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
ELEC 412 - Lecture 19 3
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
ELEC 412 - Lecture 19 4
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
ELEC 412 - Lecture 19 5
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.
ELEC 412 - Lecture 19 6
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
ELEC 412 - Lecture 19 7
Design Strategy: Approach
For power considerations, matching networks are assumed lossless
ELEC 412 - Lecture 19 8
Power RelationshipsTransducer Power Gain
ELEC 412 - Lecture 19 9
Stability of Active Device
ELEC 412 - Lecture 19 10
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.
ELEC 412 - Lecture 19 11
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
ELEC 412 - Lecture 19 12
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
ELEC 412 - Lecture 19 13
Stability Regions: Stability Circles
Where:
11 22 12 21S S S S
ELEC 412 - Lecture 19 14
Stability Regions: Output Stability Circles
ELEC 412 - Lecture 19 15
Stability Regions: Input Stability Circles
ELEC 412 - Lecture 19 16
Different Input Stability Regions
Dependent on ratio between rs and |Cin|
ELEC 412 - Lecture 19 17
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
ELEC 412 - Lecture 19 18
Constant Gain Amplifier
ELEC 412 - Lecture 19 19
Constant Gain Circles in the Smith ChartTo obtain desired amplifier gain performance
ELEC 412 - Lecture 19 20
Circle Equation and Graphical Display
ELEC 412 - Lecture 19 21
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
ELEC 412 - Lecture 19 22
Trade-off Between Gain and Noise