Adding a parallel shunt resistor across the PIN diode can may offer some diode control benefit. Here's an example of how this is achieved.

PIN diodes are devices commonly used as variable dynamic resistances in variable attenuators for RF and microwave applications. Driven to conduct such and so many milliamperes DC, they can receive that DC from any of numerous current source circuits. However, current source drive may not be the best approach. Instead, the addition of a parallel shunt resistor across the PIN diode may offer some diode control benefit.

In the example that follows, there is a shunt resistance of 150Ω in parallel with the illustrated PIN diode. The characteristic V-I curve for that diode is estimated from the general diode V-I equation where the "reverse" current value for the part is set to 7.6414E-19A so that the modelled forward voltage drop of 1V at a forward current of 50mA matches the specification of the PIN diode whose datasheet is excerpted here.

The fraction of the excitation current that reaches the PIN diode assembly is less than unity and varies versus the excitation level and versus the resistance value itself.

A sample calculation of the effect follows below.

**Figure 1:** *Current division of the PIN diode and a shunt resistance.*

Since derving an explicit form of the governing equation for the PIN diode current is not within my skill set (Maybe someone else can do that.), a small program was written using an implicity equation and iteration to solve for the PIN diode current, Id, versus the excitation current, Is.

The iteration process goes like this:

**Figure 2:** *Iteration flow chart.*

In this process (Yes, this code is in GWBASIC. I just keep on using it.), we force Idleft to be very closely equal to Idright so that the two of them converge essentially to the value of PIN diode current, Id.

The iteration process executed in GWBASIC looks like this:

10 CLS:SCREEN 9:COLOR 15,1:YSTART=100:XSTART=420:PI=3.14159265#
20 PRINT "save "+CHR$(34)+"pindiod1.bas"+CHR$(34):PRINT
30 PRINT "save "+CHR$(34)+"a:\pindiod1.bas"+CHR$(34):PRINT:PRINT
40 A$="#.## volts ####.#### mA":ON ERROR GOTO 150
50 B$="Is= #.### Id= #.### Id/Is= #.### Ir= #.#####"
60 T=300:K=1.38E-23:Q=1.602E-19:IO=7.6414E-19:GOTO 80
70 ID=IO*(EXP(Q*VD/K/T)-1):RETURN
80 R=150:FOR IS=0 TO .2001 STEP .01:IDRIGHT=IS*.9
90 IDLEFT=IO*(EXP(Q*(IS-IDRIGHT)*R/K/T)-1):IR=IS-IDLEFT
100 DELTA=.0000001:IF IDLEFT>IDRIGHT THEN IDRIGHT=IDRIGHT+DELTA
110 IF IDLEFT<IDRIGHT THEN IDRIGHT=IDRIGHT-DELTA
120 IF ABS(IDLEFT-IDRIGHT)<.000001+IS/100 THEN GOTO 130 ELSE GOTO 90
130 IF IS>0 THEN IRATIO=IDLEFT/IS
140 PRINT USING B$;IS,IDLEFT,IRATIO,IR:NEXT IS
150 END

The advantage, and admittedly the trade-off as well, of using the shunt resistance, R, is that the PIN diode current can be reduced to a very small value while the current source is still delivering a non-zero but easily controlled current value to its load. For R = 100Ω as an example, the PIN diode receives only 1mA while the current source is delivering 10mA.

Making a current source circuit that is easily controlled at a very low current can sometimes be a bit tricky but the inclusion of R makes the task of delivering very low currents to the PIN diode just a bit easier, albeit at the tradeoff of some non-linearity at that low current level as seen in the following tabulations.

R = 100Ω R = 150Ω
IS= 0 ID= 0 ID/IS= 0 IR= 0 IS= 0 ID= 0 ID/IS= 0 IR= 0
IS= .01 ID= .001 ID/IS= .092 IR= .00908 IS= .01 ID= .004 ID/IS= .368 IR= .00632
IS= .02 ID= .010 ID/IS= .510 IR= .00979 IS= .02 ID= .013 ID/IS= .668 IR= .00664
IS= .03 ID= .020 ID/IS= .665 IR= .01006 IS= .03 ID= .023 ID/IS= .772 IR= .00683
IS= .04 ID= .030 ID/IS= .743 IR= .01027 IS= .04 ID= .033 ID/IS= .825 IR= .00700
IS= .05 ID= .040 ID/IS= .791 IR= .01043 IS= .05 ID= .043 ID/IS= .857 IR= .00713
IS= .06 ID= .049 ID/IS= .824 IR= .01059 IS= .06 ID= .053 ID/IS= .879 IR= .00725
IS= .07 ID= .059 ID/IS= .846 IR= .01075 IS= .07 ID= .064 ID/IS= .914 IR= .00602
IS= .08 ID= .069 ID/IS= .864 IR= .01088 IS= .08 ID= .074 ID/IS= .925 IR= .00597
IS= .09 ID= .079 ID/IS= .878 IR= .01100 IS= .09 ID= .084 ID/IS= .935 IR= .00588
IS= .10 ID= .089 ID/IS= .889 IR= .01112 IS= .10 ID= .094 ID/IS= .942 IR= .00579
IS= .11 ID= .101 ID/IS= .917 IR= .00911 IS= .11 ID= .104 ID/IS= .948 IR= .00573
IS= .12 ID= .111 ID/IS= .925 IR= .00902 IS= .12 ID= .114 ID/IS= .953 IR= .00563
IS= .13 ID= .121 ID/IS= .931 IR= .00896 IS= .13 ID= .124 ID/IS= .957 IR= .00554
IS= .14 ID= .131 ID/IS= .936 IR= .00891 IS= .14 ID= .135 ID/IS= .961 IR= .00548
IS= .15 ID= .141 ID/IS= .941 IR= .00880 IS= .15 ID= .145 ID/IS= .964 IR= .00543
IS= .16 ID= .151 ID/IS= .946 IR= .00871

IS= .17 ID= .161 ID/IS= .949 IR= .00862

IS= .18 ID= .171 ID/IS= .953 IR= .00854

IS= .19 ID= .181 ID/IS= .955 IR= .00850

IS= .20 ID= .192 ID/IS= .958 IR= .00842

R = 200Ω IS= 0 ID= 0 ID/IS= 0 IR= 0 IS= .01 ID= .005 ID/IS= .519 IR= .00481 IS= .02 ID= .015 ID/IS= .748 IR= .00504 IS= .03 ID= .025 ID/IS= .826 IR= .00521 IS= .04 ID= .035 ID/IS= .866 IR= .00535 IS= .05 ID= .046 ID/IS= .910 IR= .00449 IS= .06 ID= .056 ID/IS= .926 IR= .00446 IS= .07 ID= .066 ID/IS= .937 IR= .00438 IS= .08 ID= .076 ID/IS= .946 IR= .00431 IS= .09 ID= .086 ID/IS= .953 IR= .00419 IS= .10 ID= .096 ID/IS= .959 IR= .00409 IS= .11 ID= .106 ID/IS= .964 IR= .00400

What combination of low current control versus what degree of non-linearity can be tolerated is a choice left up to the designer.