lecture05

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S. Yan, EE 338L Lecture 5 1 EE 338L CMOS Analog Integrated Circuit Design Lecture 5, Single-Stage Amplifiers (2) Calculations of Small Signal Input and Output Impedances How to calculate input and output impedances (or admittances) of an amplifier? In the following sections, we assume that the amplifier is a voltage amplifier, whose input and output are both voltages. But we can easily extend the principles to any other types of amplifiers, such as current amplifiers (input and output are both currents), transimpedance amplifiers (input: current, output: voltage), and transconductance amplifiers (input: voltage, output: current). 1. Input impedance Method A: i) Apply tst v at the input (* see note below), draw the small signal diagram. ii) Calculate ) ( tst tst v f i = . iii) The input impedance is given by tst tst in i v z = , and the input admittance is tst tst in v i y = . Method B: i) Apply tst i at the input, draw the small signal diagram. ii) Calculate ) ( tst tst i f v = . iii) The input impedance is given by tst tst in i v z = , and the input admittance is tst tst in v i y = . * Note: If the amplifier requires an output termination, we should terminate the output accordingly. The load condition may affect the input impedance. small signal circuit v out i tst v tst ( v in ) small signal circuit v out i tst v tst ( v in )

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S. Yan, EE 338L Lecture 5 2 2. Output impedance Method A: i) Set 0 = in v , or if the input is a signal current, set 0 = in i (** see note below). ii) Apply tst v at the output, draw the small signal diagram. iii) Calculate ) ( tst tst v f i = . iv) The output impedance is given by tst tst out i v z = , and the output admittance is tst tst out v i y = . Method B: i) Set 0 = in v , or if the input is a current, set 0 = in i (** see note below). ii) Apply tst i at the output, draw the small signal diagram. iii) Calculate ) ( tst tst i f v = . iv) The output impedance is given by tst tst out i v z = , and the output admittance is tst tst out v i y = ** Note: If the amplifier requires some input termination, we should terminate the input accordingly. The input termination may affect the output impedance. Example : Calculate the output impedance of the following circuit, assuming both M1 and M2 work in saturation region. The small signal parameters of M1 and M2 are shown in the following table. Transconductance Bulk transconductance Drain-source conductance M1 g m1 g mb1 g ds1 M2 g m2 g mb2 g ds2 Note: As g mb is not 0, we take bulk (or body) effect into consideration. As g ds is not 0, we also consider channel length modulation effect. small signal circuit (v out ) ( v in =0 ) i tst v tst small signal circuit (v out ) ( v in =0 ) i tst v tst
S. Yan, EE 338L Lecture 5 3 Solution: (1) Set v in =0, and draw the small signal digram. (2) Apply i

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