# How to read BJT output characteristic

What's the propper way to read current graph / output characteristic of BJT? Is it useful if there is no DC load line plotted?

I'm very beginner and I thought that only possible operating points lay on the DC load line. I was assuming that Ic and Vce are resulting from circuit Vcc, resistance and Ib.

Recently I've watched the lecture, where they were concidering changing of Ib for constant Vce to see changes in Ic. Is there any point in doing that? For my understaing, there is only one operating point for a given Vce, for a given circuit.

• A resistive load leads to a straight load line. But there's no law that says the load has to be resistive, or even that it has a well-defined unique V/I relation. Nov 18, 2023 at 0:52
• This kind of chart is useful for learning or understand bjt behaviour, but not much useful for practice designing. You doesn’t even see the linearity from it. Constant Vce is used in shunt reference designs, not much in amplifiers. Nov 18, 2023 at 5:35

Is it useful if there is no DC load line plotted?

The load line tells you the characteristic of the load, not of the BJT.

The characteristic of the BJT is useful because it tells you, for any choice of load, what the operating point will be.

You can't tell this with just the load line (the load characteristic) or with just the BJT characteristic curve. You need to have both and find the intersection point to get the operating point of the circuit.

I'm very beginner and I thought that only possible operating points lay on the DC load line.

True, but you won't always use this BJT with a 10 kohm load. In another circuit you might use a 5 kohm load, and in a third circuit you might use 47 kohms.

Knowing the full characteristic of the BJT allows you to know what the operating point will be in all three of these circuits.

When you're designing the circuit, you have to choose what load resistor to use. Having the full characteristic of the BJT and not just its behavior when used with one particular load allows you to do that.

they were concidering changing of Ib for constant Vce to see changes in Ic. Is there any point in doing that?

Yes, this gives the very important $$\\beta\$$ or $$\h_{fe}\$$ characteristic of the BJT (two names for essentially the same thing).

For my understaing, there is only one operating point for a given Vce, for a given circuit.

For a given base current bias, yes. But when designing the circuit you might have the ability to choose the base current. This will let you adjust the power consumption of the circuit. In an analog amplifier it will let you control the dynamic range of the circuit. In a digital buffer it will let you be sure the circuit fully switches for the desired input levels.

Also remember, your load line diagram gives the behavior of one particular circuit: the common-emitter amplifier. This isn't the only possible circuit you can build with a BJT, and different specifications of the BJT will be important for different circuits.

• Why a common-base amplifier? Nov 19, 2023 at 19:39
• @Circuitfantasist, brain lapse. Edited. Nov 19, 2023 at 21:15
• Thanks for answer but I don't get beta / hFE part. I mean if have a defined circuit (with some resistance etc) so we can plot a DC load line then we can controll a BJT (Ib) to see the output (Ic, Vce) - we don't have independent controll of either Ic or Vce. What's the point of assuming constant Vce and analyzing what will be the change of Ic due to Ib changes. Here is the lecture youtube.com/watch?v=65jKlnoXrWY and part I'm describing is 3:45 - 4:30. I thought we approach saturation from south-east as the DC line is, not from south (constant Vce). Could you please elaborate on that? Nov 20, 2023 at 8:01
• @Brutal, if you already have a defined circuit, you don't need to know anything about the transistor at all. The datasheet is for people who are designing the circuit, not for people who are handed a circuit already designed. Nov 20, 2023 at 15:53