ECE344_3 - Transistors Transistors are three-terminal...

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Transistors Transistors are three-terminal devices . Essentially, they are resistors (so, devices with two terminals, say ‘1’ and ‘2’) whose resistance can be varied over a large range of values by means of a third terminal (say ‘3’), as shown in the Fgure below. 1 2 3 R controlled by V 3 0 2 4 6 8 10 0 2 4 6 8 10 V 3 = 1 V V 3 = 2 V V 3 = 3 V V 3 = 4 V I V 2 – V 1 While in the right frame of this Fgure we have shown the linear characteristics of a perfect resistor, usually transistor have non linear characteristics, as seen in the right frame of the Fgure below. The usefullness of transistors lies in two possible functions they can play: Amplifers and switches . To see how this works, lets consider the simple circuit in the Fgure below. In a Feld-e±ect transistor terminals ‘1’, ‘2’, and ‘3’ are called source , drain , and gate , respectively. In a bipolar junction transistor they are known as emitter , collector , and base contacts. ECE344 ²all 2009 150
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V ext R i D =f(v D ) v D + I D V D V ext /R V ext i D v D A two-terminal device is assumed to have nonlinear characteristics, that is, the current i d fowing acorss the device varies with the voltage v d applied to the two terminals oF the device in some complicated way, i D = f ( v D ) , as we see at right above. IF we place the device in the circuit shown in the ±gure, in series with a resistor R and a battery supplying a voltage V ext , we can write the equation expressing the toltal voltage drop in the ‘loop’: V ext = Ri D + v D . (233) This equation is not su²cient by itselF to determine the actual current I D fowing in the circuit, because we have two unknowns, i D and v D and only one equation. So, we must supplement Eq. (233) with the equation describing the characteristics oF our nonlinear device, i D = f ( v D ) . Now we have two equations in two unknowns. It is instructive to solve the problem graphically: The right Frame oF the ±gure above shows the i D - v D characteristics oF the device crossing the ‘load line’, that is, the i D - v D straight line corresponding to the circuit equation, Eq. (233). The intersection oF the two curves clearly corresponds to the solutions, I D and V D , giving us the current fowing in the circuit and the voltage drop across the nonlinear device. Let’s now consider a transistor in the same con±guration, but with an extra battery supplying voltage to the ECE344 ³all 2009 151
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third terminal (labeled G for gate , word whose meaning we shall see later) which controlls the resistance (or, more generally, the characteristics) of the device. This situation – shown in the Fgure below – will be similar, the main di±erence being that now we have a full set of characteristics, i D = f ( v D ,v G ) , parameterized by the ‘gate bias’ v G . ²or a given voltage v G (=2.0 V in the example shown in the Fgure), we Fnd the current and voltage across the transistor, I D and V D , by looking at the intersection between the load line and the device characteristics.
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This note was uploaded on 09/13/2010 for the course ECE ECE344 taught by Professor Polizinni during the Spring '10 term at University of Massachusetts Boston.

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ECE344_3 - Transistors Transistors are three-terminal...

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