Lecture 12: Distortion Metrics

EE142 Lecture12

Amin Arbabian Jan M. Rabaey

EE142 – Fall 2010

Oct. 5th, 2010

University of California, Berkeley

Gain Compression/ Expansion

. The large signal input/output relation can display gain compression or expansion. Physically, most amplifier experience gain compression for large signals. . The small-signal gain is related to the slope at a given point. For the graph on the left, the gain decreases for increasing amplitude.

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1dB Compression Point

. Gain compression occurs because eventually the output signal (voltage, current, power) limits, due to the supply voltage or bias current. . If we plot the gain (log scale) as a function of the input power, we identify the point where the gain has dropped by 1 dB. This is the 1 dB compression point. It’s a very important number to keep in mind. EE142-Fall 2010 3

Apparent Gain

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Apparent Gain (2)

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1dB Compression Point

We will talk about IIP3 in the next lecture

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Review: Intermodulation Distortion

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Second Order Intermodulation

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Practical Effects of IM2

. This term produces distortion at a lower frequency ω1-ω2 and at a higher frequency ω1 + ω2 . Example: Say the receiver bandwidth is from 800MHz to 2.4GHz and two unwanted interfering signals appear at 800MHz and 900MHz. . Then we see that the second-order distortion will produce distortion at 100MHz and 1.7GHz. Since 1.7GHz is in the receiver band, signals at this frequency will be corrupted by the distortion. . A weak signal in this band can be “swamped” by the distortion. . Apparently, a “narrowband” system does not suffer from IM2? Or does it ?

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Low-IF Receiver

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IM2 Examples

. Broadband systems susceptible to IM2 distortion – Audio Amplifiers: 20Hz-20KHz – E.g. applying two tones at 3KHz and 4KHz,… – Cable TV: 5MHz-900MHz . Generally IM2 not a problem in cell phones (if not using low IF,…) – 880MHz-900MHz the IM2 products fall outside the band

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Cubic Intermodulation

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IM3 Components

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In-band IM3 Distortion

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IM3 Definitions

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Overall Picture of a Two-Tone Response

. 1 . 1

. 1

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Example: Distortion in BJT Amplifiers

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Distortion in BJT Amplifiers (2)

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Cross Modulation

. Assume we have an AM modulated signal and a desired signal coming into our receiver :

. 1 12 1 2

m: Incoming Modulation Index

. Cross-Modulation is where the modulation from the AM signal moves to the other carrier. We will have terms like:

. Here is the Transferred Modulation index

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Cross Modulation (2)

INPUT:

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CM Definitions

. We have a term:

. Cross-Modulation is defined as:

12 Or: 3 123 3 22

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Intercept Points: IP2

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Properties of IP2

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