ELCT564
Spring 2012
Diodes, Transistors and Mixers
4/13/2015
ELCT564
1
Diodes
• Diodes are non-linear devices
Passive diodes --- signal detection, phase shifting
• Non-linear devices
Active FET, MESFET, HBT, BiCMOS -- signal generation
and amplification
• Rectifying properties of semiconductor materials
Known since 1874 --- created by the establishment of electrostatic barrier in
the path of the current flow
• Electrostatic barrier can be created by junctions
Conductivity when charge carriers have enough energy to overcome barrier
(usually thermionic emission)
PN Junctions
• When an n-type and a p-type semiconductor are brought into contact a
pn junction is formed --- acts as a diode
• Carriers: electrons and lack of electrons (holes)
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Diodes
• n-type: majority electrons, p-type: majority holes
• When in contact electrons + holes diffuse across the interface and they
recombine when they reach the opposite layer—until thermal equilibrium
• Electric field that stops diffusion is set up – V0 contact potential depends on
doping concentrations on each side
Depletion region is formed (no carriers)
• If external voltage is applied --- junction is biased, net flow of charge and energy
• Forward bias: reduces potential difference from |V0| to |V0 | - |Vext|
net current flow from p-side to n-side
• Reverse bias: increases potential difference from |V0 | to |V0 | + |Vext|
carriers cannot overcome the additional barrier--- little current due to the charge
densities
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Diodes
• When an ohmic contact is placed on the surface of the n-doped semiconductor
only majority carrier diffusion dominates the junction performance
• Free electrons that enter the metal have positively charged region depleted of
carriers just underneath the metal contact
• This positive charge gives rise to an electrostatic barrier V0
• Fwd-bias diffusion current from electrons in semiconductor dominates
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Practical Diodes
• Finite resistivity of the bulk semiconductor----voltage drop--- reduces the fraction
of the applied voltage that appears across the junction
• Current shows a saturation tendency
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Transistor
GaAs MESFET
BJT
Minimum transit time from emitter to collector
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BJTs are planar and made of si
in the npn type
Low cost and reliable solution
below 4 GHz
Applications: small signal
amplifiers, linear power
amplifiers, low noise amplifiers
and oscillaors
Use ion-implantation for
fabrication and self-alignment:
Multi-finger emitter-base
construction
Transistor Model
More advanced hybrid-p model
Model for packaged transistors
Lp ~ 0.2 to 1nH
Cp ~ 0.01 to 0.05 pF
Advanced CAD programs allow designers to
compare measured s-parameters of transistor
with those obtained from model.
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Field Effect Transistors
Metal- semiconductor field-effect transistor
Gate terminal uses a Schottky barrier gate
Microwave FETS with GaAs– greater e- mobility– f>4GHZ
Excellent freq. response & noise performance
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Mixers and Detectors
Use non-linear characteristic of device to generate an output signal containing many
frequency components
Detection
Diode is receiving only one high frequency ωRF
DC term which includes information about the amplitude of the RF signal
Diode sensitivity = PDC/ PRF
Frequency diagram of a detector diode
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Mixing
Required in order to get amplitude +phase information
Need two high (RF) frequencies close to each other RF frequency and LO frequency
(produced by a local oscillator)
Goals for the design of a good mixer
Optimum match for the RF and LO signals at RF & LO ports
Suppression of the DC component at RF, LO, IF ports
Optimum match for the IF at the IF port
Optimum termination of higher harmonics at RF, LO & IF ports
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Mixer Diode
Let’s assume that the incoming signal to
the diode is the superposition of the RF &
LO as shown below:
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Single Diode Mixers
Fundamental to the design of all mixers
Every mixer can be scaled down to a single diode one
At very high frequencies exhibit best performance in terms of Lc
But not that good in terms of RF/LO isolation
Problems : Leakage of IF through RF/LO ports, leakage of RF/LO through IF port, leakage of
DC through all ports, coupling between RF and LO, mismatch between RF/LO and IF ports and
the diode --- need to modify circuits
•
Transfer of RF, LO powers to the diode is optimized
by using matching networks
• Transfer of IF power from diode to connecting circuits is
optimized by appropriate design + choice of diode
• Image frequency has to be determined & isolated
from RF/LO & IF ports
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Microstrip Single Diode Mixer
Single diode mixers are not used effectively or frequently at microwave
frequencies
Found more often in single balanced mixers
Single stub match for ZRF /LO
fLO =20 GHz, fIF =1 GHz
fRF fLO+ fIF
IF filter extracts only desired frequencies
and matches ZIF
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Mixer Measurements
Conversion loss: Lc =10log(PRF /PIF)
(if using BJT & FET – conversion gain!!)
Calibrate RF, LO powers before measurement
Find cable losses
Calibrate Spectrum analyzer
Noise Figure
Use N.F. meter
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Balanced Mixers
Single diode mixers are easy to make but have difficulty in isolating the RF from LO
ports --- balanced mixers overcome this problem
Also give cancellation of AM noise from the local oscillator
Single balanced mixers
• 2 complete single diode mixers connected
to mutually isolated ports of a hybrid
• RF & LO connecting to mutually isolated
ports
• IF outputs are usually connected in parallel
or combined through another hybrid
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180o Hybrid Balanced Mixer
Microstrip configuration
Vd1=VRF + VLO
Vd2=VRF - VLO
In to get a non-zero IF --- 2 diodes are connected in an
anti-parallel configuration.
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Rat-race Mixer
not appropriate for all applications but have simple
design, low LO requirements and simple
bias circuit
Example of a mixer used in an integrated low-noise
receiver
IF output is filtered directly from the ring and low
frequency blocks are used on the RF and LO lines
Anti-parallel diode mixer
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Double Balanced Mixer
• Suppresses even harmonics of both LO + RF
• Four diodes in configuration
• Very low conversion loss
Image Rejection Mixer
Fabricated Monolithic FGC Mixer
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