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Dr. Ahmed Heikal – Lecture 2 Electronic Devices Ninth Edition Floyd Chapter 6 Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary AC Quantities V AC quantities are indicated with a italic subscript; rms rms avg values are assumed unless Vce V Vce otherwise stated. ce VCE Vce The figure shows an example of vce a specific waveform for the collector-emitter voltage. Notice the DC component is VCE and 0 t 0 the ac component is Vce. Resistance is also identified with a lower case subscript when analyzed from an ac standpoint. Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary Linear Amplifier A linear amplifier produces an replica of the input signal at the output. +VCC Ic Vb ICQ V BQ R1 RC Vce C2 VCEQ Rs C1 Ib IBQ Vs R2 RE RL For the amplifier shown, notice that the voltage waveform is inverted between the input and output but has the same shape. Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary AC Load Line Operation of the linear amplifier can be illustrated IC Q I B using an ac load line. b I The ac load line is different Ic I than the dc load line because CQ Q a capacitor looks open to dc but effectively acts as a short to ac. Thus the 0 VCE collector resistor appears to Vce be in parallel with the load VCEQ resistor. Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary Transistor AC Model The five resistance parameters (r-parameters) can be used for detailed analysis of a BJT circuit. For most analysis work, the simplified r-parameters give good results. The simplified r-parameters are C C shown in relation to the transistor model. βacIb βacIb An important r-parameter is r '. It e B B r ′ appears as a small ac resistance e Ib between the base and emitter. r e′ ' 25 mV re = E E IE Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary The Common-Emitter Amplifier In the common-emitter (CE) amplifier, the input signal is applied to the base and the inverted output is taken from the collector. The emitter is common to ac signals. VCC RC C3 R1 Vo ut C1 Vin RL R2 RE C2 Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary The Common-Emitter Amplifier What is re' for the CE amplifier? Assume stiff voltage-divider bias. VCC 27 kΩ +15 V VB = 15 V = 4.26 V 68 kΩ+ 27 k Ω RC C3 R1 3.9 kΩ V = 4.26 V – 0.7 V = 3.56 V 68 kΩ E C 1 10 µF VE 3.56 V = = = 1.0 µF RL IE 1.62 mA 3.9 kΩ R 2.2 kΩ R2 E Ω 27 k RE C2 2.2 kΩ 100 µF ' 25 mV 25 mV re = = = 15.4 Ω IE 1.62 mA Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary The Common-Emitter Amplifier Notice that the ac resistance of the collector circuit is RC||RL. What is the gain of the amplifier? VCC +15 V Vout R c RRC L A = = = v '' RC C3 Vrin e r e R1 3.9 kΩ 68 kΩ 3.9 kΩΩ 3.9 k C1 A = = 127 10 µF v Ω 15.4 1.0 µF RL Ω R2 3.9 k 27 kΩ C The gain will be a little lower if the RE 2 2.2 kΩ 100 µF input loading effect is accounted for. Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary The Common-Emitter Amplifier Greater gain stability can be achieved by adding a swamping resistor to the emitter circuit of the CE amplifier. The gain will be lower as a result. VCC +15 V RC What is the gain with the addition C3 R1 3.9 kΩ 68 kΩ of the swamping resistor? (Ignore C 1 10 µF the small effect on re'.) 1.0 µF R Vout Rc RRC L E1 R A = = = 33 Ω L v ''++ 3.9 kΩ Vin rR e E1 rRe E1 R 2 Ω 27 k RE2 C2 3.9 kΩΩ 3.9 k 2.2 kΩ A = = 38.2 100 µF v 15.4 Ω+33 Ω Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary The Common-Emitter Amplifier Multisim is a good way to check your calculation. For an input of 10 mVpp, the output is 378 mVpp as shown on the oscilloscope display for the swamped CE amplifier. input output Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary The Common-Emitter Amplifier In addition to gain stability, swamping has the advantage of increasing the ac input resistance of the amplifier. For this amplifier, Rin(tot) is given by Rin(tot) = R1||R2||βac(re' + RE1) VCC +15 V RC C3 R1 3.9 kΩ What is Rin(tot) for the amplifier if 68 kΩ C βac = 200? 1 10 µF 1.0 µF Rin(tot) = R1||R2||βac(re' + RE1) R E1 R 33 Ω L 3.9 kΩ = 68 kΩ||27 kΩ||200(15.4 Ω + 33 Ω) R 2 Ω 27 k RE2 C2 = 6.45 kΩ 2.2 kΩ 100 µF Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary The Common-Collector Amplifier The common-collector amplifier (emitter-follower) has a voltage gain of approximately 1, but can have high input resistance and current gain. The input is applied to the base and taken from the emitter. +VCC R1 C1 Vin C2 Iin Vo ut R2 RE RL Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary The Common-Collector Amplifier The power gain is the ratio of the power delivered to the input resistance divided by the power dissipated in the load. This is approximately equal to the current gain. That is, Ap ≈Ai. V You can also write power gain CC as a ratio of resistances: 2 VL R1 PRR C1 LL2 in() tot V AApv= =2 = in PRin Vin L C2 Vout Rin() tot R2 RRin() tot in () tot RE RL ≅=1 RRLL The next slide is an example… Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary The Common-Collector Amplifier Calculate the power gain to the load for the CC amplifier using a ratio of resistances. Assume Av = 1 and βac = 200. Use re' = 2 Ω. VCC +15 V Rin(tot) = R1||R2||βac(re' + RE||RL) R Ω Ω Ω Ω 1 = 39 k ||220 k ||200(2 + 500 ) C1 39 kΩ Vin = 24.9 kΩ C2 µ Vout Ω 0.22 F RL = 1.0 k R2 3.3 µF 220 kΩ R RL R E in() tot 24.9 kΩ 1.0 kΩ 1.0 kΩ Ap = = = 24.9 RL 1.0 kΩ Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary The Common-Collector Amplifier The input voltage-divider in the previous example is not “rock-solid” but the overall power gain is good. A “rock solid” stiff voltage-divider is not always the best design. Can you spot the problem illustrated here? Rin(tot) = R1||R2||βac(re' + RE||RL) VCC +10 V = 10 kΩ||10 kΩ||200(25 Ω + 3.0 kΩ) = 4.96 kΩ R1 C1 10 kΩ β = 200 V RL = 10 kΩ in C2 R 4.96 kΩ Vout in() tot R Ap = = = 0.496! 2 Ω R RL 10 kΩ 10 k RE L 4.3 kΩ 10 kΩ The problem is the power gain is less than 1! Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary The Darlington Pair A Darlington pair is two transistors connected as shown. The two transistors act as one “super β” transistor. Darlington transistors are available in a single package. Notice there are two diode drops from base to emitter. VCC VCC R1 RC C1 Vin Q1 Q2 C2 Vo ut R2 R E RL CE Amplifier Darlington CC amplifier Load Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary The Sziklai Pair Another high β pair is the Sziklai pair (sometimes called a complementary Darlington), in which a pnp and npn transistor are connected as shown. This configuration has the advantage of only one diode drop between base and emitter. +VCC What is the relation between IE2 and IB1? Vin βDC1 IB1 βDC2 The DC currents are: IC1 IE2 IC1 is βDC1 x IB1 and is equal to IB2 RE IE2 is approximately equal to βDC2 x IC1 Therefore, IE2 ≈ βDC1βDC2IB1 Electronic Devices, 9th edition © 2012 Pearson Education. Upper Saddle River, NJ, 07458. Thomas L. Floyd All rights reserved. Summary The CB Amplifier The common-base (CB) amplifier is used in applications where a low input impedance is acceptable.
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