b b Fig. 1 Transistor symbols

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1 TRANSISTORS Transistors have three terminals which are referred to as emitter (e), base (b) and collector (c). Fig 1 shows the symbols used for the two types of transistors in common use. c c b b e e npn type pnp type Fig. 1 Transistor symbols Both types operate in a very similar way but differ in the polarity of the power supply required to power them. We shall concentrate our study and practical investigations upon npn types. Operation A transistor is a device that amplifies current; it should not be confused with the operational amplifier that amplifies voltage. A small current in the base is amplified to produce a larger current in the collector. The amplification or gain of the transistor is dependant upon the type used. A transistor can be considered as a diode that has a method of current control from the base. As with a diode a 'turn-on' voltage of approximately 0.6V is required between the base and the emitter to allow any collector current to flow. 1

2 CURRENT FLOW THROUGH A TRANSISTOR The arrow on the emitter shows the direction in which conventional current can flow through a transistor. In our switching circuits, the emitter will be connected to the line. There are two current paths through the transistor. They are from base to emitter (I B ), and from collector to emitter ( ). I B I E = I B + Fig. 2 SWITCHING ACTION OF A TRANSISTOR Fig 3 shows a circuit where a transistor is being used as a switching device. +V CC LAMP (load) INPUT (from sensing unit) OUTPUT V in Fig. 3 Transistor switch Current can only flow through the output transducer in the collector circuit when a small current flows in the base circuit. A small base current is used to switch a much larger current in the collector circuit. The voltage provided by an input signal sensing unit can be used to drive the small base current. Resistor protects the transistor by limiting the base current. Let us now consider what happens as the voltage provided by an input sensor increases from. There are three regions to consider: 2

3 REGION 1: CUT-OFF (V in <0.6V) The base-emitter junction of a transistor is very similar to a diode. It does not turn on until a voltage of 0.6V is applied across it. If the signal from the input sensor is less than 0.6V, no base current flows and the transistor is cut-off. In Fig 4 the load has been represented by a resistor and V out is the voltage at the collector of the transistor. No current flows through the collector load resistor, R C, and the collector will be at the supply voltage of +V CC. +V cc +V cc = 0 = h FE I B R C R C V out = V CC I B = 0 I B = Vin V Fig. 4 Transistor cut-off (Vin <0.6V) Fig. 5 Transistor partly on (Vin >0.6V) REGION 2: LINEAR REGION When the signal provided by the input sensor reaches 0.6V current will start to flow in the base circuit. The transistor behaves like a current amplifier and, over a limited range, the collector current ( ) and the base current (I B ) are linked by the equation: = h FE x I B where h FE is the DC current gain of the transistor. The value of h FE varies from transistor to transistor and from type to type. It is usually in the range Voltage across = V in I B = V in = h FE x I B V out = V CC - I c X R C 3

4 The equation shows that as V in increases, the value of V out decreases i.e. the transistor produces an inverting action. Over a small region, the value of V out falls linearly as V in increases and the transistor is partly on. REGION 3: SATURATION If we keep increasing the input voltage, a point will be reached where the collector current drops the full supply voltage across the load. The output voltage is then near and the transistor is said to be saturated. Further increase in input voltage will increase the base current but not the collector current. Fig. 6 illustrates the voltage transfer characteristics of a transistor. CUT OFF +V CC LINEAR V out REGION 0 SATURATION 0.6V V in Fig. 6 When the transistor is used as a switching device, the input voltage should change quickly between the cut-off and saturation regions. The higher the gain, h FE, the sharper the switching action. In practice you will find that the output voltage does not fall to when the transistor is saturated. A typical value would be about 0.1V. In our calculation shall assume an ideal transistor where V out falls to in saturation. Applying current at a junction rule: = I E + I B Over the linear region: = h FE x I B Since h FE is usually >100, we can assume that I E =. 4

