Radio Frequency Electronics Active Components I Harry Nyquist Born in 1889 in Sweden Received B.S. and M.S. from U. North Dakota Received Ph.D. from Yale Worked and Bell Laboratories for all of his career Known for Nyquist rate, Nyquist plots, Nyquist stability criterion, Johnson-Nyquist noise, Nyquist-Shannon Sampling heorem, and others Died in 1976 Image from Wikipedia 1
Ideal Current-Voltage Relationship heoretical ( ideal ) diode equation: i D vd nv I S e 1 i D = diode current I S = reverse-bias saturation current v D = voltage across diode V = hermal voltage V = k/e 26 mv at = 300 K n = Emission coefficient 1 n 2, unless otherwise noted, assume n = 1 In many/most cases a good approximation for a forward-biased diode is i I D S e v V D 2
AC equivalent Circuit i D g d I S e di dv v d / V D D D d Isd e d v / V I s i gd V D e Ise dv d v d / D V v dv vd / V D / V vd / V 1 r d I DQ 1 g d I D V DQ Slope V I DQ Chain rule Small-signal incremental resistance r d r d = V I DQ = 1 40I DQ 3
Forward-Biased Diode & Diffusion Capacitance More refined small-signal model r d C d 1 g d dq dv Even more complete small signal model D V I DQ ransit time I D V g d Change in minority carrier stored charge with time-varying voltage superimposed on dc quiescent voltage. he change in stored charge leads to a diode diffusion capacitance. Diffusion capacitance is normally much larger than junction capacitance r d C j C 1 g d d V I C V 1 V I D V j0 DQ R bi m g d 4
he Reverse-Biased pn Junction depletion p (conductive) n (conductive) Reverse voltage s electric field aids built-in electric field => increases depletion region C J = C J0 1 V A V bi 1 m+2 Varactor or varicap diodes C J0 = junction capacitance at zero applied voltage, and m is the junction grading coefficient. 5
Schottky Barrier Diode Semiconductor/metal junction I qv I o e / nk n is the ideality factor: 1-2 I o is much larger than in Si/Ge diodes Significantly lower turn-on voltage Fast switching when compared to pn junction diode. Storage time t s is essentially zero. Larger reverse saturation current Lower reverse breakdown voltage Widely-used in reverse-protection circuits Widely used in switching dc-dc converters Widely-used in RF as mixers 6
Schottky Barrier Diode Circuit model for Schottky diode under forward bias 7
Varactors uning diodes are manufactured to take advantage of the voltage dependence of the junction capacitance. hese diodes are called varactors or varicaps. C J = C J0 1 V A V bi 1 m+2 he tuning ratio (R) is a figure of merit, indicating how much one can change the capacitance. R C J(V A1 ) C J (V A2 ) V A1 V A2 1 m+1 Rs range from 2.4 to 25. Hyperabrupt junctions (m = 1) has largest R and is of significant commercial interest. Large, expensive mechanical variable capacitors have been made obsolete by varactors hrough-hole varactor 8
Example Varactor SD199E6327 1.25 mm Cost less than $1 9
Varactor Application Variable capacitors are used in radio tuning circuits to select various stations and channels. Cellphones, WiFi, V,. 10
Varactor Diodes Coupling capacitors oscillator uning voltage Resonant tank More than 99% of radios (cell phones) use varactor diodes for tuning 11
Varactor Applications Pulses are on the order of 1 ns or less 12
Reverse Recovery Stored charge Current Question: What will happen to stored charge when diode is suddenly reverse-biased? Reverse recovery time for general-purpose diodes (1N4001) is a few μs Switching diodes can have reverse recovery times of few ns Schottky diodes can have reverse recovery times of 100 ps 13
Diode Reverse Recovery 10 V 60 Hz Circuit work in normal fashion 10 khz Circuit works in normal fashion, but reverse-recovery effect is starting to appear 14
Diode Reverse Recovery 10 V 500 khz he diode conducts, even when reverse biased 10 MHz Diode conducts in reverse direction for most of the cycles 15
RF PIN Diodes PIN diodes exploit the storage of carriers at the junction. hey consist intrinsic (I) semiconductor, sandwiched between a p and n material. hus, they are called p-i-n diodes or simply PIN diodes. When reverse-biased, PIN diodes behave much like other diodes, except that the capacitance does not change much with applied voltage. hey would make poor varactors. When PIN diodes are forward-biased, the intrinsic region is flooded with carriers. hey have poor reverse-recovery time, so that at high frequencies the stored charge does not have time to empty the diode. hus, the diode behaves as a resistor to RF. he value of the resistance can be controlled with the dc current. Images from RF Global Net 16
PIN Diodes Check out www.youtube.com/watch?v=xpyscm_wf50 17
PIN Diode Biasing Opens for RF Blocks dc Blocks dc Opens for RF Blocks dc Opens for RF Blocks dc Opens for RF Connects to ground for RF 18
V ctrl = 10 V V ctrl = +10 V Diode is forward-biased and functions as a resistor, shunting signal to ground V o = 6.6 V V o = 18 mv R D 0.1 Ω 19
RF PIN Diodes Applications PIN diodes are used in transceivers, protecting the receiver when the transceiver is transmitting. Remember how a λ 4 transformer works - one end sees the dual impedance at the other end. Apply dc when transmitting 20
RF PIN Diodes Applications PIN diodes are used in transceivers, protecting the receiver when the transceiver is transmitting. Remember how a λ 4 transformer works - one end sees the dual impedance at the other end. Apply dc when transmitting his diode is forwardbiased, and become a lowvalue resistance 21
RF PIN Diodes Applications PIN diodes are used in transceivers, protecting the receiver when the transceiver is transmitting. Remember how a λ 4 transformer works - one end sees the dual impedance at the other end. Apply dc when transmitting his diode is forwardbiased, and become a lowvalue resistance he λ 4 transformer makes the low impedance at the other end look like an open and blocks the transmitter RF 22
RF PIN Diodes Applications PIN diodes are used in transceivers, protecting the receiver when the transceiver is transmitting. Remember how a λ 4 transformer works - one end sees the dual impedance at the other end. Apply dc when transmitting his diode is also forwardbiased, and become a lowvalue resistance his diode is forwardbiased, and become a lowvalue resistance he λ 4 transformer makes the low impedance at the other end look like an open and blocks the transmitter RF 23
RF PIN Diodes Applications PIN diodes are used in transceivers, protecting the receiver when the transceiver is transmitting. Remember how a λ 4 transformer works - one end sees the dual impedance at the other end. Apply dc when transmitting his diode is also forwardbiased, and become a lowvalue resistance his short provides additional protection his diode is forwardbiased, and become a lowvalue resistance he λ 4 transformer makes the low impedance at the other end look like an open and blocks the transmitter RF 24
RF PIN Diodes Applications PIN diodes are used in transceivers, protecting the receiver when the transceiver is transmitting. Remember how a λ 4 transformer works - one end sees the dual impedance at the other end. Apply dc when transmitting his diode is forwardbiased, and become a lowvalue resistance he λ 4 transformer makes the low impedance at the other end look like an open and blocks the transmitter RF 25
RF PIN Diodes Applications PIN diodes are used in transceivers, protecting the receiver when the transceiver is transmitting. Remember how a λ 4 transformer works - one end sees the dual impedance at the other end. With no dc here both diodes have very high resistance, effectively disconnecting the transmitter from the antenna and connecting the receiver to the antenna 26
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