Hsms-482b Pin And Hsms-282b Schottky Diodes For Wideband Pin Diode Limiters With A Very Low Turn-on Threshold

Avago Technologies

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HSMS-482B PIN and HSMS-282B Schottky Diodes
for Wideband PIN Diode Limiters
with a Very Low Turn-on Threshold
Application Note 5443
Introduction
Limiters protect wireless Receiver front-end stages, such
as the low noise amplifier (LNA), from irreversible damage
caused by input signal overload. The most common
limiter configuration is the shunt-connected PIN Diode
which biases itself (“self-biasing PIN Diode limiter”) in
the presence of large signals. Its popularity lies in the
minimal part count because the PIN Diode performs the
dual functions of rectifying the incoming RF signal and
then using the rectified current to bias itself to a low value
of effective series resistance (Rd). An Inductor completes
the loop for the bias current flow while presenting a high
impedance path to RF. However, the turn-on threshold
(P
TH
) for the self-biasing PIN Diode limiter is around 11
dBm, and this may be higher than what is needed to
protect small geometry LNA Transistors
The limiting turn-on threshold, P
TH,
can be lowered by
using a Schottky Diode to generate the bias current for the
PIN Diode Since the Schottky Diode has a lower turn-on
threshold voltage than the PIN Diode the combined PIN-
Schottky limiter has an overall P
th
that is 10 dB lower than
the PIN-only limiter.
RF input
Theory of Operation
The Schottky Diode rectifies the incoming RF and produces
a current (I
F
) that is proportionate to the incident RF power
(Pi). The PIN Diode which is connected anti-parallel to the
Schottky Diode provides a return path for I
F
. The PIN Diode
behaves like a current-controlled Resistor with an equiva-
lent junction resistance (R
d
) which is controlled by I
F
:
R
d
=
W
2
2P,
F
W
W is the PIN diode’s I-layer height,
P
is the ambipolar
mobility of electrons and holes, and
W
is the minority
carrier lifetime
[1]
.
At power levels below the Schottky detection threshold,
the unbiased PIN diode’s R
d
is considerably higher than the
transmission line characteristic impedance (Z
0
).Therefore,
most of the incident power will pass through the limiter
with almost no attenuation (A).
At power levels above the Schottky detection threshold,
the RF rectification generates a current that lowers R
d
of
the PIN Diode The corresponding limiter attenuation is
given by:
§
Z
0
·
A (dB) = 20log
¨1
+
¸
©
2R
d
¹
PIN
Conventional PIN
Diode limiter
RF input
At very low values of R
d
, most of the incident power is
reflected back to the source (e.g. aerial) and only a tiny
fraction is dissipated in the PIN Diode As a result, the
limiter device is able to handle Pi much larger than the
PIN diode’s rated maximum power.
PIN
Schottky
Schottky enhanced limiter
Circulating self-bias
current shown in red.
Figure 1. Self-biased PIN limiter and Schottky-enhanced PIN limiter circuits