Simple Introduction to Transistor (BJT) AmplifierBipolar Junction TransistorBJT OperationBJT Water ModelCurrent Gain of BJTMain Rules of npn BJT OperationBiasing a Typical BJT AmplifierGain Analysis of BJTBJT Amplifier in Real Life1/10Simple Introduction to Transistor (BJT) Amplifier2/10• Made out of n-type and p-type silicon• There are two “flavors” of BJT’s– pnp & npnBipolar Junction Transistornpn TransistorNote: Current Directions Shown are Arbitrary!!To Learn More!!EECE 332 (Semiconductors)“Whydo they work?”pnp TransistorFig. 28.6 from Electrical Engineering Uncovered3/10• Recall FET: current flow controlled by the gate voltage• BJT: current flow controlled by the base current– Acts like a current amplifier• But… with the right circuitry around it can also be a voltage amplifierBJT OperationTo Learn More!!EECE 315 (Electronics)“Howdo they work?”Fig. 28.4 from Electrical Engineering UncoveredConceptual Model of npn TransistorCollectorEmitterBaseCurrent MeterVariable R• Base Current deflects current meter• Meter linked to variable R– Controls the Collector currentÎ Voltage supply between C & E provides large Collector currentcontrolled by small Base current4/10• While this electrical conceptual model is good… we can also use a water modelBJT Water ModelFig. 28.5 from Electrical Engineering UncoveredBase Current Spins Small Turbine (Turbines: no friction & no momentum) Must exceed Diode “drop”before base current flowsLarge Turbine spins @ same rateas small one…BUT larger size causes more water to flow Regulator (like scuba tank’s) ensures that current flows only due to the turbine’s “suck”… not the pressure difference between C-E5/10• So… the main characteristic of a BJT is its current gain (called β… also called hFE) Typical β: 50 – 200 • In our water model… β is set by the relative size of the two turbines•Warning… a designer’s knowledge of β is almost always imprecise!!!!– That seems to limit the usefulness… but can be handled through proper circuit design• Emitter Current = (Collector Current) + (Base Current)Current Gain of BJTBBBCBEIIIIII)1( +=+=+=ββShown opposite book’s direction!!KCL!!!Remove | |Ic= βIB6/101. The B-E voltage must be at a “diode drop” of 0.7V– If not… no base current flows… and no collector current flows– So… if not, the transistor is “cut off”2. Collector must be more positive w.r.t. emitter3. Can’t exceed certain maximum values on IB, IC, & VCE4. When rules 1 – 3 are obeyed, IC= βIBMain Rules of npn BJT OperationFrom The Art of Electronics, by Horowitz and Hill, Cambridge University PressLike an open switch from collector to emitter7/10Biasing a Typical BJT AmplifierBefore we can use a BJT as an amplifier we need to “set it up for use”… called biasing the transistor20V10kΩ1kΩ10kΩ110kΩ1.0V2. Diode drop makes this 1 V Now we can “inject” the signal we want to amplify1 mA3. Ohms law sets emitter current1.0v / 1000Ω = 1mAVC= 10V4. Ohms law says 10V across RC, putting the collector at VC= 10 V1mA × 10k Ω = 10v1.7V1. Voltage Dividermakes this 1.7V20v×[10/(110 + 10)] = 1.67vAssumes that current into base is negligibly small8/101.7 VDCInput Signal is “capacitivelycoupled”Output Signal is “capacitivelycoupled” too!10 VDCInjecting Signal to a BJT AmplifierThe Input vintypically wiggles around a level of 0 VDC. But we want to make the base voltage wiggle around the 1.7 VDC level. Roughly… Because the cap is like an open circuit to a DC voltage it keeps these two different “DC center values” from “working against” each other… Butit passes the input wiggles to the base!!See EECE 260 Circuits to learn methods to analyze this idea9/10Gain Analysis of BJT1. A small wiggling vincauses a small wiggle ΔVBhere in VBcentered around 1.7 V1.0V1.7V2. The small wiggle at the base causes the same amount of wiggle here around 1V… ΔVE= ΔVB.That causes a wigglingemitter current ΔIEaround 1mA 3. The collector current wiggles about the same amount: ΔIC ≈ΔIE.Causes the collector voltage to wiggle around 10V…But VCgoes downwhen VB goes up(inversion!!!) 10kΩ1kΩECBCEBCECREBERRVVRVRIRVRVIC−=ΔΔΔ−=Δ−≈ΔΔ=ΔGain!!InversionNote that the transistor’s current gain, β, has no direct effect on the gain… a good design!10/10Partial Circuit for the Boss DS-1 Distortion Box for GuitarFrom Pedal Power Column by Robert Keeley, in Musician’s Hotline MagazineBJT Amplifier in Real Life Guitar distortion boxes work (in part) by amplifying the guitar signal before it reaches the actual amplifier… this overdrives the amplifier to give more distortion than it would otherwiseVariation of amplifier circuit we looked atGuitar