7.6 Transistors

The transistor is a fairly complicated, yet essential part of most modern electronics systems. The word transistor comes from transfer of resistance, which we will discuss later in this unit.

There are two types of transistors: npn transistors, and pnp transistors. Npn transistors are made of two n-charged (negatively charged) layers around a thin p-charged (positively charged) layer, whereas a pnp transistor has a thin n-charged layer surrounded by two p-charged layers. The npn transistor is much more commonly used, so all transistors we discuss will be of this type.

The three layers in a transistor each have names, which indicate their function.

The "top" n-layer is the collector, the p-layer is the base, and the bottom n-layer is the emitter. The collector-base connection is forward-biased, while the emitter-base connection is reverse-biased. This means that in a transistor, current travels from the collector, through the base, to the emitter.

A transistor is similar to a faucet. When the base-emitter voltage, V_BE, is above a certain level, the transistor, or symbolic faucet, is turned on. This then allows a current (or stream of water) to flow from the collector to the emitter. V_BE, the voltage required to do this, is around 0.6 to 0.7 Volts. Without the 0.6 or 0.7 Volts, no current will flow, and the transistor is considered off.

There are three key parts to current involved in a transistor system: I_B, the current flowing in at the base, I_C, the current flowing in at the collector, and I_E, the current flowing out the emitter. These three quantities are related in a very simple, yet useful, equation:

More than 99% of the emitter current is made up of the collector current, while less than one percent of it is made up of the base current.

This leads us to an important transistor function: current amplification. Since the emitter current is a dependent amount, it varies with collector current and base current. In a transistor, the input always comes from the base, or the knob or valve on the faucet, which determines whether it is on or off. Because the collector current is about 100 times greater than the base current, the collector current is amplified by a large amount when the base current is only slightly increased. The ratio between the change in collector current and the change in base current is called the current gain of the transistor, and can be expressed thus:

Note: h_FE , the symbol for current gain in transistors, tells more than we might expect. The "F" stands for forward current, and "E" means the transistor is connected in the emitter mode (the only one we will be discussing). The fact that both the "F" and the "E" are capitalized indicates the current gain is in D.C..

For a common transistor - a silicon npn - h_FE is usually between 100 and 200. It is important to remember that this amplifying action is controlled by current, not voltage, and also that it is a ratio, and therefore has no units.

The transistor also has another, perhaps more important function: that of a switch. We know that the transistor is controlled by the input of the base, but more importantly, it is the base-emitter voltage that determines whether the transistor is on or off. It is possible to control this using a simple system of resistors

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In a series circuit, the ratio between the potential difference of different parts of the circuit is equal to the ratio between their respective resistances, or :

From this equation, we find:

The proportion of a resistance to its p.d. is the same throughout the circuit, which means that the greater the resistance for a certain part of the circuit, the greater the voltage (p.d.) for that part will be.

Let’s say that we want to design a circuit that turns a streetlight on when it gets too dark. We would need to make sure that the base emitter voltage is at least 0.7 V when it is dark. One way of setting this up is to make sure that the resistance R_2, is greater than the resistance R₁ in darkness. This way, V₂ (in this case V_BE) would be greater than V₁. Since only 0.7 Volts are required to turn on the transistor, it is therefore almost certain that whenever V₂ > V₁, the transistor will turn on.

However, in order not to waste electricity, we would want to make sure that the streetlight is not on during the day as well. This means that while it is light out, R₁ needs to be greater than R₂ in such a proportion that V₂ coulds not possibly reach 0.7 V. A resistor that changes its resistance with light would be needed to detect the point at which the streetlight should go on. Since Light Dependent Resistors (LDR’s) usually increase their resistance as it gets darker, we would want an LDR to be R₂. But we definitely want to be able to decide at which point the streetlights should go on, which means deciding at which point R₂ should become greater than R₁. We can do this by using a variable resistor, which will allow us to set a specific resistance for R₁. We should set the resistance of R₁ just under the resistance of R₂ when it is in darkness. When R₂ allows V₂ to be high enough, current can pass through R₃ to activate whatever result is desired, in this case, the streetlights.

Similar switch functions can be set up using other forms of resistors, such as a water-detecting device for a sprinkler system. Devices like these, that convert a non-electrical signal into an electrical one are called transducers. Another example is a motion sensor that can send a signal (via a transistor!) to start a burglar alarm.