ELEC 412 - Lecture 191 ELEC 412 RF & Microwave Engineering Fall 2004 Lecture 19.

ELEC 412 - Lecture 19 1

ELEC 412

RF & Microwave Engineering

Fall 2004

Lecture 19

ELEC 412 - Lecture 19 2

Stub Tuner Matched RF Amplifiers• Stub tuners can be used to match sources and load

to S11* and S22* of the RF BJT or FET• Either open or short circuit stubs may be used• When using short circuit stubs, place a capacitor

between the stub and ground to produce RF path to ground – Do not short directly to ground as this will affect transistor DC biasing

• High resistance /4 transformers or RFC’s may be used to provide DC path to transistor for biasing without affecting the RF signal path

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Stub Tuner Matched RF Amplifier

01

resonant resonantL C

Series Resonant Ckt at Operating Frequency: Short Ckt at Resonance, Open Circuit at DC

/4 Transformer: Transforms Short Circuit at Resonance to Open circuit at BJT Collector Thus Isolating RC from RF Signal Path

Stub tuners of two types:Base-Side: Open Circuit Stub w/ Isolation from DC Bias Circuit Using RFC.Collector-Side: RF Short Circuit Stub via By-Pass Capacitor

The BJT “Self-Bias” Configuration Is Shown Which Produces Excellent Quiescent Point Stability

Power Supplies Are Cap By-Passed and RF Input and Output are Cap Coupled

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Stub Tuner Matched RF AmplifierSimpler method of bias isolation at BJT collector: CBP is RF short-circuit which when transformed by the Quarter-Wave Transformer is open circuit at the Single Stub Tuner and provides DC path for the Bias Network

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Design Strategy: RF Amplifiers• Objective: Design a complete class A, single-stage

RF amplifier operated at 1 GHz which includes biasing, matching networks, and RF/DC isolation.

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Design Strategy: RF Amplifier

• Design DC biasing conditions

• Select S-parameters for operating frequency

• Build input and output matching networks for desired frequency response

• Include RF/DC isolation

• simulate amplifier performance on the computer

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Design Strategy: Approach

For power considerations, matching networks are assumed lossless

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Power RelationshipsTransducer Power Gain

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Stability of Active Device

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Stability of Amplifiers

• In a two-port network, oscillations are possible if the magnitude of either the input or output reflection coefficient is greater than unity, which is equivalent to presenting a negative resistance at the port. This instability is characterized by

|in| > 1 or |out| > 1

which for a unilateral device implies |S11| > 1 or |S22| > 1.

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Stability Requirements

•  Thus the requirements for stability are

and

• These are defined by circles, called stability circles, that delimit |in | = 1 and | L | = 1 on the Smith chart.

12 2111

22

S +in 1 = < 1L

L

S S

S

out 22| | = S +l < 1

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Stability Regions: Stability Circles• Regions of amplifier stability can be

depicted using stability circles using the following:Output stability circle:

**22 1112 21

2 2222222

,out out

S SS Sr C

SS

Input stability circle: **

11 2212 212 222

1111

,in in

S SS Sr C

SS

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Stability Regions: Stability Circles

Where:

11 22 12 21S S S S

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Stability Regions: Output Stability Circles

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Stability Regions: Input Stability Circles

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Different Input Stability Regions

Dependent on ratio between rs and |Cin|

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Unconditional Stability

Stability circles reside completely outside |S| =1 and |L| =1. Rollet Factor: 2 2 2

11 22

12 21

11

2

S Sk

S S

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Constant Gain Amplifier

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Constant Gain Circles in the Smith ChartTo obtain desired amplifier gain performance

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Circle Equation and Graphical Display

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Gain Circles

• Max gain imax =1/(1-|Sii|2) when i = Sii* ; gain circle center is at dgi= Sii* and radius rgi =0

• Constant gain circles have centers on a line connecting origin to Sii*

• For special case i = 0, gi = 1-|Sii|2 and

dgi = rgi = |Sii|/(1+|Sii|2) implying i = 1 (0 dB) circle always passes through origin of i plane

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Trade-off Between Gain and Noise