Transistors and Biasing

A diode can only pass current or block it. A transistor can control it — a tiny current at one terminal governing a much larger one at another — and that is the whole of amplification, oscillation and switching. Section A tests this lesson harder than almost any other: the doping of the three regions, the two current relations, which junction is biased which way, and the three configurations. None of it is difficult once you can picture what is physically happening inside the device.

Two junctions, back to back

A bipolar junction transistor (BJT) is a single crystal grown as three layers: either N-P-N or P-N-P. That gives two PN junctions sharing a middle region. The three terminals are the emitter, which emits the carriers, the base in the middle, and the collector, which collects them.

“Bipolar” means both kinds of carrier take part: electrons and holes. A PNP transistor works exactly like an NPN with every supply polarity reversed, so everything below is written for the NPN and you may flip the signs for a PNP.

The three regions are not built alike

Memorise that order — heavy, light, moderate — because the paper asks it directly, and it also offers “heavy, moderate, light” as a plausible-looking distractor. And note the separation between doping and size: the collector is the biggest lump of silicon but it is not the most heavily doped; the emitter is.

Transistor action, and why the base must be thin

Bias an NPN so that the base-emitter junction is forward biased and the base-collector junction is reverse biased. Now follow one electron.

1. The forward-biased base-emitter junction has its depletion layer collapsed, so the heavily doped emitter floods electrons into the base.
2. Once in the base those electrons are minority carriers, in P-type material full of holes. They ought to recombine and leave as base current.
3. But the base is only a fraction of a micrometre thick and has very few holes in it. Before an electron finds a hole, it has wandered into the reach of the base-collector junction.
4. That junction is reverse biased, and its field points exactly the way to sweep an electron across into the collector. Reverse bias blocks majority carriers, but it accelerates minority carriers — which is what these electrons now are.

The result: 98 to 99.5 per cent of what the emitter injects arrives at the collector, and only a small remainder leaves through the base. Thicken the base or dope it more heavily and more electrons recombine on the way, the base current rises, the collector current falls and the gain is destroyed. The base must be thin and lightly doped.

The current relations

Everything that leaves by the base and the collector must have entered by the emitter. Kirchhoff's current law, applied to a three-terminal device:

So the emitter current is always the largest of the three. The two gain figures are simply ratios taken from that equation:

Alpha is always a little less than one — typically 0.95 to 0.995 — because the collector can never receive more than the emitter sent. Beta, the current gain of the ordinary common-emitter stage, is typically 20 to 500 and varies enormously between samples of the same part number and with temperature. Design that depends on a precise value of beta is bad design.

Worked example

A transistor passes 20 µA of base current and 2 mA of collector current.

Convert first: 20 µA = 0.02 mA.

Ie = Ib + Ic = 0.02 + 2 = 2.02 mA

β = Ic ÷ Ib = 2 ÷ 0.02 = 100

α = Ic ÷ Ie = 2 ÷ 2.02 = 0.990

Notice how close alpha is to unity, and how much larger beta is. If a multiple-choice answer for alpha is greater than 1, it is wrong on sight.

Biasing

A transistor on its own does nothing useful with a signal. Bias is the steady DC you apply to set a no-signal operating point — the quiescent point, or Q-point — so that the incoming AC signal has room to swing both up and down without running out of transistor at either end. Bias it too low and the negative half of the signal is clipped at cut-off; too high and the positive half is clipped at saturation. The purpose of biasing is to set a steady operating point so the signal is amplified without distortion. That is close to the examiner's own wording.

Saturation and cut-off are useless for an amplifier and perfect for a switch. Which of the three you are in is decided entirely by the bias.

Fixed bias versus potential-divider bias

Fixed bias is one resistor from the supply to the base. It sets the base current, and then leaves beta to decide the collector current: Ic = β × Ib. Since beta varies by a factor of three between samples and climbs as the device warms, the operating point wanders. Cheap, and unstable.

Potential-divider bias uses two resistors to fix the base voltage and adds a resistor in the emitter lead. The divider is made stiff enough that the base sits at a fixed voltage whatever the transistor does; the emitter then sits about 0.7 V below that; and the emitter current is simply that voltage divided by the emitter resistor. Beta hardly enters into it. This is why potential-divider bias is the standard arrangement in almost every amplifier you will meet.

Thermal runaway, and the resistor that stops it

Recall the negative temperature coefficient from the diode lesson. A warm transistor passes more collector current. More collector current means more power dissipated in the device, which makes it warmer, which makes it pass more current. That loop is thermal runaway, and left alone it ends with a destroyed transistor.

The emitter resistor Re breaks it. Rising emitter current raises the voltage at the emitter. The base is held at a fixed voltage by the divider, so the voltage across the base-emitter junction — which is what actually controls the current — falls. Less forward bias, less current, and the loop is self-correcting. This is negative feedback doing a DC job, and it is the single most important reason for that resistor being there.

The three configurations

One terminal is common to both input and output, and its name gives the configuration.

Why does common emitter invert? A rising base voltage increases collector current, which increases the drop across Rc, which pulls the collector down. Note also that “an amplifier always inverts” is not a safe rule — the other two configurations do not. And the emitter follower, with a voltage gain of less than one, amplifies no voltage at all; what it gives you is high input impedance with low output impedance, which is precisely the definition of a buffer. That is the property the oscillator lesson needs.

The transistor as a switch

Drive the base hard enough and the transistor saturates: collector to emitter becomes a few tenths of a volt, effectively a closed switch. Remove the base drive and it cuts off: an open switch. Held anywhere between the two it behaves as a variable resistor, and that is how it amplifies.

The FET and the MOSFET

The field-effect transistor does the same job by a different mechanism. Instead of a base current, a voltage on the gate produces an electric field that widens or narrows a conducting channel between the source and the drain. The gate is either a reverse-biased junction (a JFET) or a metal plate insulated from the channel by a layer of oxide (a MOSFET), so in both cases it draws essentially no current.

A JFET is a depletion-mode device: with zero volts on the gate the channel is open and the FET is normally on, and reverse gate bias pinches the channel off. That is the opposite of a BJT, which needs bias before it conducts at all. Enhancement-mode MOSFETs are normally off and need gate voltage to turn on.

FETs are preferred in receiver RF front ends and mixers for two reasons the exam likes. First, the very high input impedance barely loads the tuned circuit feeding it, so the selectivity of that circuit is preserved. Second, a FET's transfer characteristic is close to a square law, which produces far fewer unwanted mixing products than a BJT does — so a FET mixer generates less cross-modulation and copes better with a strong signal on a nearby frequency. The dual-gate MOSFET, with the signal on one gate and AGC on the other, was the standard receiver front end for a generation and is still common.