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?

Re: What is a load line - intuitively
veek wrote:

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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?

Re: What is a load line - intuitively
On 4/30/2017 12:57 AM, veek wrote:
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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.

--  

Rick C

Re: What is a load line - intuitively
rickman wrote:

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

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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?

Re: What is a load line - intuitively
On 4/30/2017 3:23 AM, veek wrote:
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Yes, Rc (c for collector) is the load resistor in this case.


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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.

--  

Rick C

Re: What is a load line - intuitively
rickman wrote:

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cool - thanks!

Re: What is a load line - intuitively
veek wrote:

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https://qph.ec.quoracdn.net/main-qimg-752573d7d6bc8c20319db701ddc539d3
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 Ic50% cuts the load  
line?

Re: What is a load line - intuitively
On 4/30/2017 6:36 AM, veek wrote:
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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.


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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.


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


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

--  

Rick C

Re: What is a load line - intuitively

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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.




--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: What is a load line - intuitively
snipped-for-privacy@highlandtechnology.com says...
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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.

.  



Re: What is a load line - intuitively

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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.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


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