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Lecture #16 Bipolar transistors

Lecture #16 Bipolar transistors. Reading: transistors Bipolar: chapter 6 MOS: chapter 14. Topics. Today: Bipolar transistors IV curve Making an amplifier. Electron flow.

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Lecture #16 Bipolar transistors

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  1. Lecture #16 Bipolar transistors Reading: transistors Bipolar: chapter 6 MOS: chapter 14 EE 42 fall 2004 lecture 16

  2. Topics Today: Bipolar transistors IV curve Making an amplifier EE 42 fall 2004 lecture 16

  3. 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 16

  4. 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 IC=αIE Alpha can be close to one, 0.99 is not uncommon. • Since IB+IC=IE IB=(1-α)IE • we define β=(1-α) EE 42 fall 2004 lecture 16

  5. 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. EE 42 fall 2004 lecture 16

  6. 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 16

  7. 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. EE 42 fall 2004 lecture 16

  8. Currents and voltages • The currents are labeled by the letter for the terminal they come into 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 VCE The voltage between the base and the emitter is called VBE IC IB IE EE 42 fall 2004 lecture 16

  9. 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 IB+IC=IE • And of course if you know any two of the voltages you can calculate the third. We generally use VBE and VCE EE 42 fall 2004 lecture 16

  10. 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 16

  11. Common Emitter configuration R 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 VCE The voltage between the base and the emitter is called VBE IC IB + Vout - + Vin - IE EE 42 fall 2004 lecture 16

  12. 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 16

  13. The NPN bipolar as a current amplifier • The bipolar transistor is naturally a current amplifier, because the voltage VBE 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 IB instead. • The current from the collector is IC=βIB, so we amplify the input current by the factor β EE 42 fall 2004 lecture 16

  14. 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 RC IC IB + Vout - + Vin - RB IE EE 42 fall 2004 lecture 16

  15. Voltage amplification The current into the base IB is: And the current into the collector is: And if we have a 5 volt supply rail, the output voltage is: EE 42 fall 2004 lecture 16

  16. 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? EE 42 fall 2004 lecture 16

  17. 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. EE 42 fall 2004 lecture 16

  18. The bipolar transistor in a logic device • Bipolar devices have also been used to make logic circuits: an example of a NOR gate: Output A B 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 EE 42 fall 2004 lecture 16

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