Fun with Bipolar Transistors Physics 117 Lecture 4 Winter 2016 Prof Pietro Musumeci Active vs Passive Circuits Up until now passive circuits Active circuits add power to the signal Unlike a transformer V I or V I But always such that Pout Pin To add power to the signal conservation of energy need to add power from somewhere typically from a power supply What we want A small current controls modulates a larger current faucet analogy A small force on the handle can control a large stream of water causing a large force Vacuum Tube Triode valve Electrons emitted thermioncally from Cathode filament Small changes of voltage on Grid cause large changes in electron flow from Cathode filament to the Anode plate Bipolar Transistor is solid state analogue E g NPN N P N Collector Like filament Base Like Grid Emitter Like plate Solid State Much smaller lower power better behaved Transistor Nomenclature VB voltage from base to ground VC voltage from collector to ground VE voltage from emitter to ground VBE voltage from base to emitter typically about 0 6 to 0 9V VCB voltage from collector to base VCE voltage from collector to emitter IB current into base IC current into collector IE current out of emitter VCC voltage of power supply if hooked up to C VBB voltage of power supply if hooked up to B rare VEE voltage of power supply if hooked up to E Bipolar Transistor Microscopic picture N P small signal N big supply Presence of VBE applied voltage causes IB to flow but also disturbs depletion zone of P N junction so electrons diffuse through it IC IB 50 250 bipolar both electrons and holes are moving Aligned with circuit element drawing N big supply P N small signal Transistor Rules VBE always about a diode drop 0 6 to 0 9 V IC IB 50 250 IE 1 IB IB Note that VCE can be large but not always Notes Zin of a circuit does not typically equal Zout of the same circuit example our voltage divider Zin or Zout can also be considered instead to be V I Allows us to ignore DC offsets For previous weeks circuits there was no difference Input output impedance R V I is in Ohms low resistance big change in current flow per volt high resistance small change in current flow per volt Vin Vsource R1 looks into input forward Vout R2 Sees input impedance of R1 R2 wants small I per V ie wants large Zin Input output impedance with resistors Same for input and output Vin R1 Vout R2 looks into output backward Rload Sees output impedance of R1 R2 wants small V per I ie wants small Zout Transistor input output impedance Zin Zout IC IB V Zin out V I 100 Vin RB IE IB iB RE Vout Done at the board two ways Can think of previous circuit as an impedance changer VE VB except for 0 6 V drop VIN VOUT except for 0 6 V and RB drop name of circuit emitter follower but still a very important circuit element High input impedance small output impedance Zin Zout 2 RE RB 100 000 in this week s labs Black Box equivalent of emitter follower Zin large Zout small also Vout shifted down by 0 6 to 0 9 V Vocabulary quiescent point State of your active circuit voltages on base and emitter if you have no signal on the input Beware it is not always zero volts nor do you often want it to be Biasing Typically one power supply is used and biasing is required to pull the quiescent voltages off center to avoid clipping The 4 basic single transistor circuits 1 Emitter Follower labs 4 2 4 4 already did on board 2 Current source lab 4 6 3 Amplifier lab 4 7 4 Transistor switch lab 4 9 2 Constant current source lab 4 6 You are most familiar with constant voltage sources such as batteries Why does this circuit provide a constant current through the load Hint calculate the current for different loads VCC eg 10V load Vin 5 6V 5k Notice no Vout other polarity PNP P N P Just like PNP but generally more useful for negative voltages VC must be large NEGATIVE in normal operation i e reverse biased the fwd biased junction in normal operation PNP transistor used as current source 3 Common Emitter Amplifier lab 4 7 What happens if you connect Vout to the Collector instead of the Diagram and calculation on board Emitter of a transistor More on Common Emitter amplifier Circuit in lab 4 7 shifts baseline output quiescent point to amplify both positive and negative voltages 1 mA quiescent current 10 V 20 V 10k 1 mA quiescent voltage 10x amplifier for frequencies 4 Transistor Switch lab 4 9 This is the basic unit of digital electronics 5V 5V could be a different voltage lamp 1k Push Pull lab 5 6 Vin Vout You will build this circuit in lab Advantages Disadvantages relative to a single NPN transistor