Common Collector (Emitter Follower) Amplifier
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Common Collector (Emitter Follower) Amplifier • Gain is never better than unity, however, has some desirable input and output impedance characteristics --- acts as a buffer VCC RC RS C3 C1 C2 vs RB RE RL -VEE Lecture 15-1 Common Collector (Emitter Follower) Amplifier • Without RC there is no need for C3 VCC RS C1 C2 vs RB RE RL -VEE Lecture 15-2 Emitter FollowerExample • Calculate the voltage gain, current gain, input resistance and output resistance • Assume capacitors are infinite • 1) Calculate dc operating point +10V 10kΩ vs β = 100 VA = 100 100kΩ 10kΩ 10kΩ -10V Lecture 15-3 Emitter Follower Example • 2) Establish small signal model 10kΩ vs 100kΩ rπ βib 10kΩ ro 10kΩ Lecture 15-4 Amplifier Input Resistance, Rib • 3) Calculate input resistance 10kΩ vs 100kΩ rπ βi b 10kΩ ro 10kΩ Lecture 15-5 Transistor’s Input Resistance, Rib • “Reflect” impedances into base from emitter for simplified input resistance calcuation 100kΩ Rib 3kΩ βib 10kΩ 119kΩ 10kΩ Lecture 15-6 Amplifier Input Resistance, Ri • RB is part of the amplifier circuit, and adds to the input resistance to the transistor Ri 100kΩ 3kΩ βib 10kΩ 119kΩ 10kΩ • Amplifier input resistance is limited by RB in this circuit. Why not make it bigger? Lecture 15-7 Emitter Follower --- Buffer • Even though the load resistance is 10kΩ, the input resistance is much higher • This is a desirable feature of a buffer amplifier, especially if RS is large, or RL is small • Is Rin big enough for our example? What is vi? RS vs 100kΩ vi rπ βib 10kΩ ro RL Lecture 15-8 Emitter Follower --- Buffer • Further voltage division for the amplifier stage • Most easily seen using other small signal model αie vi re 10kΩ ro RL Lecture 15-9 Analysis by Inspection • Experienced analog designers just analyze the circuit directly by inspection +10V 10kΩ vs 100kΩ 10kΩ 10kΩ -10V Lecture 15-10 Analysis by Inspection • What if RL is much less than RE? +10V 10kΩ vs 10kΩ RL -10V Lecture 15-11 Emitter Follower Current Gain • No voltage gain, but acts as a buffer to drive small impedance loads • Provides good current amplification 10kΩ vs 100kΩ rπ βi b 10kΩ 119kΩ 10kΩ Lecture 15-12 Current Gain 10kΩ vs 100kΩ rπ βi b 10kΩ 119kΩ 10kΩ Lecture 15-13 Output Resistance, Ro • 4) Calculate output resistance 10kΩ 0 100kΩ rπ βi b 10kΩ ro Ro Lecture 15-14 Output Resistance, Ro Lecture 15-15 SPICE Results dc solution VCC 10V +- IC 839.882 µA RS 10E3Ω C2 1E-6F + Q1 Custom IIN 0.000 pA VINAC + 0.000 pV - + VSSIN SIN VIN + -758.300 mV - C1 1E-6F + RB 100E3Ω IO 0.000 pA RE 10E3Ω VOUT + 0.000 pV - RL 10E3Ω + VEE -10V Lecture 15-16 SPICE Results voltage gain 0.0 0.1 0.2 0.3 0.4 0.5 time 0.6 ms 10 mV 0 -10 VINAC VOUT Lecture 15-17 SPICE Results current gain 0.0 0.1 0.2 0.3 0.4 0.5 time 0.6 ms 1 uA 0 -1 IO IIN Lecture 15-18 Common Base Amplifier • Gain is close to unity • Very low input impedance --- great for impedance matching (e.g. 50 ohms) • Large output impedance -- approximately RC • Great high frequency behavior -- more on this later... VCC RC vo RS C1 I vs -VEE Lecture 15-19 Current Source Biasing • Instead of resistors, a current source is used to bias the transistor in this example • Current sources can be used to bias other amplifier types too • Building a current source is less expensive than building a resistor on an IC --- we’ll be addressing IC issues such as this much more extensively beginning with the next lecture VCC RC vo RS C1 I vs -VEE Lecture 15-20 Current Buffer • Can be used as a current buffer • Macromodel: ii vi io Ri αi i Ro vo Lecture 15-21 Small Signal Equivalent Circuit • Since the base is grounded, we would probably select the T-model from Lecture 13 ic C αi e ib B ie re E • What parameters are being ignored in this simplified T-model? And what is the potential impact? Lecture 15-22 Common Base Small Signal Analysis Lecture 15-23 Common Base Small Signal Analysis Lecture 15-24
