Electronic – Difference between GTO and BJT

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.

The current through a BJT is controlled by the base-emitter voltage VBE, it depends exponentially on that voltage. From this point of view the BJT is a voltage-controlled device.

The way a BJT works makes it necessary to also have a small current through the base. This base current is only a small fraction of the collector current. The ratio between these currents can be used to define a current gain and it is possible to regard the BJT as a current-controlled device.

So we have two ways to deal with a BJT and depending on the situation one is better than the other.

For a CB circuit we see that input and output current are almost equal and it could be used as a current buffer. Apart from that current gain doesn't help much to gain further insight.

Using the voltage-controlled operation approach, we see that the emitter gives direct access to VBE of the transistor in almost the same we as for the CE configuration, only the sign is different. So a change of the emitter voltage causes a change of VBE. This also changes the current through the transistor which is then converted into a voltage by Rc. So it is possible to achieve voltage amplification.

However, usually voltage amplifiers with a larger input impedance are wanted and therefore the CE stage is preferred over the CB configuration. This leaves the use as a current buffer like in a cascode circuit.