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What exactly is the definition of an operating point in the context of Bipolar junction transistors. What is their physical significance? I was going through few books on electronics and found that a load line is drawn (Linear relation between collector emitter voltage and collector current) and the point of intersection with the output characteristics provides us with the operating points. Why is it necessary to fix the operating point in the linear part of the output characteristics so as to operate the transistor as a D.C. amplifier?

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Re: Why is it necessary to fix the operating point in the linear part of the output characteristics so as to operate the transistor as a D.C. amplifier?

The AC signal can be regarded as a waveform superimposed on the DC operating point, causing positive and negative displacements from that middle position.

The operating point is chosen so that there is maximum "headroom" in either direction before the onset of clipping. If the operating point is too close to saturation or cutoff, then that limits the amount of gain that can be achieved prior to the onset of distortion.

The middle point of the load line is chosen because it is the middle point, not because the behavior is linear there. It's not really any more or less linear than away from the center.

A transistor isn't precisely linear. The use of negative feedback makes the amplifiers (more) linear. If you want a linear device with, say, a voltage gain of 100, then you cascade two "swamped" common emitter stages with gain 10, rather than a single stage with gain 100.

Note that, for instance, in class AB push-pull amplifiers, the biasing is different. The quiescent point is such, in fact, that both transistors in the AB stage are close to cut-off. Considered individually, the behavior of each transistor is very nonlinear because it amplifies only half of the signal! And yet, the amplifier stage as a whole is linear.

You have been studying a particular kind of amplifier stage in those text books with particular biasing; that is not the full story. Have a look at other kinds of BJT topologies.

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When using a transistor to amplify a AC signal, the output has to go both up and down from some center point. You also want the output to be reasonably linear with respect to the input, unlike in switching applications.

To allow a transistor circuit to go both up and down from a center point, the transistor has to be partially on. You turn it on more to go one way, and off more to go the other way. Either way, some headroom is required. The operating point is the choice of DC bias voltages and currents to keep the transistor in the partially on state. For linear operation, you want this to be far enough into the linear range of the transistor so that it is still in the linear range when it gets turned off a little at one and, and not too far so that it has room to turn on more at the other end.

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Look at the output load-line below. With the operating point set at 30mA, as the base current changes, we move between the blue traces and Vce & Ic change accordingly. If we were to choose 55mA as the operating-point (about 1V Vce), all the blue traces converge so that changes in base current have little effect on Vce or Ic.

enter image description here

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