MICROWAVE AND RF DESIGN Case Study Amp1: Narrowband …€¦ · Case Study Amp1: Narrowband Linear...
Transcript of MICROWAVE AND RF DESIGN Case Study Amp1: Narrowband …€¦ · Case Study Amp1: Narrowband Linear...
MICROWAVE AND RF DESIGNMICROWAVE AND RF DESIGN
Based on material in Microwave and RF Design: A Systems Approach, 2nd
Edition, by Michael Steer. SciTech Publishing, 2014.
Presentation copyright Michael Steer
Case Study:Amp1Narrowband Linear Amplifier Design
Presented by Michael Steer
Reading:Chapter 17, Section 17.10
Index: CS_Amp1
Case Study Amp1:Narrowband Linear Amplifier Design
Slides copyright 2013 M. Steer.
Design of a stable 8 GHz pHEMTAmplifier
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Design Specifications Gain: maximum gain at 8 GHz
Topology: three two-ports (input and output matching networks, and the active device)
Stability: broadband stability
Bandwidth: maximum that can be achieved using two-elementmatching networks
Source impedance: ZS = 50 Ω
Load impedance: ZL = 50 Ω
2
Block diagram of an RF amplifier including biasing networks.
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M1 M2TRANSISTOR
INPUTMATCHINGNETWORK
OUTPUT
NETWORKMATCHING
RFINPUT
RFOUTPUT
GATE
COLLECTOR OR DRAINFILTERLOW-PASS
FILTERLOW-PASS
AMPLIFIER
CONTROLCHIP
BIASOR BASE
Linear amplifier with input and output matching networks
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S /[ ]
INPUTMATCHINGNETWORK
M1 INPUTMATCHINGNETWORK
M2[S]
21S
12S
NETWORKFEEDBACK
ISTORTRANS-
SOURCE LOAD
AMPLIFIER
SZ
Z L
S
L
General amplifier configuration
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Transistor Choice
A pHEMT is a JFET (jFET), junction field effect transistor.
IEEE symbol Common symbol
6
JFET
Semi-insulatingn-type substrate
Reverse-biased junction
GS Dn-type channel
Applying a voltage at the gate (a gate-source voltage) closes off the channel by extending the space-charge region (i.e no charge region) of a reverse-biased junction.
The GS voltage changes the resistance of the channel.– Not quite accurate as for high DS voltages it changes the drain current.
This is called an depletion mode of operation.
A GaAs jFET is called a MESFET (Metal-Epitaxy Semiconductor Field Effect Transistor.) Largely replaced by pHEMT.
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thV
ID
GV
n-type channel
Semi-insulatingn-type substrate
Reverse-biased junction
GS D
Current-voltage characteristic of a depletion-mode JFET
There are n‐type and p‐type JFETs but the performance of a pJFET is poor. So designs mostly use only nJFETs.
= 0.0 V
= -0.2 V
= -0.3 V
= -0.1 V
VDS
ID
GSV
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n-type channel
Semi-insulatingn-type substrate
Reverse-biased junction
GS D
Current-voltage characteristics of an enhancement-mode JFETs
thV
ID
GV
Special doping in channel that results in a built-in reverse bias that shuts that must be overcome by positive gate-source voltage.
= 2.0 V
= 0.25 V= 0.5 V
= 1.5 V
= 1.0 V
VDS
ID
GSV
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Current-voltage characteristics of depletion-mode and enhancement-mode JFETs
thV
ID
GV
thV
ID
GV
Note the need for a negative gate voltage.
= 2.0 VENHANCEMENT
= 0.25 V
= 0.5 V
= 1.5 V
= 1.0 V
VDS
ID
GSV = 0.0 V
= -0.2 V
= -0.3 V
= -0.1 V
JFET DEPLETION
VDS
ID
GSV
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thV
ID
GV
n-type channel
Semi-insulatingn-type substrate
Reverse-biased junction
GS D
Current-voltage characteristics of depletion-mode JFETs
There is not a useful p‐type pHEMTbut there are p‐type JFETs but they do not work well.
