1. 10/4/2004EE 42 fall 2004 lecture 151 Lecture #15 Basic amplifiers, Intro to Bipolar transistors Reading: transistors (chapter 6 and 14)

2. 10/4/2004EE 42 fall 2004 lecture 152 Topics Today: Examples, circuit applications: Diode circuits, Zener diode Use of dependent sources Basic Amplifier Models

3. 10/4/2004EE 42 fall 2004 lecture 153 NODAL ANALYSIS WITH DEPENDENT SOURCES Finding Thévenin Equivalent Circuits with Dependent Sources Present Method 1: Use V oc and I sc as usual to find V T and R T (and I N as well) Method 2: To find R T by the “ohmmeter method” turn off only the independent sources; let the dependent sources just do their thing. See examples in text (such as Example 4.3) and in discussion sections. Pay most attention to voltage-dependent voltage sources and voltage-dependent current sources. We will use these only.

4. 10/4/2004EE 42 fall 2004 lecture 154 NODAL ANALYSIS WITH DEPENDENT SOURCES Example : Find Thévenin equivalent of stuff in red box. With method 2 we first find open circuit voltage (V T ) and then we “measure” input resistance with source I SS turned off. Verify the solution: I SS R 3 V a +  A v V cs R 6 V c R 2

5. 10/4/2004EE 42 fall 2004 lecture 155 EXAMPLE: AMPLIFIER ANALYSIS USING THE AMPLIFIER MODEL WITH R i = infinity: A: Find Thévenin equivalent resistance of the input. B: Find the Ratio of the output voltage to the input voltage (“Voltage Gain”) Method: We substitute the amplifier model for the amplifier, and perform standard nodal analysis You find : R IN and V O /V IN +  +  V0V0 AV 1 +  V1V1 RiRi Circuit Model in linear region AMPLIFIER MODEL +  A V-V- V+V+ V0V0 RFRF RSRS V IN

6. 10/4/2004EE 42 fall 2004 lecture 156 EXAMPLE: AMPLIFIER ANALYSIS USING THE AMPLIFIER MODEL WITH R i = infinity: How to begin: Just redraw carefully! Method: We substitute the amplifier model for the amplifier, and perform standard nodal analysis Verify the solution: R IN = V O /V IN = +  AV 1 - + V1V1 V-V- V+V+ V0V0 RFRF RSRS V IN +  A V-V- V+V+ V0V0 RFRF RSRS

7. 10/4/2004EE 42 fall 2004 lecture 157 Bipolar transistors Bipolar transistors are made from two PN junctions that are very close together. The name bipolar comes from the fact that both carrier types play roles in its function The connection to the middle slice, called the base, can control the current without having to supply much itself NPN

8. 10/4/2004EE 42 fall 2004 lecture 158 Symbol The symbol for an NPN transistor is: B E C Most of the current in an bipolar transistor flows between the collector and emitter, but the amount of current is controlled by the base-emitter junction. In an NPN transistor, the current is conducted by electrons moving from the emitter (emitter of electrons) to the collector (collector of electrons)

9. 10/4/2004EE 42 fall 2004 lecture 159 Reversed biased Base-Collector junction To understand how the transistor works, lets just look at the base- collector junction under a reverse bias. The collector is doped more lightly than the base, so to balance the charge, the depletion extends further into the collector than it does into the base. The electric field is as shown, holding back the electrons and holes from the junction electrons holes collector base

10. 10/4/2004EE 42 fall 2004 lecture 1510 Electrons in the base & depletion region What would happen if we were to “beam in” some electrons into the base? –They would be swept into the collector by the electric field. If we were to inject these electrons by absorbing light, this device would be a light detector. (Photodiode) If we inject these electrons by another junction, it is a NPN bipolar transistor electrons holes collector base - -

11. 10/4/2004EE 42 fall 2004 lecture 1511 Minority carriers diffuse Outside the depletion region, there is no electric field, so the carriers move by diffusion only. Diffusion is just the name for the effect that happens when randomly moving particles spread from areas where they are concentrated to areas where there are fewer of them.

12. 10/4/2004EE 42 fall 2004 lecture 1512 Injection of minority carriers To see how we would inject minority carriers into a region, consider a forward biased junction base Emitter If we forward bias a PN junction, then the internal field is reduced, holes are injected into the N side, and electrons are injected into the P side. Since we only desire to have electrons injected into the base, we heavily dope the emitter so that most of the current comes from electrons going from the emitter into the base, rather than from holes going into the emitter

13. 10/4/2004EE 42 fall 2004 lecture 1513 How a bipolar transistor works So a one sentence description of how a bipolar transistor works is: A forward biased junction injects minority carriers which can then go through a nearby reverse biased junction.

14. 10/4/2004EE 42 fall 2004 lecture 1514 Gain How does a bipolar transistor act as an amplifier? The Emitter-Base junction current is just the current of a PN diode, i.e. the voltage from the base to the emitter will give a large current as soon as the voltage exceeds.7 volts. But since most of the carriers (99%) go to the collector, the base only needs to deliver 1% of the total current in order to control the voltage (and therefore the current)

15. 10/4/2004EE 42 fall 2004 lecture 1515 Electron flow So the forward bias on the emitter-base junction induces the electrons to flow, but most of them make it across to the collector instead of stopping in the base and flowing to the base terminal Collector Base Emitter

16. 10/4/2004EE 42 fall 2004 lecture 1516 Device model As long as the Base-collector junction is reverse biased, and the Emitter-base junction is forward biased, a good model of the NPN transistor is: Emitter Collector Base

17. 10/4/2004EE 42 fall 2004 lecture 1517 Modes of operation Cut-off: If the Emitter-base junction (the one controlling the current) is not forward biased, then the transistor is said to be in cut-off. A small amount of current will still flow, usually negligible Saturation: If the Base-collector junction sees so much current flow that it is no longer forward biased, then the device will no longer behave as described. Breakdown: If a high enough voltage is applied, the transistor junctions will break down, and a high current can flow.

18. 10/4/2004EE 42 fall 2004 lecture 1518 IV curve Since the transistor is a three terminal device is a three terminal device, you might think that 6 variables would be important: Vbc – the voltage between the base and the collector Vbe – the voltage between the base and the emitter. Vce- The voltage between the collector and the emitter. Ib- the current into the base. Ie- the current into the emitter. Ic- the current into the collector. But the transistor has no net charge, so I b +I e +I c =0 And of course if you know any two of the voltages you can calculate the third.

19. 10/4/2004EE 42 fall 2004 lecture 1519 Transistor circuit configurations Typically we will want to use the transistor as a device which has an input and an output. Since one of the terminals must be shared, we call that a common terminal The voltages with respect to the common terminal are then used to describe the operation of the transistors There are three types of connections: –Common emitter, –Common collector, –Common base

20. 10/4/2004EE 42 fall 2004 lecture 1520 IV curve for common emitter To show the IV curve for a NPN transistor in a common emitter configuration, we plot the voltage from the collector to the emitter V ce vs the current from the emitter I c The base current is shown by setting several values and then plotting a curve for each of them (called steps) IcIc V ce Saturation Forward Active Breakdown Cutoff