6.070 Transistors Dr. James Bales Transistors: Transistors are at the heart of the electronics revolution of the past seven decades. Transistors are three-terminal devices that are typically used either as electronic switches or as amplifiers. As a switch, a transistor allows an electrical signal at one of its three terminals to either permit or prohibit the flow of current into a second terminal and out of the third. As an amplifier, the signal at one terminal is presented in an amplified form at another terminal. There are two major classes of transistors—bipolar junction transistors (BJT’s or bipolar) and field effect transistors (or FETs). We will consider only bipolar transistors in this subject1. There are two types of bipolar transistor, they are called NPN and PNP. The letters refer to how the devices are constructed. We will consider both types. You can use an NPN and a PNP together to form a complementary pair2. A bipolar transistor is a three-terminal device, that is, it has three connections to a circuit, rather than the two of resistors or diodes. The three terminals are called the emitter, collector, and base. Figure 1 shows the schematic symbol for the NPN and PNP transistors, with the leads labeled. Fig 1 Schematic symbols for NPN (L) and PNP (R) transistors. The bipolar transistor tries to follow two rules (analogous to the golden rules of the op-amp). 1) 2) The base-emitter junction acts like a diode, with forward voltage of 0.7 V, and The transistor will adjust the voltage difference from collector to emitter so that the collector current (IC) will equal a constant (b) times the base current (IB), that is, Eqn. 1 IC = β × I B Note the arrowhead on the schematic symbols. It is meant to show you the direction of the baseemitter diode, and it is reversed between the NPN and the PNP transistors. The parameter b (sometimes called hFE) is typically between 20 and 200 or so. Many characteristics are reversed between NPN and PNP transistors, from here on we note the PNP performance in parenthesis. The bipolar transistor is a current amplifier, that is, a small current flowing into (out of) the base of the NPN (PNP) transistor, controls the much larger current--by a factor of b--into (out of) its collector. Rule 2 can fail if the circuitry attached to collector is incapable of supplying (consuming) that much current to (from) the NPN (PNP) transistor. 1 An important class of FETs are the Metal-Oxide-Semiconductor (MOS) FETs, or MOSFETs. MOSFETS also come in two types, called n-channel and p-channel. You can combine them into a complementary pair of MOSFETs, making what is called a CMOS device. 2 Page 1 of 4 6.070 Transistors Dr. James Bales Transistor Man The book, The Art of Electronics, by Horowitz and Hill, is a detailed electronics textbook designed for Physics undergraduates. They introduce the “Transistor man” model of the transistor: “Transistor Man” sits inside the transistor and measures the base current. He then adjusts a variable resistor that lies between the collector and the base and tries to adjust that resistance so that IC = β × I B . (I think that, in the language of the Transistor People, “b ” is spelled, “hFE”.) The maximum value of that potentiometer is effectively infinite. There is a minimum value, at which point the transistor is letting the maximum possible current flow between collector and emitter. When a transistor is in that state (i.e., maximum possible ICE) we say the transistor is “fully on” (aka saturated), or that it “acts like a closed switch”. If we only use a transisor as “fully on” or “fully of” we are using it as a switch. Digital electronics only uses transistors in this way Alternatively we can use the transistor in the region where it gives current amplification. Analog electronics usually use transistors in this way, but will also use them as switches as well. Page 2 of 4 6.070 Transistors Dr. James Bales An Example Suppose I’m building a night-light that, when the room is too dark, turns on an incandescent light bulb (instead of an LED). Let us assume that my comparator gives low voltage when the room is lit and a high voltage when it is dark. Table 1 shows the specifications of my components. Table 1. Specifications of components for example. Comparator Light Bulb* High = 5 V Voltage Voltage Required 4-6 Volts Low = 0 V 150 mA (at 3 V) Source 20 mA (at 5 V) 200 mA (at 4 V) Current Capability Current Required 250 mA (at 5 V) Sink 20 mA (at 0 V) 300 mA (at 6 V) * You might notice that my “light bulb” acts like a 20 Ohm resistor. This is a bad model of a real light bulb! I’d like to connect the output of the comparator directly to the light bulb, unfortunately, I can’t. Why? Because the voltages match, but the currents don’t. The comparator can only provide some 30 mA - the bulb needs 250 mA at 5 V. The way out of this bind is to partition the problem into three parts. First is the sensing part, second is the deciding part (the comparator), whose job is to take the sensor output and turn it into a clear, unambiguous, but relatively weak (meaning limited ability to drive current), electrical signal. Last is the actuating part, the light bulb and a transistor that can take the unambiguous but weak signal from the comparator and use it allow or prohibit current from flowing through the light bulb. The circuit is shown in Figure 2. Deciding Circuit +5 V 10k +5 V +5 V +5 V Sensing Circuit + - C 1.1k B E 1k Actuating Circuit Figure 2. Comparator circuit to drive bulb. Page 3 of 4 6.070 Transistors Here we use an NPN transistor as our electrical switch, its specifications are in Table 2. Let us analyze the circuit. We begin by noting that the emitter is grounded, as is commonly done with NPN transistors. Then, by Rule 1, the base must be at 0.7 V when the comparator output is high. 3 Dr. James Bales Table 2. Transistor Specifications Example Specifications of Transistor 50 b VSATCE 0.3 V Maximum Power Dissipation 0.5 W We now know the voltage at the ends of the 1.1 kW resistor, and can calculate the current through it from Ohm’s law, namely, I B = ΔV RB = 4.3V /1100Ω ≈ 4mA . Eqn. 2 With IB known, we can then calculate the collector current, which is also the current passing through the light bulb, as simply b ´ IB = 50 ´ 4 mA = 200 mA. This will light the lamp. Finally, let us see where the voltage drops occur. The lamp has a 4 V drop across it when 200 mA passes through it, so the remaining 1 V must appear between the collector and emitter of the transistor (VCE). Because there is a 1 V drop from collector to emitter, and 200 mA flow into the collector and out of the emitter, the transistor dissipates 1 V x 0.2 A = 0.2 W, a significant fraction of the power rating of the transistor. This usage of the transistor is called a low-side switch. In practice, we would make the base resistor much smaller than 1,000 Ohms. That way, the transistor would try to get even more current to flow through the light bulb, and it would do so by reducing the voltage drop from collector to emitter as low as it possibly can. We call this saturation, and when we use transistors as switches, we try to operate them in saturation when we want them to look like a closed switch. PNP Transistors Figure 3 shows a sample PNP circuit that turns current on or off through the 220 W resistor. Note that here the current flows out of the base, and out of the collector. This is known as a high-side switch. +5 V E RB B C 220 The logic here is the reverse of the NPN, that is, applying a high voltage to the base resistor (i.e., > 4.3 V) turns the transistor off, while lowing that voltage below 4.3 V turns the transistor on more and more. Figure 3. A PNP transistor as a high-side switch. 3 This shows why we need the base resistor, RB, for without it, the comparator would try to hold the base at 5 V while the transistor would try to pull the base voltage down to 0.7 V. If you tried this, most likely the comparator would lose and the magic blue smoke might come out of it. Page 4 of 4