Electronics 2a - Bipolar junction transistors

Question 1

What are the two basic functions of transistors?

Answer 1

1. Signal amplifiers: transistors can control its output in proportion to its input
2. Switches: can be used to turn current on or off in a circuit

Question 2

What does BJT stand for?

Answer 2

Bipolar Junction transistor (BJT)

Question 3

What are the two types of BJTs?

Answer 3

• pnp transistor
• npn transistor

Question 4

How does an npn transistor (BJT) work?

Answer 4

We call each section of the npn something:
* top n = collector region (lightly doped)
* middle p = base region (lightly doped and narrow)
* bottom n = emitter region (heavily doped)

What happends when applying a current to base region?
* The current across the base drags away some electrons and there is a forward bias between the np junction at the bottom, and a reveresed bias at the np junction at the top
* Then while some electrons are remevoved at the base, sudden build-up of electrons at the reversed bias section (np junction at the top) allow electrons to jump the large depletion region to the holes on the other side and they are quickly expelled round the circuit.
* Therefore a small current at the base leads to a large flow of current across the collector region. The emitter current, IE = IC + IB (Collector current + base current). Although usually IB is so small we can say IE ≈ IC

We usually can work out the value of the collector current, IC based on the Base current, IB:
* Usually, Ic = (100) IB
* This value is usally 100 (the gain) but can be larger.

**Note: **
* An npn or pnp transistor can only act as an amplifier in the linear activation region, anything higher than it is saturated and collector current is fixed at a specific current.

The symbol for an npn transistor is shown, as well as an pnp transistor.

Question 5

How can we configure a npn BJT transistor into a voltage amplifier for both DC and AC signals?

Answer 5

For DC:
* We set the circuit up as shown.
* By supplying a base current that can change within the active region (where no clipping occurs, that is less than V+). We cause an increase in collector current, that causes a bigger voltage drop across Rc.
* Therefore, as IB increases, so does IC by the gain factor, causing bigger drops in Vout as voltage is taken away at higher levels in the Rc resistor. This means we get a inverted output compared to the input.
* AC doesn’t work in this circuit, as for the circuit to work, the np junction at the bottom region of the npn BJT must be forward bias, and by supplying negative current we then reverse bias it. Therefore, we can’t have negative violtages which are found in AC.

For AC:
* To supply AC signals into an npn BJT, we must ‘bias’ the circuit. We add another resistor, Rb which will feed some of the voltage supply back into the npn BJT, (usually bias is half the voltage supply, so rather than the signal waveform oscilating around 0, it is now oscillating around Vc / 2, so it shouldn’t go negative).
* We can work out the resistance of Rb, by understaning that for a forward bias we drop 0.7v across that np junction in the npn BJT, thus resistance, Rb = (Vc - 0.7) / IB, where we can find IB, by understanding its gain compared to IC.
* We can work out the ristance of Rc, by also using IC, which we can just guess for now, Rc = (Vc / 2) / IC.
* The two (high reactance )capacitors are coupling capactitors (act as high-pass filters).
* The capacitor close to the input makes sure that only the AC signal is entering the npn BJT, so the bias isn’t thrown off, this is because capactiors only allow voltage flow through when current is changing.
* The capacitor at the output does a similar function, and removes the bias voltage signal, vc at the output before it becomes Vout.
* The output signal will be higher gain than the input, but will be inverted

Note:
* If Vout > Vc (supply voltage): then clipping or saturation will occur where current signal can’t exceed that signal and we get distortion.
* This arraingement is called a common-emitter amplifier