# ECE344_3 - Transistors Transistors are three-terminal...

This preview shows pages 1–4. Sign up to view the full content.

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

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
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
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.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

## 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.

### Page1 / 96

ECE344_3 - Transistors Transistors are three-terminal...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online