# transistor: active, cut-off, saturation, and pinch off!

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#### PG1995

##### Full Member level 5
Hi

I was searching if a transistor is linear device or not. A linear device has a linear relationship between current and voltage (Ohm's law). I'm a beginner in this electricity and electronic stuff. So, please keep your reply simple. Thanks.

While searching I found this:
We will operate transistor mainly in 4 regions, namely active , cut-off, saturation and pinch off region, depending on the type of biasing. If it is under active region then transistor is a linear device (linearity in the sense if the output is proportional to input then it is said to be linear).

So far I only know that a transistor has a collector, base, and emitter points.

It would be extremely nice of you if you could explain it a bit. I would be much grateful. Thanks.

PG1995

### PG1995

points: 2

#### LvW

Even in the active region the BJT is - strictly speaking - not a "linear device".
However, in practice the BJT can be "treated" as a linear device under the condition that the drivng amplitudes are within certain limits. This enables us to describe its properties and to calculate gain values based on a set of linear equations.

#### PG1995

##### Full Member level 5
yadavvlsi, LvW, thanks a lot. I will ask some follow-on questions soon.

Best wishes
PG

Sure! Whenever you have a query or doubt just post on EDAboard. There are a lot of experts to answer you!

PG1995

### PG1995

points: 2

#### PG1995

##### Full Member level 5
Hi again, ;-)

1: I was reading that a transistor is an active component. How is it one? It doesn't produce any power. We feed 'power' to it. It is just like a resistor which is a passive component. Could you please explain this to me in simple words? At the bottom you will find the quoted passage from a Wikipedia page on this which I was not able to understand. So far I have been told any component which adds power to the circuit is an active component. In this context only battery is an active component. We feed the current to the base of a transistor which in turn lets more current to pass from the collector to the emitter. We can say adding current to the base makes the bore of the 'transistor pipe' a little wider. In other words adding current to the base decreases the resistance the transistor.

2: The word transistor is portmanteau of words "transfer resistor". How does it transfer resistance? I don't get it. Perhaps you could help.

3:
I can understand the following voltage vs. current graph of a diode:

I was unsuccessfully trying to understand this load-line (I think this is what it's called!) of a transistor. I don't even get what that 'dark' line AB is. Please help me in simple words.

Electronic component:
A component may be classified as passive or active. The strict physics definition treats passive components as ones that cannot supply energy themselves, whereas a battery would be seen as an active component since it truly acts as a source of energy.

However electronic engineers performing circuit analysis use a more restrictive definition of passivity. When we are only concerned with the energy due to signals it is convenient to ignore the so-called DC circuit and pretend that the power supplying components such as transistors or integrated circuits is absent (as if each such component had its own battery built in) although it may in reality be supplied by the DC circuit which we are ignoring. Then the analysis only concerns the so-called AC circuit, an abstraction which ignores the DC voltages and currents (and the power associated with them) present in the real-life circuit. This fiction, for instance, allows us to view an oscillator as "producing energy" even though in reality the oscillator consumes even more energy from a power supply, obtained through the DC circuit which we have chosen to ignore. Under that restriction we define the terms as used in circuit analysis as follows:

Passive components are ones which cannot introduce net energy into the circuit they are connected to. They also cannot rely on a source of power except for what is available from the (AC) circuit they are connected to. As a consequence they are unable to amplify (increase the power of a signal), although they may well increase a voltage or current such as is done by a transformer or resonant circuit. Among passive components are familiar two-terminal components such as resistors, capacitors, inductors, and most sorts of diodes.

Active components rely on a source of energy (usually from the DC circuit, which we have chosen to ignore) and are usually able to inject power into a circuit although this is not part of the definition[1]. This includes amplifying components such as transistors, triode vacuum tubes (valves), and tunnel diodes.

Passive components can be further divided into lossless and lossy components:
Lossless components do not have a net power flow into or out of the component. This would include ideal capacitors, inductors, transformers, and the (theoretical) gyrator.

Lossy or dissipative components do not have that property and generally absorb power from the external circuit over time. The prototypical example is the resistor. In practice all non-ideal passive components are at least a little lossy, but these are typically modeled in circuit analysis as consisting of an ideal lossless component with an attached resistor to account for the loss.
[Wikipedia]

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