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pHEMT pseudomorphic High Electron Mobility Transistor
Generally depletion-mode
Enhancement-mode possible
Only n-type
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n-type channele.g. AlGaAs Semi-insulating e.g. GaAs
Reverse-biased junction
GS D
= 0.0 V
= -0.2 V
= -0.3 V
= -0.1 V
VDS
ID
GSV
JFET summaryTwo modes of operation
Enhancement modeDepletion mode
n-type channel
Semi-insulatingn-type substrate
Reverse-biased junction
GS D
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Circuit model of fundamental operation
Scattering parameters of an enhancement mode pHEMT transistor biased at VDS = 5 V,ID = 55 mA,VGS = −0.42 V.
Extract from the data sheet of the FPD6836P70 discrete transistor.
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Current-voltage characteristic of pHEMT
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= 0.0 V
= -0.4 V
= -0.6 V
= -0.2 V
VDS
ID
GSV
5 V
55 mA
VDD
VDD
R L
L D
DS, min
VD
DDV2
V
Amplifier classes
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Input characteristic
AB
AA
A
C B
AB
B
ABB
C
C
ID
IG
VGS
Output characteristic
Conventionalload line
High efficiencyload line
BC
A AB
VDS
ID
AB
A
Extract from Manufacturer’s Datasheet
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S parameters of pHEMT transistor at VDS = 5 V, ID = 55 mA,VGS = −0.42 V.
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Definition of power measures
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M1 M2SOURCE
1 2
LOAD
PORT 1 PORT 2
POWERINPUT SIGNALAVAILABLE
POWER POWER POWER
ACTUALACTUAL
DEVICEINPUTMATCHINGNETWORK
OUTPUTMATCHINGNETWORK
ACTIVE
INPUT SIGNALACTUAL DEVICE
OUTPUT SIGNALAVAILABLE DEVICE
OUTPUT SIGNALAVAILABLE
OUTPUT SIGNALPOWER
INPUT SIGNALPOWER
AMPLIFIER
n1
Vin
n4
n5n2
n3ZS
Z L
PAi Pin PADo Ao
PLPin
PD
So many definitions are needed as it is necessary to describe the amplifier before the matching networks have been designed and to know the ultimate performance at different stages.
Definition of gains
The input and output matching networks are lossless so that the actual device input signal power, PinD, is the power delivered by the source.
Similarly, the actual output signal power delivered to the load, PL, is the power delivered by the active device.
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M1 M2SOURCE LOAD
POWERINPUT SIGNALAVAILABLE
POWER POWER POWER
DEVICEINPUTMATCHINGNETWORK
OUTPUTMATCHINGNETWORK
ACTIVE
INPUT SIGNALACTUAL DEVICE
OUTPUT SIGNALAVAILABLE DEVICE
OUTPUT SIGNALAVAILABLE
PAi Pin PADo Ao
PLPin
PD
Definition of powers
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M1 M2SOURCE LOAD
POWERINPUT SIGNALAVAILABLE
POWER POWER POWER
DEVICEINPUTMATCHINGNETWORK
OUTPUTMATCHINGNETWORK
ACTIVE
INPUT SIGNALACTUAL DEVICE
OUTPUT SIGNALAVAILABLE DEVICE
OUTPUT SIGNALAVAILABLE
PAi Pin PADo Ao
PLPin
PD
Most useful gains
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Transducer GainPower GainSystem Gain
Power actually delivered to the load relative to the input power delivered by the source.
G but with the effect of M1removed. G with optimumM1.
M1 M2SOURCE LOAD
POWERINPUT SIGNALAVAILABLE
POWER POWER POWER
DEVICEINPUTMATCHINGNETWORK
OUTPUTMATCHINGNETWORK
ACTIVE
INPUT SIGNALACTUAL DEVICE
OUTPUT SIGNALAVAILABLE DEVICE
OUTPUT SIGNALAVAILABLE
PAi Pin PADo Ao
PLPin
PD
Most useful gains
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Transducer Gain Available Gain
G with optimumM1.
M1 M2SOURCE LOAD
POWERINPUT SIGNALAVAILABLE
POWER POWER POWER
DEVICEINPUTMATCHINGNETWORK
OUTPUTMATCHINGNETWORK
ACTIVE
INPUT SIGNALACTUAL DEVICE
OUTPUT SIGNALAVAILABLE DEVICE
OUTPUT SIGNALAVAILABLE
PAi Pin PADo Ao
PLPin
PD
G with optimumM1and M2
Most useful gains
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Transducer Gain
G with optimumM1.
