BJT Amplifiers - Loadline question

In summary: EECE 251 course, explains that in order to determine the Q point of a transistor, both the DC and AC loadlines must be considered. The DC loadline shows the collector current for a given collector-emitter voltage, while the AC loadline shows the maximum possible output swing. The Q point is selected on the DC loadline, and the AC loadline automatically crosses it. A capacitive load can cause an elliptical loadline, but at the operating frequency, it is negligible. There should be a circuit diagram provided for better understanding.
  • #1
taylorwinston
1
0
Hello,

I had a basic analog electronics course this summer. Now that I'm trying to design my own circuit with a bipolar junction transistor (both for fun and to prepare for the future), I'm finding I don't really understand how and why I have to follow certain procedures to get maximum output voltage swing.

I don't understand why you have to consider the AC and DC loadlines' intersection to get the quiescent (Q) point of a transistor, for example of a bipolar junction transistor (BJT) amplifier.

The DC loadline tells me how the collector current will look for a given collector-emitter voltage. It is a negatively linear relationship meaning if I increase the collector-emitter voltage, I get a proportional decrease in collector current. If I want a different result, I have to change the collector and/or emitter resistors.

The AC loadline tells me my maximum possible output swing. Similarly, if I want a different characteristic, I need a different set of impedances or perhaps my input signal could be changed.

Question: Why do I need the intersection to determine the Q point? Why can't I just use the DC loadline's middle to give me the quiescent (Q) point?
 
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  • #2
This video explains it EECE 251 - BJT Tutorial A (optimum Q point)

 
  • #3
At first, it would be very helpful (even necessary) to have a circuit diagram.
Otherwise, we do not know where the capacitor is which is responsible for the ac load line.
One can assume that the emitter path consists of a resistor RE (DC feedback) and a parallel capacitor, correct?

Secondly, you have not to "consider" the "intersection" between both load lines to get the Q-point. The other way round: You select a proper Q-point on the DC load line - and the AC load line AUTOMATICALLY crosses this point because - without any ac input - there is no other point which fulfills the voltage-current relationships.
 
  • #4
I recall that a capacitive load gives an elliptical load line. So I presume C is large enough to have neglible reactance at the operating frequency.
 
  • #5
tech99 said:
I recall that a capacitive load gives an elliptical load line. So I presume C is large enough to have neglible reactance at the operating frequency.
Elliptical? The load line does not involve any frequency variation and, hence, is really a straight "line".
 
  • #6
It is in textbooks on amplifiers. I suppose that a capacitive load draws current in quadrature to a resistive load, hence the elliptical load line. In other words, it draws more current when the voltage is not maximum.
 
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Likes cabraham
  • #7
Please, can you provide a reference (or better: An excerpt) ?
Thank you
(By the way - you are Claude A.?...nice to meet you again!))
 
  • #8
LvW said:
Please, can you provide a reference (or better: An excerpt) ?
Thank you
(By the way - you are Claude A.?...nice to meet you again!))
Greetings LvW:
No, I, Claude A., have not participated in this thread so far. I will look it over later & reply if I feel like contributing.
Nice to meet you again, as well. BR.

Claude Abraham
 

1. What is a BJT amplifier?

A BJT (Bipolar Junction Transistor) amplifier is an electronic circuit that uses a bipolar junction transistor to amplify an input signal. It is commonly used in audio and radio frequency (RF) applications.

2. How does a BJT amplifier work?

A BJT amplifier works by using the amplifying properties of a bipolar junction transistor. The transistor is biased to operate in its active region, where small changes in the base current can result in larger changes in the collector current. This allows the input signal to be amplified.

3. What is the loadline in a BJT amplifier?

The loadline in a BJT amplifier is a graphical representation of the relationship between the collector current and collector-to-emitter voltage. It is drawn on a graph of the transistor's characteristic curves and is used to determine the operating point of the amplifier.

4. How do you determine the operating point of a BJT amplifier using the loadline?

The operating point of a BJT amplifier is determined by finding the intersection of the loadline and the transistor's characteristic curves. This point represents the collector current and collector-to-emitter voltage at which the amplifier is operating.

5. What factors affect the shape of the loadline in a BJT amplifier?

The shape of the loadline in a BJT amplifier is affected by the value of the load resistance, the DC biasing voltage, and the transistor's characteristics such as beta and the Early voltage. Changes in any of these factors can cause the loadline to shift, resulting in a different operating point for the amplifier.

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