What is a load line - intuitively

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That's a typical graph of Vce vs Ic for a bunch of Ib as provided by the manufacturer. Is my understanding correct:

We create a loadline so that we can operate the transistor's collector ckt in a linear fashion for a varying base input.

Basically there are an infinite set of Vce vs Ic curves for each and every base current value possible - obviously manufacturer's can't plot all that so they give us certain typical curves.

Q represents the DC operating point for some base current but when an input signal is fed for amplification, the base current changes, and we basically move to a different Vce vs Ic curve (the new Q point on this curve represents the output for the new base current flowing as a result of changed Ib due to signal).

Because the load line is linear, for every change in Ib we get a linear change in Ic; if the load line was somehow a sinusoid we'd get a sinusoidal amplification action for Ib?

Reply to
veek
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Also, would it be fair to say: Once the transistor is amplifying a signal.. the Q point would wander/walk all along the load-line and that the current Q-point position represents the Vce*Ic output for corresponding Ib?

Reply to
veek

I'm not sure of all the things you wrote, but the load line is the line drawn *on* the Ic vs Vce curves to show the possible operating points. With a constant power supply voltage the voltage across the load resistor will be the difference of power supply minus Vc, so the load line is drawn from zero current and max Vcc to zero voltage and max Ic.

Where the load line intersects the transistor curves for a given base current is where the transistor will operate. I would not say the operation of the transistor is necessarily linear. To see that clearly you need to plot Ic vs Ib for a given load. In general it won't be completely linear.

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Rick C
Reply to
rickman

Is Rc the load resistor? Normally load's Rl .. (anyway yeah - that's the math procedure for computing loadline - plug and chug thing]

Ah.. so the transistor operates in a range between Qh and Ql on the load line? For a sine wave, first half it would operate between Q and Ql? The operating point would shift from Q to Ql and then back to Q for the zero crossover?

Reply to
veek

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In this graph/image:

  1. Can the transistor work at A? No - right? The curved part represents a non-linear part where Vce is not large enough to draw all electrons from the emitter - you'd be leaving behind part of th signal (so to speak) for some particular Ib?
  2. Note the Q Point (active REGION). The transistor is active across all Ib on the load-line?

As in: the Q point (that point on graph) is where the transistor stays when it has no input signal because of DC biasing. When there is an input signal the Q point moves along the load-line between A and B driven by the input signal (assuming the signal is large enough to drive the Q point that far) - correct?

If it goes all the way to the curved region near A - the Ic value will no longer be proportional to Ib like it was near say where Ic=50 cuts the load line?

Reply to
veek

Yes, Rc (c for collector) is the load resistor in this case.

I was looking at the schematic for the other post with 100 kohm and 10 kohm resistors. This one with 500 kohm base and 3 kohm collector resistors is *not* in saturation.

I don't remember the Q point thing, at least the name. But the idea is that the bias puts the static point somewhere which you are calling Q and the AC signal creates the movement on the load line between Ql and Qh. So I think you get it.

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Rick C
Reply to
rickman

I think you are referring to "working" as amplifying a signal with linearity. No, biasing the transistor to point A will not let it swing both ways so an input signal of any amplitude will be clipped.

But amplifying an AC signal is not the only reason to use a transistor. There are switching uses where bias at point A would be a good idea.

Not sure what this implies, "active across all Ib"? There are limits to the Ib range you can use no matter where you bias it. But yes, this bias point will give you a good range of operation.

Yes, when you apply an input signal the collector voltage moves across the load line.

Yes, it only has to get close and the non-linearity gets bad.

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Rick C
Reply to
rickman

cool - thanks!

Reply to
veek

If you're designing a linear transistor amplifier, you usually don't need to think about load lines. Just assume that static collector current will be constant at some value, and pick the collector resistor accordingly, typically to park midway between VCC and transistor saturation.

Tubes, especially triodes, had very slopey plate curves, and load line analysis was one easy way to define the operating point.

And sure, if the collector load is nonlinear, the gain will be nonlinear.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Two small points:

Beta is poorly controlled, so using Ib to set Ic is hazardous. One generally designs a circuit to force some desired Ic indepentent of beta.

"Q-point" is, I think, an amateur audio term. We talk about bias current and collector voltage. Or more generally, maybe bias point.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Beta is often not very quality controlled. Often the values of the passive components are set so that beta does not play much of a part as long as it is enough.

Q-Point has been used to describe a point for many simiconuctors, and not just transistors for over 50 years. Referred to a shortened "quiescent point". It is often near the middle of a transistor load line. It is a point where when a signal changes from a larger current to a smaller one, or the other way depending on the signal. For example in a transistor if you have no signal the transistor will put out a certain current. If you put a sine wave signal to it, over the positive part of the cycle the transistor will draw more current and the negative part it will draw less current.

Q can be used in electronics to describe how sharp a resonate circuit is in simple terms.

For hams there is a a list of what is called Q signals. Usually 3 letters starting with Q. It was really started by commercial stations as a short form of statements or questions as everything was sent by Morse code and at a speed of about 20 words per minuit it was slow to get messages through. For example it is lots faster to send QRO instead of telling the other station to increase power or asking if I should increase power.

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Reply to
Ralph Mowery

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