M1 M2SOURCE LOAD
POWERINPUT SIGNALAVAILABLE
POWER POWER POWER
DEVICEINPUTMATCHINGNETWORK
OUTPUTMATCHINGNETWORK
ACTIVE
INPUT SIGNALACTUAL DEVICE
OUTPUT SIGNALAVAILABLE DEVICE
OUTPUT SIGNALAVAILABLE
PAi Pin PADo Ao
PLPin
PD
Unilateral Transducer Gain
GT with S12 = 0.
GTU
Maximum Unilateral Transducer Gain
GTU with optimumM1 and M2
GTUmax
Development of gain expressions
Developed using generalized scattering parameters (which can be defined different and complex load and source impedances).
Then refer back to using transistor’s 50- S parameters
Different gains useful at different stages of design.
E.G. GTUmax used in selecting transistor, estimating design challenge.
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Amplifier Gain in Terms of Transistor S Parameters
Transducer Gain
For a unilateral two-port S12 = 0– Unilateral transducer gain
– Maximum unilateral transducer gain
– GTUmax IS THE MOST IMPORTANT FIGURE OF MERIT THAT GUIDESINITIAL DESIGN (design is very hard if required gain is greater).
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Maximum unilateral transducer gain, GTUmax, of the pHEMT transistor
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Even more gains
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M1 M2SOURCE LOAD
POWERINPUT SIGNALAVAILABLE
POWER POWER POWER
DEVICEINPUTMATCHINGNETWORK
OUTPUTMATCHINGNETWORK
ACTIVE
INPUT SIGNALACTUAL DEVICE
OUTPUT SIGNALAVAILABLE DEVICE
OUTPUT SIGNALAVAILABLE
PAi Pin PADo Ao
PLPin
PD
Maximum Available Power Gain
GT with optimum M1 and
M2
Maximum Stable Gain
GMA at edge of stability k = 1.
Transducer Gain
G with optimumM1.
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12MS
SGS
221
12
1MASG k kS
2 211 22
12 21
12
S Sk
S S
Stability consideration
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Activedevice
LS IN OUT
IN
S
NOISE
Unstable if |S IN | > 1.
L
NOISE
OUT
Unstable if |L OUT | > 1.
Input Output
Amplifier stability
For stable amplification– |SIN| must be less than one at all frequencies– |LOUT| must be less than one at all frequencies– For passive source and load |S| < 1, |L| < 1– Thus for unconditional stability require
– |IN| < 1– |OUT| < 1
IN
S
NOISE
Unstable if |S IN | > 1.
L
NOISE
OUT
Unstable if |L OUT | > 1.
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Amplifier Stability
Amplifier is unstable if
Unconditional stability if (as long as source and load are passive)
Note: magnitudes of complex numbers describe circles in the complex plane.
Formulas have been developed for a stability circle (center and radius).
ACTIVEDEVICE
LS IN OUT
or
and
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OutputInput
Amplifier Stability Amplifier is unstable if Output stability circles on the L plane:
STABLEin
LcrL
UNSTABLE in
L
UNSTABLE
cLr
STABLE
|S11| < 1 |S11| > 1
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Representations of stability circles
using shading to indicate the unstable region
using a dashed line to indicate the unstable region
stability circle of an unconditionally stable two‐port
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Output stability circles for |S11| < 1
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8 GHz. 16 GHz.1 GHz.
inSTABLE
UNSTABLE
in
STABLE
in
STABLE
UNSTABLE
L must be in stable region for amplifier to be stable
Input stability circles for |S22| < 1
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8 GHz. 16 GHz.1 GHz.
outSTABLE
UNSTABLE
STABLEout
STABLEout
UNSTABLE
S must be in stable region for amplifier to be stable
Unconditional stability criterion
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The amplifier is stable for any source and load (provided that |S11| < 1 and |S22| < 1 .
STABLE
UNSTABLE
STABLE
UNSTABLE
Conditionally stable
Unconditionally stable
k-factor of pHEMT
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Edwards-Sinsky Stability Criterion, factor
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STABLE
UNSTABLE
is the distance from the origin to the nearest point of the unstable region.
> 1 for unconditional stability
factor of pHEMT
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> 1 for unconditional stability,The greater the more stable.
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Input and output factor of pHEMT
Summary of stability considerations If an amplifier is unconditionally stable design is considerably
simplified. Even if an amplifier is not “unconditionally stable” it could still be
stable. Design is then tricky and is only done in special circumstances.
