ELCT564 Spring 2012

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Diodes, Transistors and Mixers

Diodes

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• Diodes are non-linear devices

• Non-linear devicesPassive diodes --- signal detection, phase shifting

Active FET, MESFET, HBT, BiCMOS -- signal generationand amplification

• Rectifying properties of semiconductor materials Known since 1874 --- created by the establishment of electrostatic barrier inthe 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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• 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

Diodes

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• 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

Diodes

Practical Diodes

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• 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

Transistor

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GaAs MESFET

BJT

Minimum transit time from emitter to collector

BJTs are planar and made of si in the npn typeLow 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

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More advanced hybrid-p modelModel for packaged transistorsLp ~ 0.2 to 1nHCp ~ 0.01 to 0.05 pF

Advanced CAD programs allow designers to compare measured s-parameters of transistor with those obtained from model.

Field Effect Transistors

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Metal- semiconductor field-effect transistor

Gate terminal uses a Schottky barrier gate

Microwave FETS with GaAs– greater e- mobility– f>4GHZExcellent freq. response & noise performance

Mixers and Detectors

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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

Mixing

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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 portsOptimum match for the IF at the IF port

Optimum termination of higher harmonics at RF, LO & IF ports

Mixer Diode

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Let’s assume that the incoming signal to the diode is the superposition of the RF & LO as shown below:

Single Diode Mixers

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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

Microstrip Single Diode Mixer

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Single diode mixers are not used effectively or frequently at microwave frequencies

Found more often in single balanced mixers

IF filter extracts only desired frequenciesand matches ZIF

Single stub match for ZRF /LO

fLO =20 GHz, fIF =1 GHz

fRF fLO+ fIF

Mixer Measurements

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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 FigureUse N.F. meter

Balanced Mixers

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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

180o Hybrid Balanced Mixer

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Microstrip configuration

Vd1=VRF + VLO

Vd2=VRF - VLO

In to get a non-zero IF --- 2 diodes are connected in ananti-parallel configuration.

Rat-race Mixer

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not appropriate for all applications but have simple design, low LO requirements and simplebias 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

Double Balanced Mixer

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• Suppresses even harmonics of both LO + RF

• Four diodes in configuration

• Very low conversion loss

Image Rejection Mixer

Fabricated Monolithic FGC Mixer