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 where  = q /nkT, q =charge, k=Boltzmann’s constant, T = temperature, n = ideality factor and I s = saturation current. Package components RsRs Junction components Contact resistance

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

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

7. Rectifier application If the diode voltage consist of DC and small RF signal V = V o +    cos   t where Vo is a DC bias voltage and  cos  o t is a small RF signal voltage. Then by substituting into (400) RF inDC out

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

9. Detector application Modulated signal representation where m = modulation index  m = modulation frequency  o = RF carrier frequency

10. continue

11. Trigonometry relationship

12. continue From the eq. above we have several harmonics as shown with relative amplitude. k km/2 k=  o G d /(  o 2 G d ’/4) =4/(  o  ) As linear detector ( ~  o G d ) As squared detector ~  o 2 G d ’/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  o 2. If we want to measure voltage, then the linear region is the choice. For linear detector,we choose frequency at  o and for square-detector at 2  o.. Using filter we can filter out the modulating frequency  m.

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,  r,  o,  r -  o,  r +  o. For IF, we filter out all frequencies except  r -  o. For upconverter, we filter out all lower frequencies and allow only  r +  o. Combiner either T-junction or directional coupler Matching network is to match the output of combiner to the diode circuitry.

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

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

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

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  i =  r -  o then the IF current is where v n << v o. 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(  r ) 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  r =  o +  i will also produce the IF (  i ) when mixed with LO. The frequency produced will be USB(  r =  o +  i ) and LSB(  r =  o -  i ). 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,  o, The IF’s produced by mixer are.

25. Analysis Both IF, then combined in the 90 o 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 RF in RF out ON =No RF out OFF= RF out ON= RF out OFF=No RF out Note: C 1 and C 2 are dc block

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: C j = 0.1pF, R r = 1 , Rf= 5 , L i = 0.4nH. If the operating frequency is 5 GHz and Z o = 50  which circuit (series or shunt) should be used to obtain the greatest ratio of off-to-on attenuation? Solution Series switch Shunt switch ON OFF state Ratio 10.05dB Ratio 7.04dB

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

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

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

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 90 o, 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 /2 and frequency is shifted due to diode capacitance.

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

44. 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.  2 is 90 o phase shift, /4 is 45 o phase shift and  8 is 22.5 o phase shift.

45. Switching equivalent phase shift AB C Phase shift

46. PIN diode application Bridged T attenuator where 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). Attenuation factor is defined as

47. PIN diode application  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 V in 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.

51. Attenuator (transmission mode) Attenuator (Reflection mode) Diode ON-attenuated Diode OFF- transmitted