Fast-setting, low power, jammer insensitive, biasing...

Details Fast-setting, low power, jammer insensitive, biasing... Fast-setting, low power, jammer insensitive, biasing...

2001-03-01

2002-09-17

Shingleton, Michael B (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including particular biasing arrangement

C330S302000, C330S288000, C330S283000

Reexamination Certificate

active

06452456

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to semiconductor devices, and, more particularly, to a fast-settling, low power, biasing circuit for single-ended circuits that are insensitive to jammers.
BACKGROUND OF THE INVENTION
Radio Frequency (RF) receivers include preamplifiers to boost an incoming signal level prior to the frequency conversion process. The presence of intermodulation products produced by large interfering signals compromises the receiver's ability to process very weak signals. This is what is conventionally known as desensitization. Third-order intermodulation occurs when two interfering signals at differing frequencies combine in the amplifier third-order nonlinearity to produce an intermodulation product close to the desired signal.
Desensitization may also occur when a single large interfering signal (i.e. a blocker or jammer) is present. The reduction in sensitivity arises through two separate mechanisms. The first, gain compression, is caused by third-order nonlinearity in the circuit, allowing the existing noise source in the amplifier and mixer to exert a relatively larger influence, thus degrading the overall signal to noise performance. The second mechanism, second-order nonlinearity in the circuit, promotes mixing between relatively low-frequency noise sources in the amplifier and the interfering signal. As a result, low-frequency noise is up-converted to the desired signal frequency which degrades the circuit noise performance. More on the study of blocking and desensitization can be found in “Blocking and Desensitization in RF Amplifiers,” R. G. Meyer and A. K. Wong, IEEE (1995), which is incorporated by reference herein.
Particularly, desensitization occurs in single-ended circuits, such as single-ended front-end Low-Noise Amplifiers (LNAs) for wireless receivers, that operate in the presence of jammers due to an increase in the noise floor at the output of the circuit and gain compression. Essentially, distortion in the circuit causes an up-conversion of low-frequency noise into the band of interest. This added noise increases the noise-floor of the circuit, that causes the Signal-to-Noise Ratio (SNR) at the output of the amplifier to degrade considerably in the presence of large amplitude jammers.
A typical single-transistor LNA is shown in
FIG. 1
a.
This single-transistor LNA circuit includes a transistor Q
in
as its primary gain device. As shown, power supply P
s
applies a voltage input to the circuit. An output is observed at output node P
out
. The bias circuit is composed of transistors Q
1
and Q
2
, that act as mirror devices. If the device area of transistors Q
in
and Q
1
is A
in
and A
1
, respectively, then the current flowing through transistor Q
in
is I
ref
(A
in
/A
1
). A resistor R
x1
is added in series with the bias circuit, to ensure that the incoming Radio Frequency (RF) power is not diverted into the bias circuit and is supplied primarily to transistor Q
in
for proper amplification. Since a base current flows into transistor Q
in
, a static voltage drop develops across resistor R
x1
. In order to balance this drop, a resistor R
x2
is added in series with the base of transistor Q
1
. Note that the base current in transistor Q
in
equals the base current of transistor Q
1
multiplied by A
in
/A
1
. Therefore in order to balance the base-current drops, resistor R
x2
must equal R
x1
(A
in
/A
1
).
In practice, an RF signal represented by v
a
cos(&ohgr;t) may be applied to the input of the LNA circuit; meanwhile a jammer signal of strength v
a
′cos((&ohgr;+&Dgr;&ohgr;)t) may be applied to the input of the LNA as well. The jammer is much larger than the incoming RF signals in most wireless standards. For example, the Global System for Mobile Communication (GSM) standard requires that a sensitivity of −102 dBm be maintained for the receiver. The receiver is required to demonstrate an identical signal-to-noise ratio at the output when the LNA input power is 99 dBm and a jammer is present at 3 MHz at a power level of −23 dBm this requires minimal degradation of the receiver noise figure in the presence of the jammer. With this level of jammer signal, low-frequency noise can be up-converted as explained above. At node
1
, there exists low-frequency noise from the bias circuit output. Due to second-order harmonic distortion inherent in the LNA, the jammer tone beats with low-frequency noise at frequency &Dgr;&ohgr;, and translates the noise to frequencies to &ohgr;+&Dgr;&ohgr;+&Dgr;&ohgr; and &ohgr;+&Dgr;&ohgr;−&Dgr;&ohgr;. The latter term is at the same frequency as the desired signal. Thus, the SNR ratio at the desired output frequency suffers. It should be noted that this effect scales with the strength of the jammer, such that is the SNR degrades more for larger jammer strengths. This effect is shown in
FIGS. 1
b
and
1
c.
The following noise sources impact the total noise at low frequencies in the bias circuit: a) the noise of the reference bias (I
ref
), b) the noise of bias resistor R
x2
, c) the base shot-noise of transistor Q
1
, and d) the collector shot-noise of transistor Q
1
. Several other noise sources, however, may exist in a LNA circuit; yet, their impact is negligible. As a consequence of the tightly coupled feedback loop formed by transistors Q
1
and Q
2
and resistor R
x2
, the impedance seen by the reference current source, I
ref
, is of the order of the inverse of the transconductance g
m
of transistor Q
1
. This is a small quantity in most bias circuits. Consequently, the noise of the reference bias circuit is small at its output, node X. In addition, as a result, the collector shot-noise of transistor Q
1
is mitigated. The remaining noise sources noted above in b) and c) are major noise sources in the LNA circuit shown in
FIG. 1
a,
since this circuit presents these noise sources with a relatively high impedance at the base of transistor Q
1
. Thus, noise current at this node develops a large noise voltage, which is effectively amplified by transistor Q
1
at its collector node. Since transistor's Q
2
placement with adjacent elements is such that it represents a voltage follower circuit, any noise at its base appears on its emitter with little attenuation. Hence, a large noise voltage develops at the base of transistor Q
in
. As explained above, this low-frequency noise can be up-converted to RF frequencies.
An approach that has been used to mitigate the noise up-conversion, is the use of external passive LC filters at the input node of the LNA circuit, or at the collector of transistor Q
1
is shown in FIG.
2
. The indicated LC circuit including inductor L
n
and capacitor C
n
creates a notch in the frequency domain at the frequency equal to the difference between the jammer and the signal-frequency. Thus, any noise on the bias line is filtered off at this frequency. The notch LC filter is so designed, that it appears as a very high impedance at the radio-frequency, and hence has a minimal impact on circuit performance. The LC notch circuit is effective in reducing the influence of the jammer.
This approach, however, has several disadvantages. First, it requires the use of external inductor and capacitor elements which add to the total cost of the solution. Second, the value of the capacitor in the notch filter is relatively high, since the filtering action is required at low frequencies. As a consequence, when the amplifier is powered on, it requires a long time to settle to its steady state, often in the order of hundreds of microseconds, which may be unacceptable in the overall system. Third, parasitics introduced by the large external components can degrade RF performance.
It should also be pointed out, that the solution of using a tuned series LC circuit applied at the input of the amplifier is very effective in suppressing the noise of resistor R
x1
and the base shot noise of transistor Q
in
as well, in addition to suppressing the noise of the bias circuit. However, there are several practical problems with this impleme

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