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ECE 442 – Jose Schutt-Aine 1 ECE 342 Solid-State Devices & Circuits 14. CB and CG Amplifiers Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois 1

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Common Gate Amplifier Substrate is not connected to the source must account for body effect Drain signal current becomes And since Body effect is fully accounted for by using

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ECE 442 – Jose Schutt-Aine 3 Common Gate Amplifier

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ECE 442 – Jose Schutt-Aine 4 Common Gate Amplifier

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ECE 442 – Jose Schutt-Aine 5 Taking r o into account adds a component ( R L /A o ) to the input resistance. The open-circuit voltage gain is: The voltage gain of the loaded CG amplifier is: Common Gate Amplifier

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ECE 442 – Jose Schutt-Aine 6 CG Output Resistance

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CG Amplifier as Current Buffer G is is the short-circuit current gain

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- Include C L to represent capacitance of load - C gd is grounded - No Miller effect High-Frequency Response of CG

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ECE 442 – Jose Schutt-Aine 9 f P2 is usually lower than f p1 f P2 can be dominant Both f P1 and f P2 are usually much higher than f P in CS case 2 poles: High-Frequency Response of CG

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CB Amplifier

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ECE 442 – Jose Schutt-Aine 11 High-Frequency Analysis of CB Amplifier The amplifiers upper cutoff frequency will be the lower of these two poles. From current gain analysis

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ECE 442 – Jose Schutt-Aine 12 Common source amplifier, followed by common gate stage – G 2 is an incremental ground MOS Cascode Amplifier

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ECE 442 – Jose Schutt-Aine 13 CS cacaded with CG Cascode Very popular configuration Often considered as a single stage amplifier Combine high input impedance and large transconductance in CS with current buffering and superior high frequency response of CG Can be used to achieve equal gain but wider bandwidth than CS Can be used to achieve higher gain with same GBW as CS MOS Cascode Amplifier

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ECE 442 – Jose Schutt-Aine 14 MOS Cascode Incremental Model

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ECE 442 – Jose Schutt-Aine 15 KCL at v s2 MOS Cascode Analysis

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ECE 442 – Jose Schutt-Aine 16 Two cases MOS Cascode Analysis

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ECE 442 – Jose Schutt-Aine 17 CASE 1 The voltage gain becomes MOS Cascode Analysis

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ECE 442 – Jose Schutt-Aine 18 CASE 2 The voltage gain becomes MOS Cascode Analysis

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ECE 442 – Jose Schutt-Aine 19 MOS Cascode at High Frequency The upper corner frequency of the cascode can be approximated as:

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ECE 442 – Jose Schutt-Aine 20 Capacitance C gs1 sees a resistance R sig Capacitance C gd1 sees a resistance R gd1 Capacitance (C db1 +C gs2 ) sees resistance R d1 Capacitance (C L +C gd2 ) sees resistance (R L ||R out ) MOS Cascode at High Frequency

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ECE 442 – Jose Schutt-Aine 21 If R sig is large, to extend the bandwidth we must lower R L. This lowers R d1 and makes the Miller effect insignificant If R sig is small, there is no Miller effect. A large value of R L will give high gain MOS Cascode at High Frequency

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Effect of Cascoding (when R sig =0)

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ECE 442 – Jose Schutt-Aine 23 Cascode Example

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ECE 442 – Jose Schutt-Aine 24 Cascode Example The cascode circuit has a dc drain current of 50 A for all transistors supplied by current mirror M3. Parameters are g m1 =181 A/V, g m2 =195 A/V, g ds1 = 5.87 A/V, g mb2 =57.1 A/V, g ds2 = A, g ds3 = 3.76 A/V, C db2 = 9.8 fF, C gd2 = 1.5 fF, C db3 =40.9 fF, C gd3 = 4.5 fF. Find midband gain and approximate upper corner frequency Therefore, we use Case 2 to compute the gain The internal conductance of the current source is:

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ECE 442 – Jose Schutt-Aine 25 Cascode Example Gain can be approximated by Upper corner frequency is approximated by

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ECE 442 – Jose Schutt-Aine 26 Common emitter amplifier, followed by common base stage – Base of Q 2 is an incremental ground BJT Cascode Amplifier

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ECE 442 – Jose Schutt-Aine 27 BJT Cascode Incremental Model

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ECE 442 – Jose Schutt-Aine 28 BJT Cascode Analysis Ignoring r x2

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ECE 442 – Jose Schutt-Aine 29 BJT Cascode Analysis

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ECE 442 – Jose Schutt-Aine 30 BJT Cascode Analysis If R s << r x1 +r 1, the voltage gain can be approximated by

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ECE 442 – Jose Schutt-Aine 31 BJT Cascode at High Frequency Define r cs as the internal impedance of the current source. R includes generator resistance and base resistance r x1 base of Q1

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ECE 442 – Jose Schutt-Aine 32 There is no Miller multiplication from the input of Q2 (emitter) to the output (collector). BJT Cascode at High Frequency

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ECE 442 – Jose Schutt-Aine 33 BJT Cascode at High Frequency

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ECE 442 – Jose Schutt-Aine 34 BJT Cascode at High Frequency

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ECE 442 – Jose Schutt-Aine 35 Cascode Amplifier – High Frequency High-frequency incremental model

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ECE 442 – Jose Schutt-Aine 36 Applying Kirchoffs current law to each node: Find solution using a computer Cascode Amplifier – High Frequency

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ECE 442 – Jose Schutt-Aine 37 g m =0.4 mhos =100 r =250 ohms r x =20 ohms C =100 pF C =5 pf G L =5 mmhos G S =4.5 mmhos s a =8.0 ns-1 s d = nsec-1 s b = j5.99 s e = s c =-2.02 – j5.99 s f =-4.05 s g = POLESZEROS As an example use: Cascode Amplifier – High Frequency

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ECE 442 – Jose Schutt-Aine 38 If one pole is at a much lower frequency than the zeros and the other poles, (dominant pole) we can approximate 3dB For the same gain, a single stage amplifier would yield: Second stage in cascode increases bandwidth Cascode Amplifier – High Frequency

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ECE 442 – Jose Schutt-Aine 39 CE Cascade Amplifier Exact analysis too tedious use computer CE cascade has low upper-cutoff frequency

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ECE 442 – Jose Schutt-Aine 40 Cascode Amplifier First stage is CE and second stage is CB If R s << r 1, the voltage gain can be approximated by

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ECE 442 – Jose Schutt-Aine 41 Cascode Amplifier – High Frequency High-frequency incremental model

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ECE 442 – Jose Schutt-Aine 42 Applying Kirchoffs current law to each node: Find solution using a computer Cascode Amplifier – High Frequency

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ECE 442 – Jose Schutt-Aine 43 g m =0.4 mhos =100 r =250 ohms r x =20 ohms C =100 pF C =5 pf G L =5 mmhos G S =4.5 mmhos s a =8.0 s d = s b = j5.99 s e = s c =-2.02 – j5.99 s f =-4.05 s g = POLES (nsec -1 )ZEROS (nsec -1 ) As an example use: Cascode Amplifier – High Frequency

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ECE 442 – Jose Schutt-Aine 44 If one pole is at a much lower frequency than the zeros and the other poles, (dominant pole) we can approximate 3dB For the same gain, a single stage amplifier would yield: Second stage in cascode increases bandwidth Cascode Amplifier – High Frequency

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