I'm covering BJT's in my semiconductor class and in lecture, the professor said that BJTs are used as amplifiers in the linear region. I'm confused because I thought the gain was proportional to the base current.
By that logic wouldn't the highest gain be in the saturation region?
Can someone clear up this confusion?
If you´re just going to switch a large current on/off with a small controlling current (at the base) then you want the transistor to go into saturation.
But if you want the collector current to follow, for instance an oscillating signal at the base, then you want the transistor to stay in the linear region.
Consider the characteristic curves of a typical small-signal NPN transistor:
Referring to the right-hand graph, we can see that the DC Current Gain hFE is almost constant with collector current provided Vce is maintained constant at 1V. It starts to drop as you approach Ic of 100mA and (not shown) it will also drop at very low collector current.
If you now look at the left-hand graph, the intersection of Vce = 1.0V corresponds to a point on the right-hand graph for each of the base currents from 0.1mA to 1.0mA. As the collector current increases the gain is less, so the space between the evenly spaced base currents becomes less. Now, imagine the transistor is entering saturation, so Vce is less than 1V, approaching zero. For example at 1mA base current, and 0.1V Vce you can see that the collector current is only about 25mA- that means the current gain has dropped to 25 from more than 200. At 0.1V and 0.1mA base current, the gain is more like 50, still a far cry from the 300 at 1V Vce.
As Vce approaches zero, the gain current drops to zero. This makes sense because the transistor needs some voltage to work. At some point very near zero it will even turn slightly negative as increasing base current causes current to flow out of the collector.
The other thing you might find interesting is that at low base current the curves are fairly flat- the gain does not change much with Vce provided it sees "enough".
Please note that the "saturation region" for a BJT is the region where Vce < Vce_sat. In this region of operation, Ic is not only determined by Ib and Vbe but also by Vce. If you would determine a small signal model of the BJT in the saturation region you would find an extra component "eating up" part of the collector current resulting in less gm (what you call gain).
The linear region is where Vce > Vce_sat. In this region of operation Ic is mainly determined by Ib and Vbe but not so much by Vce (only through the Early effect). This results in a higher output impedance (at the collector) if compared with the saturated region, resulting in more usable gm.
Although a signal can be amplified by a BJT in either region, the linear region is more convenient to use as the Vce dependancy is (in 1st order) removed.
The (small signal) transconductance gm = dIc/dVbe of a BJT is mainly dependent on Ic but since beta = Ic/Ib (assuming we're in the linear region !) you could say that the gm (I would not call that gain as gm has unity [A/V]) is dependent on Ib. But it is more convenient to say the gm is dependant on Ic since gm = Ic/Vt.
