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Diode Circuits

Diode Circuits. By Professor Syed Idris Syed Hassan Sch of Elect. & Electron Eng Engineering Campus USM Nibong Tebal 14300 SPS Penang. Application of diodes. Rectifier Detector Mixer switching/switch Phase shifter Attenuator. Type of diodes. Basic diode characteristic.

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Diode Circuits

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  1. Diode Circuits By Professor Syed Idris Syed Hassan Sch of Elect. & Electron Eng Engineering Campus USM Nibong Tebal 14300 SPS Penang

  2. Application of diodes • Rectifier • Detector • Mixer • switching/switch • Phase shifter • Attenuator

  3. Type of diodes

  4. Basic diode characteristic V-I characteristic Equivalent circuit Package components Contact resistance Rs Junction components where a= q /nkT , q =charge, k=Boltzmann’s constant, T = temperature, n = ideality factor and Is = saturation current.

  5. Continue Let’s say diode voltage V = Vo + u where Vo is a DC bias voltage and u is a small AC signal voltage. We expand using Taylor series Taylor series Reminder Taking f(x) = I(V), then x= Vo+u and a = Vo By substituting, we have (x-a) = ( Vo+u -Vo)= u and the Taylor series for I(V) is where and

  6. Continue By substituting and Io= I(Vo) in the first derivative where Similarly in the second derivative, we have (400) Then

  7. Rectifier application RF in DC out If the diode voltage consist of DC and small RF signal V = Vo + uocos wot where Vo is a DC bias voltage and u cos wot is a small RF signal voltage. Then by substituting into (400)

  8. continue Rearrange AC harmonics current of frequency wo and 2wo. This can be filtered off by using lowpass filter DC rectified current

  9. Detector application Modulated signal representation where m = modulation index wo= RF carrier frequency wm= modulation frequency

  10. continue

  11. Trigonometry relationship

  12. k km/2 km/2 continue From the eq. above we have several harmonics as shown with relative amplitude. k=uoGd/(uo2Gd’/4) =4/(uoa) As linear detector ( ~uoGd) As squared detector ~uo2Gd’/4

  13. Square-law region of diode detector We are measuring power , thus square-law region is to be chosen since the power measured is proportional to uo2. If we want to measure voltage , then the linear region is the choice. For linear detector,we choose frequency at wo and for square-detector at 2wo.. Using filter we can filter out the modulating frequency wm.

  14. Single-ended mixer Downconverter Upconverter The purpose of mixer is to convert either from one frequency to higher frequency or vice versa. The advantages of conversion are (i) to reduce 1/f noise when convert to lower frequency (ii) for easy tuning for a wide band with fixed IF and (iii) frequency off-set between transmitter and receiver by using a single LO as in Radar.

  15. Simplest Single-ended mixer • Uses nonlinearity of a diode property • The output generated consist of frequencies spectrum dc component, wr,wo,wr-wo, wr+wo. • For IF, we filter out all frequencies except wr-wo. • For upconverter, we filter out all lower frequencies and allow only wr+wo. • Combiner either T-junction or directional coupler • Matching network is to match the output of combiner to the diode circuitry.

  16. analysis Let’s and Then substituting into equation (400) and we have for the second term as DC Figure of merit in mixer is its conversion loss, defined as

  17. Single Balanced Mixer Circuit * Note that, although it is not shown, the diodes required biasing and matching network. • Advantages • For either better input SWR or better RF/LO isolation • Cancellation of AM noise from LO

  18. analysis Let’s and Where vr<<vo and vn(t)<<vo Vn is a small random noise voltage The voltages across the two diodes of 90o out of phase is given as Diode 1 Diode 2

  19. Diode current Assuming identical diodes so that diode currents can be represented as and (reverse polarity) Dc and lower frequency bands

  20. IF frequency band After low pass filtering, the remaining terms are dc and IF frequency terms, thus Written the IF frequency wi = wr- wo then the IF current is where vn << vo . This show that the noise in the first order is cancelled by the mixer while the desired IF signal combined in phase.

  21. Anti parallel diode mixers The LO is one-half of usual LO, I.e The non-linearity of diode generates 2nd harmonic of LO to mix with RF(wr) to produce desired IF. The anti parallel diode creates symmetrical V-I characteristic that suppresses the fundamental product of RF and LO. It also suppresses AM noise.

  22. Double Balanced mixer Single -ended mixer produces output consisted of all harmonics. The balanced mixer using hybrid suppresses all even harmonics of the LO. Double balanced mixer suppresses all even harmonics both LO and RF.

  23. Image rejection mixer The RF with frequency wr= wo +wi will also produce the IF (wi) when mixed with LO. The frequency produced will be USB(wr= wo + wi ) and LSB(wr= wo - wi ) . The undesired frequency either USB or LSB is called image frequency. The mixer can produce one single side band is used as modulator.

