10/6/2004EE 42 fall 2004 lecture 161 Lecture #16 Bipolar transistors Reading: transistors Bipolar: chapter 6 MOS: chapter 14

10/6/2004EE 42 fall 2004 lecture 162 Topics Today: Bipolar transistors IV curve Making an amplifier

10/6/2004EE 42 fall 2004 lecture 163 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

10/6/2004EE 42 fall 2004 lecture 164 Beta (β) and alpha (α) When the base-emitter junction is forward biased, and the base-collector junction is reverse biased, approximately a fixed portion of the electrons will make it across to the collector rather than coming from the base contact. The ratio current from the electrons that make it across to the total current is alpha I C =αI E Alpha can be close to one, 0.99 is not uncommon. Since I B +I C =I E  I B =(1-α)I E we define β=(1-α)

10/6/2004EE 42 fall 2004 lecture 165 Both of these definitions for the bipolar transistor are only approximately true, but for most bipolar transistors in the active mode, they are reasonable approximations.

10/6/2004EE 42 fall 2004 lecture 166 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

10/6/2004EE 42 fall 2004 lecture 167 Other 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.

10/6/2004EE 42 fall 2004 lecture 168 Currents and voltages The currents are labeled by the letter for the terminal they come into IBIB IEIE ICIC The voltages are labeled with a double subscript, with the subscripts referring to the two terminals the voltage difference is taken between: Example, the voltage difference between the collector and emitter leads is called V CE The voltage between the base and the emitter is called V BE

10/6/2004EE 42 fall 2004 lecture 169 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. Ic- the current into the collector. Ie- the current out of the emitter. But the transistor has no net charge, so I B +I C =I E And of course if you know any two of the voltages you can calculate the third. We generally use V BE and V CE

10/6/2004EE 42 fall 2004 lecture 1610 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

10/6/2004EE 42 fall 2004 lecture 1611 Common Emitter configuration The voltages are labeled with a double subscript, with the subscripts referring to the two terminals the voltage difference is taken between: Example, the voltage difference between the collector and emitter leads is called V CE The voltage between the base and the emitter is called V BE IBIB IEIE ICIC + V in - + V out - R

10/6/2004EE 42 fall 2004 lecture 1612 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

10/6/2004EE 42 fall 2004 lecture 1613 The NPN bipolar as a current amplifier The bipolar transistor is naturally a current amplifier, because the voltage V BE is pretty much clamped to.7 volts in the active mode of operation. As VBE moves slightly above 0.7 volts, the current gets very large If VBE is slightly below 0.7 volts, the current goes to zero Rather than trying to set VBE to a very precise value, we can just put in a current I B instead. The current from the collector is I C =βI B, so we amplify the input current by the factor β

10/6/2004EE 42 fall 2004 lecture 1614 The bipolar transistor as a voltage amplifier We can convert a voltage into a current by using a resistor, and we can also convert a current into a voltage, so we can make a voltage amplifier from a NPN transistor IBIB IEIE ICIC + V in - + V out - RCRC RBRB

10/6/2004EE 42 fall 2004 lecture 1615 Voltage amplification The current into the base I B is: And the current into the collector is: And if we have a 5 volt supply rail, the output voltage is:

10/6/2004EE 42 fall 2004 lecture 1616 Amplifiers Notice that when the input voltage goes up, the output voltage goes down (the voltage gain is negative This is a very common feature of single transistor amplifiers The input is referenced to the 0.7 volts of the turn on for the base-emitter diode, and must be higher than 0.7 volts. (Why?) The output is offset from the power supply voltage, and can not go higher than the power supply voltage. (Why?) Since the output is larger than the input, where does the power come from?

10/6/2004EE 42 fall 2004 lecture 1617 Biasing a transistor Setting up a transistor circuit so that it will amplify a voltage without it needing have a specific offset voltage, and producing an output referenced to a desired point instead of whatever you get in terms of an offset from the power supply, is called biasing a transistor. We will study biasing in chapter 8.

10/6/2004EE 42 fall 2004 lecture 1618 The bipolar transistor in a logic device Bipolar devices have also been used to make logic circuits: an example of a NOR gate: A B Output If A is below 0.7 volts, and B is also below 0.7 volts, then the output is near 5 volts if either A or B is high, then the output is pulled down