# Category: microelectronics by sedra and smith

# Category: microelectronics by sedra and smith

#### 10.71 Find the dc gain and the 3-dB frequency of a MOScascode amplifier operated at gm = 2 mA/V and ro = 20 k.The MOSFETs have Cgs = 20 fF, Cgd = 5 fF, and Cd b = 5 fF.The amplifier is fed from a signal source with Rsig = 100 kand is connected to a load resistance of 1 M. There is alsoa load capacitance CL of 20 fF.

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#### 10.70 An IC CG amplifier is fed from a signal source withRsig = ro/2, where ro is the MOSFET output resistance. It has acurrent-source load with an output resistance equal to ro. TheMOSFET is operated at ID = 100 μA and has gm = 1.5 mA/V,VA = 10 V, Cgs = 0.2 pF, Cgd = 0.015 pF, and Cd b = 20 fF.As well, the current-source load provides an additional 30 fFcapacitance at the output node. Find fH .

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#### 10.68 Consider a CG amplifier loaded in a resistance RL = roand fed with a signal source having a resistance Rsig = ro/2.Also let CL = Cgs. Use the method of open-circuit timeconstants to show that for gmro 1, the upper 3-dB frequencyis related to the MOSFET fT by the approximate expression

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#### 10.65 Use the method of open-circuit time constants tofind fH for a CS amplifier for which gm = 1.5 mA/V, Cgs =Cgd = 0.2 pF, ro = 20 k, RL = 12 k, and Rsig = 100 kfor the following cases: (a) CL = 0, (b) CL = 10 pF, and (c)CL = 50 pF. Compare with the value of fH obtained using theMiller approximation.

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#### 10.64 Consider a CS amplifier loaded in a current sourcewith an output resistance equal to ro of the amplifyingtransistor. The amplifier is fed from a signal source with Rsig =ro/2. The transistor is biased to operate at gm = 2 mA/V andro = 20 k; Cgs = Cgd = 0.1 pF. Use the Miller approximationto determine an estimate of fH . Repeat for the following twocases: (i) the bias current I in the entire system is reduced bya factor of 4, and (ii) the bias current I in the entire systemis increased by a factor of 4. Remember that both Rsig and RLwill change as ro changes.

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#### 10.63 A common-emitter amplifier has Cπ = 10 pF, Cμ =0.3 pF, CL = 3 pF, gm = 40 mA/V, β = 100, rx = 100 ,RL = 5 k, and Rsig = 1 k. Find the midband gain AMand an estimate of the 3-dB frequency fH using the Millerapproximation. Also, obtain another estimate of fH usingthe method of open-circuit time constants. Which of thetwo estimates would you consider to be more realistic,and why?

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