# Difference between revisions of "Transistors"

### Reference Material

• Horowitz & Hill, The Art of Electronics, 2nd Ed., Ch. 2

## Transistor Types

There are two broad classes of transistors: Bipolar Junction Transistors (BJTs), which are often used in discrete analog circuits and can typically provide higher gain over wider bandwidths, and Field Effect Transistors (FETs), which are sometimes used in front-end amplifiers because of their lower noise figures (even though they typically provide less gain over narrower bandwidths than BJTs), and are often used to act like a voltage-controlled switch, such as they do in almost all digital processors today.

### Bipolar Junction Transistors

BJT transistors have 3 terminals: the emitter, the base, and the collector. Broadly speaking, a current to/from the base (for NPN/PNP-type transistors, respectively, as described below) is used to control the flow of charge from the collector to the emitter. As long as a couple of rules are followed, the behavior of BJTs is pretty straight-forward. These rules are different for NPN and PNP transistors.

For an NPN transistor, the rules are:

An NPN bipolar junction transistor

1. In order to conduct, the voltage difference between the collector and the emitter (${\displaystyle V_{CE}}$) must be above a certain threshold (say, ${\displaystyle V_{CE}>0.2V}$).
2. In order to conduct, the voltage difference between the base and emitter (${\displaystyle V_{BE}}$) must be above a certain threshold (say, ${\displaystyle V_{BE}>0.7V}$).
3. Once it is conducting, the current flowing from the base (${\displaystyle I_{BE}}$) causes a current to flow from collector to emitter (${\displaystyle I_{CE}}$), amplified by a factor ${\displaystyle \beta \equiv h_{fe}}$.

Hence,

${\displaystyle I_{CE}=\beta I_{BE}.\,\!}$

${\displaystyle \beta }$ is often in the range of 60-100, but can vary a lot from part to part.

For PNP transistors, the rules are similar, except that the emitter is now must be at a higher voltage than the collector, and the base must be at a lower voltage than the emitter:

A PNP bipolar junction transistor

1. ${\displaystyle V_{EC}}$ must be above, say, 0.2 V, to conduct.
2. I${\displaystyle V_{EB}}$) must be above, say 0.7 V.
3. ${\displaystyle I_{EC}=\beta I_{EB}}$.

The trick to designing circuits using BJTs is to use these 3 rules to effectively regulate when and how much the transistor conducts, based on a signal applied to the base. Because ${\displaystyle \beta }$ varies so much between transistors, it is considered poor practice to rely on ${\displaystyle \beta }$ being a particular value. Rather, as is discussed in more detail with regard to amplifiers, it is better to use resistors and capacitors in circuits that regulate gain to a value less than ${\displaystyle \beta }$ on the basis of the first two rules.

## Field Effect Transistors (FETs)

A FET

All FETs have gate, drain, and source terminals that correspond roughly to the base, collector, and emitter of BJTs. FETs have a very high input resistance, on the order of 100M${\displaystyle \Omega }$ or more. This makes it effectively a voltage-controlled device, with a high degree of isolation between input and output. FETs have n-channel and p-channel varieties, which are analogous to the NPN and PNP types of bipolar junction transistors.

${\displaystyle I_{DS}}$ as a function of ${\displaystyle V_{DS}}$, for various (linearly spaced) values ${\displaystyle V_{GS}}$

FETs have essentially three operating modes that relate to a threshold voltage ${\displaystyle V_{t}}$:

1. Cutoff: ${\displaystyle V_{GS} causes no charge to flow from drain to source.
2. Ohmic: ${\displaystyle V_{GS}>V_{t}}$ and ${\displaystyle V_{DS}\leq V_{GS}-V_{t}}$ causes the FET to behave like a resistor whose value is controlled by ${\displaystyle V_{GS}}$.
3. Active: ${\displaystyle V_{GS}>V_{t}}$ and ${\displaystyle V_{DS}>V_{GS}-V_{t}}$ causes the FET to conduct, with the current ${\displaystyle I_{DS}}$ being independent of ${\displaystyle V_{DS}}$, but sensitive to ${\displaystyle V_{GS}}$.

## BJT and FET Applications

### Amplification

Both BJTs and FETs are commonly used to amplify signals. The details of amplifier design are discussed in another section, but the principle is to use a low-amplitude signal to control to a “valve” through which a lot of charge is able to flow.