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Lecture 22 Cascaded Systems and Combination BJT-FET Circuits

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Lecture 22 Cascaded Systems and Combination BJT-FET Circuits Lecture 22 Cascaded Systems and Combination BJT-FET Circuits BJT-FET 1-1 Outline Introduction to Cascaded Amplifier Systems Introduction to BJT-FET Combination Circuits BJT-FET 1-2 Cascaded Amplifier Systems A cascade amplifier is any two-port network constructed from a series of amplifiers, where each amplifier sends its output to the input of the next amplifier stage The overall voltage gain is determined by the product of gains of the individual stages The DC bias circuits are isolated from each other by the coupling capacitors The DC calculations are independent of the cascading The AC calculations for gain and impedance are interdependent BJT-FET 1-3 R-C Coupled BJT Amplifiers Input impedance, first stage: Zi R1 || R2 || re Output impedance, second stage: Zo RC 2 Voltage gain: RC1 || R3 || R4 || re Av1 re RC 2 AV 2 re BJT-FET 1-4 Av Av1 Av2 Multistage Amplifiers BJT-FET 1-5 Problem The figure shows two-stage RC coupled amplifier. If the input resistance Rin of each stage is 1 kΩ, find: (i) voltage gain of first stage (ii) voltage gain of second stage (iii) total voltage gain. BJT-FET 1-6 Cascode Amplifier Systems Cascode amplifier High frequency amplifier made up of a common-emitter amplifier with a common- base amplifier in its collector network BJT-FET 1-7 Comparisons between MOSFETs and BJTs MOSFETs BJTs Pros Cons High input impedance Low input impedance Minimal drive power, no DC current required Large drive power, continuous DC current at gate required at base Simple drive circuits Complex drive circuits as large +ve and –ve currents are involved Devices can be easily paralleled Devices cannot be easily paralleled Max. operating temp. ~ 200 oC , less temp. Max. operating temp. ~ 150 oC , more sensitive sensitive to temp Very low switching losses Medium to high switching losses (depends on trade-off with conduction losses) High switching speed Lower switching speed Cons Pros High on-resistance Low on-resistance Low transconductance High transconductance BJT-FET 1-8 BJT-FET Combination Circuits Combination of BJT and FET device in a circuit Innovative circuits that take some advantages of FETs, such as the high-input-impedance and low input power operation, and some merits of BJTs, such as high output current-driving capability How to analyze such circuits Firstly, recognize both of the devices and their current flows To make the calculation simple and easier to view, transform the circuit into the equivalent form to avoid complexity List down all the important relationships that involve for both of the devices Start with approaching the device that is closer to the ground (bottom device) BJT-FET 1-9 Example (3) Determine VD and VC BJT-FET 1-10 Example (3) – Solution We know that for the JFET device, IG = 0 making the resistor RG = 1 MΩ useless and can be remove from the circuit By analyzing the circuit, we notice that the configuration is a voltage-divider bias for both the JFET and BJT device Due to involvement of BJT, we have to check βRE ≥ 10R2 to use the approximate analysis As for βRE = (180)(1.6k) = 288k and 10R2 = 10(24k) = 240k, situation βRE ≥ 10R2 is satisfied and we can use approximate analysis for this configuration Obtaining the E : TH 24 ETH 16* 3.62 V 24 82 BJT-FET 1-11 Example (3) – Solution Transforming the circuit into its equivalent form: ETH = 3.62 V BJT-FET 1-12 Example (3) – Solution By approaching BJT (bottom device) first, we know VBE = 0.7 active operating mode From earlier calculation, we got ETH = VB = 3.62 V Obtaining VE: VIRIR 1 EEEBE 181IIBB 1.6 289.6 Obtaining IB from VBE = 0.7: ETH = 3.62 V VBE VB VE 0.7 0.7 3.62 289.6kI B IB 10.08 A BJT-FET 1-13 Example (3) – Solution From the circuit, IB is not really important but IC is very important because IC = IS = ID As for that, obtain IC: IC I B (180)(10.08) 1.81 mA Knowing the value of ID, VD can be obtained: ETH = 3.62 V 3 VD 16 I D 2.710 11.11 V BJT-FET 1-14 Example (3) – Solution From the configuration, we notice that VS = VC By obtaining VGS for the JFET, the value of VS can be achieved: VGS VG VS 3.62VS 3.62VC 2 V GS I D I DSS 1 VP 2 3.62V 1.81m 12m1 C E = 3.62 V 6 TH VC 7.29 V BJT-FET 1-15 The Darlington Pair - The Darlington circuit provides a very high current gain—the product of the individual current gains: D = 12 - The practical significance is that the circuit provides a very high input impedance. BJT-FET 1-16 DC Bias of Darlington Circuits Base current: VCC VBE = IB D 1 2 RB DRE Emitter current: IE (D 1)IB DIB Emitter voltage: VE IERE Base voltage: VB VE VBE BJT-FET 1-17 A Darlington emitter-follower used as a buffer between a common- emitter amplifier and a low-resistance load such as a speaker BJT-FET 1-18 The Sziklai Pair “Complementary Darlington” - one NPN and one PNP transistor - Current gain is similar to that of a Darlington pair - The base turn-on voltage is only about half of the Darlington's turn-on voltage BJT-FET 1-19 Current Mirror Circuits - Current mirror circuits provide integrated circuits with constant current, regardless of loading - The current mirror is used to provide bias currents and active loads to circuits - Applications: - Voltage to current converters - Current-mode analog signal processing (low-voltage operation) BJT-FET 1-20 A Simple Current Source (Current Mirror) BJT-FET 1-21 Take Home Problem Prove that the following common-collector BJT circuit has a negative feedback effect BJT 1-22 Lecture Summary Covered material Introduction to cascaded systems using BJT Introduction to BJT-FET Combination Circuits Combination of BJT and FET device in a circuit BJT-FET 1-23 .
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