# Load line of transistor

1. Dec 23, 2012

### Outrageous

The load line of transistor depends on resistor of base or resistor of collector ?
Or both?

thank

2. Dec 23, 2012

### Drakkith

Staff Emeritus
3. Dec 23, 2012

### Outrageous

depend on resistor of collector and resistor of emitter .
Then for fixed bias circuit , it can also depend on resistor of base?

Thank

4. Dec 23, 2012

### Drakkith

Staff Emeritus
I'm having a hard time understanding what you are asking, so I am sorry if this doesn't answer your question. I believe that if the bias of the base-emitter junction stays the same, then the current through the transistors emitter-collector path solely depends on the voltage across it. Adding a resistor in series with the emitter-collector would reduce the current through it. Is that what you were asking?

5. Dec 23, 2012

### yungman

What do you mean by resistor of the base? if you mean resistance at the base, then the answer is NO. The load line doesn't depend on the resistance seen by the base. Load line is PARTLY depending on the resistance at the collector and the supply voltage. But it also depends on the base current.

Load line is drawn on the graph of collector curve of Ic vs Ib at given collector voltage. You have multiple collector curve of different Ib. It is the base current you concern with, the resistance at the base is irrelevant as long as you can drive the base current.

6. Dec 24, 2012

### Outrageous

Sorry for poor description
Thank Drakkith and yungman

I think this is the answer I want. Thanks

7. Dec 24, 2012

### Staff: Mentor

For a common emitter arrangement, the load line is a straight line plot of IC vs VC, and it neatly superimposes on the transistor characteristic of IC vs VCE. The load line slope is determined by RC + RE.

http://physicsforums.bernhardtmediall.netdna-cdn.com/images/icons/icon2.gif [Broken] Neither base resistance nor base current play any part* in determining the load line.

The load line is determined by the external circuit (Vcc , RC, and RE) and is independent of the device, so with those parameters fixed the load line remains the same whether you plug in a transistor, a diode, or a resistor.

I hope that helps. http://img803.imageshack.us/img803/4666/holly1756.gif [Broken]

Last edited by a moderator: May 6, 2017
8. Dec 24, 2012

### yungman

In an indirect way, base current have a part in it, because different Ib setup different Ic curves that generates the load line.....Even if we assume linear relation between Ib and Ic as Ic=βXIb.

Load line is device dependent because different devices have different early voltage that create different slope on the flat part of the collector curve, which, determine the load line.

9. Dec 24, 2012

### Staff: Mentor

We might be talking about different things, because what I know as the load line is device independent. I need assume nothing about the device, because load line is determined not by the device but by the external linear* circuit. The external circuit constrains the device to operate along a line on the I vs V graph, and the device cannot deviate off that load line.

Example: for a simple arrangement with RE=0, then VC=VCC - ICRC, and (despite what you assumed because of the nomenclature) I actually haven't yet indicated whether the device I am using is a BJT, a diode, an SCR, a resistor, or whatever. But the load line of its operation is already precisely defined as that straight line. That's the beauty of the load line concept, it's simplicity.
http://img803.imageshack.us/img803/4666/holly1756.gif [Broken]

Last edited by a moderator: May 6, 2017
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