Q point in a common-emitter amplifier, you look at the hFE of the transistor at your operating current (aka Beta, for all practical purposes). For example, the BC300 that you linked has a hFE between 20 and 240 with between 100uA and 150mA of collector current, so a gain of 100 is reasonable (but not guaranteed!). So you drive your base current, and you know your collector current will be iC = 100*iB. With this information, you select your collector resistor such that the collector voltage is half way between the voltage rails to get your widest swing voltage possible. The problem is that hFE varies wildly, and the collector resistor often needs tweaked to set the Q point to the precise middle of the voltage rails. For this reason common-emitter amplifiers are rarely used in practice. Op-amps are much more precise in comparison and are usually used instead.

Vceo, Vcbo, and Vebo are the absolute maximum operating voltage ratings for the part; breakdown of each junction is guaranteed to be above this value.

Also note saturation voltage spec has a forced beta of 10. This is a clue that when using this part as a switch you should always design your switches to have a forced beta of 10 or less. to get those saturation voltages.

Hopefully that gets you up and running. There's a lot to learn, but you have the basics and can figure it out as you go. Don't be afraid to ask more questions.
 

neonstrobe

Joined May 15, 2009
166
Many of the older databooks and datasheets published by manufacturers did show typical transistor output characteristics. I sympathize with a beginner wanting to design a circuit based on load lines etc - that is a useful way to understand how transistor circuits work. I regret that they no longer show these curves, as it can tell you quite a bit about the transistor. For example, how does it behave near its rated Vceo, (open base by the way) and near the saturation region?
However, once you move on to designing circuit you will probably want to simulate them as much as solder them. Again I believe it is very important to know whether a simulator gives a correct interpretation of the characteristics of a transistor. You will only know that if you can compare published or preferably measured output characteristics with the simulated data.
On the other hand I can see that the output characteristics are not as useful as perhaps they were once with tubes. Tubes tended to have well defined characteristics but because transistor gains vary a lot (2:1 is optimistic) then a particular collector current:voltage sweep may apply for a range of currents for different samples of device. That is where circuit design has to consider the voltage and current biasing conditions, and then less worry about the particular base currents (but that over-simplifies designing. All effects need to be considered and circuits should be designed to work with worst case and best case transistors not just typical, which a datasheet would show anyway).
 

OBW0549

Joined Mar 2, 2015
3,566
So you don't hate me anymore ? :)
You think I hated you??? Oh, my...

Rest easy: I would never hate a newbie for calling me a <vulgar term denoting an unintelligent beast of burden> merely because of a misunderstanding. I've been earning my money in electronics for over 50 years at this point, most of them as a design engineer, so such outbursts have little impact.

This is a pretty friendly place; so just keep on keepin' on... :cool:
 

Thread Starter

MathematicianKing

Joined Nov 10, 2017
34
Could anyone tell me what is the optimal value for the quiescent current commonly ? For a discrete BJT amplifier... With a supply voltage of say 10V.... Vce at 5V... I am confused as to how to choose the Icq. I understand Vce should be at half the supply, Should Icq be at half IcMax ?

Also, when you look at a loadline graphic... What angle is usually the one that guarantees maximum Av ? Is it 45 degrees ?
 

Thread Starter

MathematicianKing

Joined Nov 10, 2017
34
You think I hated you??? Oh, my...

Rest easy: I would never hate a newbie for calling me a <vulgar term denoting an unintelligent beast of burden> merely because of a misunderstanding. I've been earning my money in electronics for over 50 years at this point, most of them as a design engineer, so such outbursts have little impact.

This is a pretty friendly place; so just keep on keepin' on... :cool:


What does a design engineer do ? What is the difference between Applications, Systems, and Design engineers? What are your tasks at work ?
 

OBW0549

Joined Mar 2, 2015
3,566
Could anyone tell me what is the optimal value for the quiescent current commonly ? For a discrete BJT amplifier... With a supply voltage of say 10V.... Vce at 5V... I am confused as to how to choose the Icq. I understand Vce should be at half the supply, Should Icq be at half IcMax ?
I don't think there's any "optimal" Icq, merely a range of "reasonable" values of Icq, in many cases somewhere between a tenth of a milliamp and a couple of milliamps. It's usually a judgement call, based on experience and the design requirements for the particular circuit.
 

Thread Starter

MathematicianKing

Joined Nov 10, 2017
34
I don't think there's any "optimal" Icq, merely a range of "reasonable" values of Icq, in many cases somewhere between a tenth of a milliamp and a couple of milliamps. It's usually a judgement call, based on experience and the design requirements for the particular circuit.

Would you tell me what is the optimal Icq for the BC300, for a supply voltage of both 5V and of 10V ? I have attached the datasheet up there ^^^

In a voltage divider structure, I biased it with Vce = 9V, Vc = 10V, Ve = 2V, and Icq =1mA, and I got Av to be just 3V/V as I remember. And in the simulator it seems like the lower Icq the higher the gain... not sure if this is so
 

Thread Starter

MathematicianKing

Joined Nov 10, 2017
34
I would like to know if Rc is the same as RL ? What I mean is the collector resistor. Is it a load resistor ? In some cases I see it in books as RL and some cases Rc. Is Rc itself a load ?

When calculating the gain of an amplifier, say a voltage divider biased common emitter, what is the load ? Is it Rc ? Or is the load something else? A capacitor connected to the collector, in series with a resistor to ground ? Is that the load ?

When my book calculates the gain, it uses Rc as the load. Is that the open loop gain ? Such that if you connect a further load to the collector the gain goes down ?

So Rc is the load, but for calculating the open gain ?



Another question...... :)

If I want a simple power amplifier, say to amplify an audio signal and feed it into a speaker... Do I use the speaker in place of Rc? So that the speaker becomes the load, directly connected in place of Rc, so that a lot of current passes through it? Or does it not matter ? Or should I AC couple the speaker to the transistor collector? However it seems the voltage will be small because Rc is large compared to the speakers impedance.
 
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