Lecture #15 Basic amplifiers, Intro to Bipolar transistors

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

2. Topics Today: Examples, circuit applications: • Diode circuits, Zener diode • Use of dependent sources • Basic Amplifier Models EE 42 fall 2004 lecture 15

3. NODAL ANALYSIS WITH DEPENDENT SOURCES Finding Thévenin Equivalent Circuits with Dependent Sources Present Method 1: Use Voc and Isc as usual to find VT and RT (and IN as well) Method 2: To find RT 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. EE 42 fall 2004 lecture 15

4. NODAL ANALYSIS WITH DEPENDENT SOURCES Example : Find Thévenin equivalent of stuff in red box. R V V a c 3 R 2 ISS R + 6 A V  v cs With method 2 we first find open circuit voltage (VT) and then we “measure” input resistance with source ISS turned off. Verify the solution: EE 42 fall 2004 lecture 15

5. Circuit Model in linear region AV1 Ri +  + + V1 V0   EXAMPLE: AMPLIFIER ANALYSIS USING THE AMPLIFIER MODEL WITH Ri = infinity: A: Find Thévenin equivalent resistance of the input. B: Find the Ratio of the output voltage to the input voltage (“Voltage Gain”) RF AMPLIFIER MODEL RS VIN V-  A V0 V+ + Method: We substitute the amplifier model for the amplifier, and perform standard nodal analysis You find : RIN and VO/VIN EE 42 fall 2004 lecture 15

6. RF RF RS RS VIN V- VIN V- +   A V0 V0 V+ + V+ AV1 - V1 + EXAMPLE: AMPLIFIER ANALYSIS USING THE AMPLIFIER MODEL WITH Ri = infinity: How to begin: Just redraw carefully! Method: We substitute the amplifier model for the amplifier, and perform standard nodal analysis Verify the solution: RIN = VO/VIN = EE 42 fall 2004 lecture 15

7. 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 N P N EE 42 fall 2004 lecture 15

8. Symbol • The symbol for an NPN transistor is: 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) B E EE 42 fall 2004 lecture 15

9. 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 collector electrons holes base EE 42 fall 2004 lecture 15

10. 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 collector electrons - - holes base EE 42 fall 2004 lecture 15

11. 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. EE 42 fall 2004 lecture 15

12. Injection of minority carriers • To see how we would inject minority carriers into a region, consider a forward biased junction base • 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 Emitter EE 42 fall 2004 lecture 15

13. 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. EE 42 fall 2004 lecture 15

14. 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) EE 42 fall 2004 lecture 15

15. 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 EE 42 fall 2004 lecture 15

16. 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: Collector Base Emitter EE 42 fall 2004 lecture 15

17. 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. EE 42 fall 2004 lecture 15

18. 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 Ib+Ie+Ic=0 • And of course if you know any two of the voltages you can calculate the third. EE 42 fall 2004 lecture 15

19. 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 EE 42 fall 2004 lecture 15

20. To show the IV curve for a NPN transistor in a common emitter configuration, we plot the voltage from the collector to the emitter Vce vs the current from the emitter Ic The base current is shown by setting several values and then plotting a curve for each of them (called steps) IV curve for common emitter Ic Breakdown Forward Active Cutoff Vce Saturation EE 42 fall 2004 lecture 15