Digital Signal Processing Reference
In-Depth Information
Untuned amplifier (DC-GHz)
Tuned low-noise amplifier (LNA)
nVdd
nVdd
I
bias
r
bias
I
bias
r
bias
v
out
v
out
Z
in,eff
Z
in,eff
L
G
i
in
r
o
r
o
node sees 1/g
m
r
bias
// r
o
C
GS
C
GS
L
S
R
S
Transistor dimensions limited to
Load impedance
can be tuned.
Inductive degeneration
driving capabilities signal source.
avoids DC voltage drop.
Wideband applications
Narrowband applications
Figure A.19.
In a narrowband application, for example an lna, the parasitics of
the transistor can be tuned to the operating frequency by embed-
ding the device in a matching network. Also, inductive degenera-
tion can be used, which avoids a dc voltage drop. In contrast, the
signal source of a wideband amplifier directly drives the capacitive
load of the transistor.
gain, its possibilities to trade gain for linearity are also limited, which makes
this type of amplifier unsuitable for use in low-distortion applications (even in
the low-frequency range).
One possible exception to this universal fact is the low-noise amplifier (lna)
in the input stage of a receiver front-end (Figure A.19). The combination of
two important conditions makes degeneration a common practice in this type
of amplifiers. First of all, the power level that is available from the antenna is
much weaker than all other signal found in the remaining part of the receiver.
Because of the fairly limited voltage levels that the lna has to cope with,
distortion at the output will automatically stay at acceptable levels.
Secondly, in most cases it turns out that it is possible to tune the amplifier to the
frequency of interest. The parasitic capacitance caused by large transistor di-
mensions can be taken into account during the design of the resonating lc-tank
surrounding the transistor. This method is only applicable when the bandwidth
of the input signal is low, compared to the center frequency (high fractional
bandwidth or
Q
-factor). This technique can be used up to considerable fre-
quencies, only limited by physical limitations such as the quality factor of the
parasitic capacitor or the self-resonance frequency of certain circuit elements.