  24. Analysis Let RF signal consist of both upper and lower sidebands Then input to mixer A and B After mixing with LO, wo , The IF’s produced by mixer are.

  25. Analysis Both IF , then combined in the 90o hybrid produces LSB and USB.

  26. Pin Diode Equivalent Circuit symbol

  27. Equivalent circuits for ON and OFF states of PIN diodes Reverse bias will provide OFF state Forward bias will provide ON state

  28. Single-pole PIN diode Switches ON= RF out OFF=No RF out RF in RF out Note: C1 and C2 are dc block RF in RF out ON =No RF out OFF= RF out

  29. Simplified switching circuits In general, the insertion loss Series switch Shunt switch where

  30. Example A single-pole switch is to be constructed using a PIN diode with the following parameters: Cj= 0.1pF, Rr= 1W, Rf= 5 W , Li= 0.4nH. If the operating frequency is 5 GHz and Zo= 50W, which circuit (series or shunt) should be used to obtain the greatest ratio of off-to-on attenuation? Solution Ratio 10.05dB Ratio 7.04dB state Shunt switch Series switch ON OFF

  31. Other Single pole single throw PIN Switches Configuration Single Pole Single Throw Note: Biasing are not shown, just diodes configuration

  32. SPST Switches performance

  33. PIN diode switching operation (Shunt diode) By putting diodes in parallel will reduce the total diode resistance and thus improves isolation as shown in graph.

  34. PIN diode switch (improving isolation) Isolation is maximum when the transmission line is exactly 90o and the effect of diodes similar to without transmission line when its length equal to 0o or 180o. 50W

  35. PIN diode switch(input impedance not 50W 50W Compensating line is a 90o transmission line to match the Rs with 50ohm line.Rs is lower than 50 ohm.

  36. All-shunt Diode Nonreflective SPST Switch

  37. PIN diode switching operation (Serial diode) By putting diodes in series will reduce the total effective series capacity, thus increase isolation. This is shown in graph below.

  38. PIN diode switching operation In this case the optimum line line is not 90o, but depend on the diode capacity.

  39. Single pole double throw PIN diode switches • Operation • One diode is biased in low impedance state with another diode in the high impedance state, so that input signal can be switched to one output to the other by reversing the diodes state or biasing. • The quarter-wave lines limit of the shunt circuit limit the bandwidth.

  40. PIN diode application(TR switch) Dc supply “ON” for transmitting. D1 and D2 will conduct, allowing the signal from transmitter to go to antenna and any signal go to receiver will be shorted. When dc supply “OFF”, D1 and D2 will not conduct, thus allowing only signal from antenna flow to the receiver.

  41. PIN diode application(Reflective phase shifter)

  42. Switched line phase shifter • Using two single pole and double throw switches to route the signal between one of two transmission lines of difference length. • The phase difference is . This circuit is a broadband & reciprocal phase shifter so that it can be used as receiver as well as transmitter. • Disadvantages-resonance when the length is multiple of l/2 and frequency is shifted due to diode capacitance.

  43. PIN diode application(8-steps phase shifter)

  44. PIN diode application(8-steps phase shifter) • When A, B or C is set “1” then D1 and D2 will conducted allowing the RF go straight. • When A,B or C is set “0” then D1 and D2 will not conducted and the RF signal will experienced phase shift according to the length of U -line. • l/2 is 90o phase shift, l/4 is 45o phase shift and l/8 is 22.5o phase shift.

  45. Switching equivalent phase shift A B Phase shift C

  46. PIN diode application Bridged T attenuator Attenuation factor is defined as Attenuation is small when D2 is forward biased (low impedance) and D1 is reverse biased (high impedance). Conversely , attenuation is large when D2 is reverse biased (high impedance) and D1 is forward biased (low impedance). where

  47. PIN diode application p attenuator Attenuation is small when D3 is forward biased (low impedance) and D1and D2 are reverse biased (high impedance). Conversely , attenuation is large when D3 is reverse biased (high impedance) and D1 and D2 are forward biased (low impedance).

  48. PIN diode application(intermodulation attenuator) At high input voltage and low attenuation, D1 tends to conduct signal.R1 and R2 set the current and isolate the dc. D2 tends to be off. At low input voltage and high attenuation, D1 tends to be off. D2 tends to bypass the signal to ground. R3 and R4 set the current and isolate the dc. R5 and R6 maintain the characteristic impedance

  49. PIN diode application (intermodulation attenuator) Attenuation of signal after applying Vin for frequency 100MHz to 400MHz

  50. PIN diode application(intermodulation attenuator) Return loss is less than 10 dB. It seem the impedance characteristic is maintain.

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