– E.g. very high frequency operation.– Consider a cell phone power amplifier connected to an antenna
Must be stable– When antenna is covered by hand.– Phone is placed on a metal surface.
Severe price if amplifier goes unstable– In a communication system the whole EM spectrum could be polluted.– Amplifier could self-destruct (thermal runaway).
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Back to design of the amplifier
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M1 M2TRANSISTOR
INPUTMATCHINGNETWORK
OUTPUT
NETWORKMATCHING
RFINPUT
RFOUTPUT
CONTROLCHIP
GATE
COLLECTOR OR DRAINFILTERLOW-PASS
FILTERLOW-PASS
AMPLIFIER
BIASOR BASE
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GMS = GMA but with k set to 1 (at edge of stability.
GT = G with optimum M1.
GMA = G with optimum
M1, M2
U = GMA with S12 = 0 .
GTUmax = G with S12 = 0, optimum M1, M2.
GT and GMA are the only gains that do not modify the device.
Gain circles of the pHEMT at 8 GHz
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12.96 dB
11.96 dB
10.96 dB
9.96 dB
13.96 dB
Plotted on the S plane
G with optimum M2
GMA = Gwith optimumM1 and M2
ACTIVEDEVICE
Z L
L
VS
Z S
S IN OUT
Output matching network design Design nearly always commences
with the output matching network.
The first design step is to choose an output matching network that will provide the appropriate impedances to ensure stability below 5 GHz and above 11 GHz.
To do this the stability circles must be considered, as the device is only conditionally stable below 5 GHz and above 11 GHz.
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> 1 for unconditional stability.
Output stability circles for |S11| < 1
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8 GHz. 16 GHz.1 GHz.
inSTABLE
UNSTABLE
in
STABLE
in
STABLE
UNSTABLE
L must be in stable region for amplifier to be stable
Capacitive at low frequency or perhaps open or short circuit.
Unconditionally stable at operating frequency of amplifier.
Look like a resistor or short at high frequencies.
pHEMT at 8 GHz
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Ignore S12
OutputMatchingNetwork
Z = R +jXOUT S S
Z = RLL Transistor
However the output of the transistor really looks like a resistor in parallel with a capacitor.
So RS = RL =
RS > RL
Consider output stability circles.
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8 GHz. 16 GHz.1 GHz.
inSTABLE
UNSTABLE
in
STABLE
in
STABLE
UNSTABLE
Consider output stability circles
49
8 GHz. 16 GHz.1 GHz.
inSTABLE
UNSTABLE
in
STABLE
in
STABLE
UNSTABLE
Alternative design.
Almost certainly will be unstable
Output matching network design
50
LoaddeviceActive
CxXSR L RL
CxXSR RLLxX
Activedevice o
o
Load
CL RL
CLSR
SX
PXRL
Xp = 152.4
Input matching network design
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Now consider S12 and loading
InputMatchingNetwork
ZIN
TransistorRS
Appropriate choice:
Input stability circles for |S22| < 1
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8 GHz. 16 GHz.1 GHz.
outSTABLE
UNSTABLE
STABLEout
STABLEout
UNSTABLE
S must be in stable region for amplifier to be stable
S
Input matching network design
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Xx
deviceActive
Source
CSR R L
LC
R LxXxX
SR L
Activedevice
Source
iiC
LR S
CL
SR
SX
PXR L
Xp = 34.23
Final amplifier schematic.
10 nH has a reactance of approximately 500 Ω at 8 GHz.
100 pF provides an RF short circuit at 8 GHz.
Simulated transducer gain is 13.2 dB– Compare to maximum available gain of 13.96 dB
– Error is because S12 was ignored in synthesizing the output matching network.
Design could also target a specified gain.54
RF OUTRF IN
VDD
L2L3
C1C2
L1
VGC3
Design for a specific gain at 8 GHz
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12.96 dB
11.96 dB
10.96 dB
9.96 dB
13.96 dB
Gain (gain circles) plotted on the Splane with optimum M2
GMA = Gwith optimumM1 and M2
Recall that simulated gain of design here is 13.2 dB.
Final amplifier
This is a surprisingly simple circuit.
Bias circuit integrated into matching networks.
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RF OUTRF IN
VDD
L2L3
C1C2
L1
VGC3