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8/11/2019 PIN Diodes
1/5
4/30/2007 PIN Diodes 1/5
Jim Stiles The Univ. of Kansas Dept. of EECS
PIN Diodes
Q: Just how do we makeswitches and voltage controlled
attenuators?
A: Typically, they are constructed withPIN diodes.
A PIN diode is simply ap-njunction diode that is designed to
have a very small junction capacitance (0.01 to 0.1 pf).
Sort of theoppositeof the varactordiode!
To see why this is important, recall diode small signal analysis
from your first electronics course.
In small signal analysis, the totaldiode voltage consists of a D.C.
bias voltage(VD) and a small,time-varying signal ( dv ):
) )( (D dD vV tv t = +
For radio engineering applications, the small signal is a
microwavesignal!!! I.E.,:
) )( (D RD F tv vVt = +
Thus, we know that the diode currentiD is:
( )exp 1RFDsD
T
V v ti I
nV
+=
8/11/2019 PIN Diodes
2/5
4/30/2007 PIN Diodes 2/5
Jim Stiles The Univ. of Kansas Dept. of EECS
Since vRFis very small, we can approximatethis diode current
iD(vD) using a Taylor Series expansion around vD=VD:
( )
( ) ( ) ( )
1 ( )
v VD D
v VD D
D
TD
T
D D
RFD D D D D
VnVV
nV SS RF
T
i v
i v i v v t v
I eI e v t
nV
=
=
+
= +
We recognize that:
D1 D.C. Bias Current ID
T
VnV
SI e
=
and thus we can write our small-signal approximationas:
( )( )
( )
sDRFD D
T
RFD
d
I Ii I v t
nVv t
Ir
+= +
= +
where we have defined the diode small-signal resistancerd as:
T
dsD
nV
r I I= +
The diode small-signal resistance is also oftenreferred to as
thejunction resistance Rjor the seriesresistanceRs.
8/11/2019 PIN Diodes
3/5
4/30/2007 PIN Diodes 3/5
Jim Stiles The Univ. of Kansas Dept. of EECS
We can further conclude that the total diode current iDis the
sum of the D.C. bias current ID, and the small-signal current
iRF(t), where:( )
( )RF
RFd
v t
i t r=
Just like Ohms Law!
To a small (i.e., low power) microwave signal, a diode looks like
a resistor.
Moreover, we can controland modifythe resistance of thediode bychangingthe D.C. bias.
Sort of a voltage-controlled resistor!
For example, if we put the diode into forwardbias ( D TV nV>> ),
the bias current IDwill be positive and big, thus the junction
resistance will be very small(e.g., rd = a few ohms).
A forwardbiased diode is very nearly a microwave
short circuit!
Iget it! If wereverse
bias our diode, such thatVD
8/11/2019 PIN Diodes
4/5
4/30/2007 PIN Diodes 4/5
Jim Stiles The Univ. of Kansas Dept. of EECS
Not so fast! The small-signal resistanceof a reverse
biased diode is in fact very large. BUT, we must
also consider the junction capacitance Cj!
Recall that in reversebias, the junction capacitance of a diodecan be significant, and in fact generally increasesas the bias
voltage becomes more negative!
As a result, a good microwave circuit model of a diode includes
both the series resistance and junction capacitance:
1d
Djd
rZ
j r C=
+
For forwardbias, where rd is very small, we find that diode
impedance ZDis approximately equal to this smallseries
resistance( D dZ r )a shortcircuit (approximately):
rdCj
0fb
D dZ r Z
8/11/2019 PIN Diodes
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4/30/2007 PIN Diodes 5/5
Jim Stiles The Univ. of Kansas Dept. of EECS
For reverse bias, where rd is very large, we find that diode
impedance ZDis approximately equal to that of the junction
capacitanceCj:
For low-frequencies (e.g., kHz), this impedance will be typically
beverylarge and thus the diode can be approximate as an open
circuit.
However, at microwave frequencies (where is very large) the
reverse bias impedance rbDZ may notbe particularly large, and
thus the reverse biased diode cannotbe considered an open
circuit.
In order for the impedance 1rb
jDZ j C=
to be very largeatmicrowavefrequencies, the junction capacitance Cjmust be
very, verysmall.
0
1rbD
j
Z Zj C
=
PINdiodes! Ibetthats
why we use PINdiodes!
Thatsexactly why! A PIN diode is approximately
a (bias) voltage controlled resistor at microwave
frequencies. We can select any value ofrdfrom a
shortto an open.
As a result, we can make many interesting devices!