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Unformatted text preview: ECE 654 Solid State Devices II Prof. S. Mohammadi  60  Chapter 3 Nonlinearity and Distortion in Electronic Devices Nonlinearity and Distortion In this chapter we discuss the importance of nonlinearity in devices and how they are modeled. We take a simplified approach to understand nonlinearity. The approach helps us understand the mechanisms responsible for device nonlinear nature. Note that in this chapter the nonlinearity is attributed to DC characteristics of the transistor. Therefore, the source of nonlinearity discussed here is the transconductance of the transistor. Transistors’ input and output capacitances can also be very nonlinear. However, that type of nonlinearity is not captured by power series method used here. One needs to rely on Volterra Series, harmonic balance or time domain techniques to understand those nonlinearities. Example of the effect of device nonlinearity on circuit performance In the following example we assume that the RF transistor used as the input stage of a receiver (as the active circuit of a low noise amplifier (LNA)) is subjected to three signals coming from three adjacent channels: your receiver receives 3 signals f Δ + = 1 f f Δ + = 2 2 f f 2 adjacent channels are strong interferers your signal is weak in your LNA you have nonlinearity so your strong interferers mix with each other due to LNA nonlinearity ECE 654 Solid State Devices II Prof. S. Mohammadi  61  This mixing generates frequencies such as ( ) ( ) 2 1 2 2 2 f f f f f = Δ + Δ + = This is what you get at the output of LNA your signal IM3 (distortion of interferers) This distortion makes your reception difficult we have to understand distortion and nonlinearity Nonlinearity Weakly nonlinear systems (such as in receivers) Strongly nonlinear systems (such as in high efficiency transmitter) LNA class BE PA s ’ also clamping to deal with weakly nonlinear system there are two approaches power series volterra series represent transfer function using a power series power series with information about phase for strongly non linear system use envelope analysis instead of frequency domain analysis use time domain analysis ECE 654 Solid State Devices II Prof. S. Mohammadi  62  * At midband frequency parasitic caps and inductors bias and bypass caps can be neglected in terms of the phase they introduce use power series * At low or high frequencies you need to know the phase of the signal that is subjected to nonlinearily use volterra series linear strong nonlinearity (cannot be modeled by power/volterra series) since at this point higher order terms become dominant in saturating the signal in S out S Note that at low to midband frequencies, the impedances of device input and output capacitances...
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This note was uploaded on 02/19/2012 for the course ECE 654 taught by Professor Mohammadi during the Spring '08 term at Purdue UniversityWest Lafayette.
 Spring '08
 Mohammadi

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