5 POWER DISSIPATION IN A TRANSISTOR SWITCH Before we deal with applications of a transistor switch we must consider power dissipation over the three regions. (a) CUT-OFF In this case, no current flows through the transistor and so no power is dissipated. (b) LINEAR REGION Power dissipated in the base-emitter junction = 0.6 x I B ( and can usually be ignored) Power dissipated in the collector-emitter = V out x The latter will be at its maximum near the middle of the linear region. (c) SATURATION REGION If we assume that V out = 0 when the transistor is saturated, no power will be dissipated due to. The only power dissipated will be 0.6I B. When a transistor is used as a switching device, the input voltage variation should be such that the transistor operates in the cut-off or saturation regions only. 5

6 CIRCUIT ANALYSIS EXAMPLE 1: The transistor shown has a current gain h FE of 100. Calculate the minimum value of V in to ensure saturation. 1k 9V V in 10k V out (a) Assume V out = 0 when transistor saturated. We can find the value of by applying Ohm s law to the 1k load resistor. = 9/1 = 9mA I B = /h FE = 9/100 = 0.09mA V in = I B = (0.09 x 10) = +1.5V 6

7 EXAMPLE 2: Calculate a suitable value for to ensure saturation. Assume h FE = V When saturated, = 60mA 6V, Min value of I B = 60/200 = 0.3mA 0.06A V in = I B V = = 2.4 = 8 kω A 6.8kΩ resistor could be used. SWITCHING INDUCTIVE LOADS If the load in the collector circuit is inductive e.g. a relay coil, a high voltage is induced, especially at switch off. The induced voltage could damage the transistor. In such a case, a diode is used to short circuit the induced voltage. V CC Relay coil 7

8 DARLINGTON PAIR The current gain h FE of a transistor will be in the range Much higher gains can be obtained using two transistors connected together to form a Darlington Pair. 1 I B1 TR1 2 TR2 I E1 = I B2 Fig. 8.7 Darlington pair Since two transistors are used, the turn-on voltage will be 1.2V If the transistors are operated in their linear regions: 1 = h FE1 x I B1 Since collector and emitter currents are almost equal: For transistor TR2: I B2 = I E1 = h FE1 x I B1 2 = h FE2 I B2 = h FE2 x h FE1 x I B1 Since 2 >> 1, the overall gain is given by: Current gain = 2 = h FE1 x h FE2 I B1 The transistors used could be an identical pair, but more often than not, TR2 is of a higher power rating than TR1 e.g. a BC108 could be used for TR1 and a BFY51 for TR2. The higher gain increases the input resistance of the unit. 8

9 Example In the following diagram Transistors TR1 and TR2 have current gains of 100 and 50 respectively. 680Ω +6V 15k TR1 TR2 V in What minimum value of input current is required to drive the Darlington pair into saturation?. ma What minimum value of input voltage will drive the Darlington pair into saturation? Answer to nearest 0.01V. +. V 9

10 TRANSISTOR PACKAGES All transistors have three connection points but their appearance can vary depending upon their power handling and current carrying capacity. Fig 8.8 shows examples of packages in common use. E-line TO18 & TO39 TO3 Fig.8 Transistor packages E-line transistors are encapsulated in a plastic case. The ZTX651 is a popular choice in this range. They can be used to switch load currents of up to 2A and can dissipate power up to 1W. Power can be dissipated through the thick connecting leads fitted on the device. The BFY51 transistor is enclosed in the large TO39 metal case. It can handle larger currents and dissipate more power than the BC108 transistor which is enclosed in the smaller TO18 case. Power dissipation can be increased by fitting a heat sink on the case. Type TO3 packages are used for very high power switching applications. Transistors enclosed in such packages can switch current of up to 50A and are capable of dissipating 100W when mounted on a suitable heat sink. Fig.8 also shows how to identify the leads and connection points on transistors. Make certain that you can identify the leads when carrying out your practical exercise. 10