Amplifiers – With semiconductor amplifying device – Including balanced to unbalanced circuits and vice versa
Reexamination Certificate
2000-04-06
2002-04-02
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including balanced to unbalanced circuits and vice versa
C330S311000
Reexamination Certificate
active
06366171
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to electrical circuits, and more particularly to a circuit and method of generating differential signals exhibiting phase accuracy at high frequencies.
BACKGROUND OF THE INVENTION
Electrical circuits are utilized in a myriad of diverse applications, for example, computers, communication devices, industrial equipment, etc. In many of the applications which employ such circuits, differential signals are utilized to effectuate various functions. In addition, in many applications, the frequency of signals within such circuits are increasing in order to provide improved speed, conforming to communication protocols or standards, etc. In high frequency applications such as RF (radio frequency) communications, circuits employing differential type signals sometimes suffer from problems relating to phase delay. That is, one of the differential signals (e.g., RF
out(+)
) is not exactly 180 degrees out of the phase with the other corresponding differential signal (e.g., RF
out(−)
). Such phase imbalances may result in various undesirable effects.
One type of circuit system which sometimes utilizes differential signals is a communications receiver in a wireless application such as a cellular phone. An exemplary portion of a conventional heterodyne type receiver is illustrated in prior art
FIG. 1. A
heterodyne receiver translates the desired RF signal to one or more intermediate frequencies before demodulation. The receiver system is composed of several active and passive function blocks and each contributes to the system's overall signal gain and noise figure (NF). The system
10
of
FIG. 1
includes an antenna
12
, a duplexer
14
, an amplifier
16
, one or more filters
18
a
and
18
b
, and a mixer
20
driven by a local oscillator
21
(LO).
The antenna
12
provides an interface between free space and the receiver input. The duplexer
14
interfaces with the antenna
12
and allows simultaneous transmitter and receiver operations with a single antenna. The duplexer
14
operates to isolate the receiver system
10
and a transmitter
22
from each other while providing a generally low loss connection to the antenna
12
for both systems.
The system
10
of
FIG. 1
also includes the amplifier
16
, typically a low-noise amplifier (LNA) that increases the amplitude of the signal received from the antenna
12
which allows for further processing by the receiver
10
. An ideal amplifier increases the amplitude of the received signal without adding distortion or noise. Real world amplifiers, however, add noise and distortion to the received signal, and attempts are made to minimize signal degradations. The LNA
16
is the first amplifier after the antenna
12
in the system
10
and contributes most significantly to the system noise figure, consequently the amplifier
16
is typically designed to minimize noise, and hence the name LNA. The LNA
16
is typically constructed using active devices which operate in their linear range, however, the LNA output signal is not always perfectly linear, and distortion is added to the amplified signal due to nonlinearities of the one or more transistors therein.
The system
10
also includes one or more filters
18
a
and
18
b
, respectively. The filters form one or more networks which allow a range of RF frequencies to pass therethrough (oftentimes called bandpass filters). The filters block RF signals outside of their designed passband. When used, the RF filter
18
a
which is located before the LNA
16
is called a preselect filter and the post-amplifier RF filter
18
b
is often called the image-reject filter. The preselect filter
18
a
prevents signals far outside of the desired passband from saturating the front end and producing intermodulation distortion products related to those signals at far away frequencies, while the image-reject filter
18
b
rejects spurious response type signals. Lastly, the system
10
includes the mixer
20
which translates the received, filtered and amplified RF signal to both a higher and lower intermediate frequency (IF) value. One of the intermediate frequencies is passed while the other is rejected (e.g., called either up-conversion or down-conversion, respectively), using translation with the LO signal that mixes with the RF signal.
Many conventional mixers are designed to receive a differential input because differential signals help in decoupling the system
10
from noise in the integrated circuit substrate, thereby lowering the system NF, and aid in facilitating high device integration. Because the mixer
20
is designed to receive a differential signal input and the antenna
12
generates a single received signal, the system
10
must transform the single-ended signal into a differential signal somewhere between the antenna
12
and the mixer
20
.
Conventional solutions which perform a transformation from a single-ended signal to differential signals before the LNA
16
have been found undesirable because prior to amplification the received signal is weak and the transformation results in too much loss, thereby degrading the integrity of the received signal. Similarly, conventional post-LNA transformation solutions have been found to be undesirable because of linearity issues. For example, a post-LNA solution sometimes utilizes a unity-type buffer coupled to the output of the LNA. Since the received signal exiting the LNA has been amplified (e.g., by about 20 dB), the post-LNA buffer must operate in a linear range for a substantially higher power signal, which results in an undesirable increase in power consumption. Since both pre-LNA and post-LNA single-to-differential transformation solutions are unsatisfactory, attempts have been made to integrate the transformation of a single RF signal to a differential signal within the LNA
16
.
Therefore there is a need in the art for a circuit and method which provides a single-to-differential signal transformation functionality integrated within a low-noise amplifier or other type circuit arrangements such as buffers, etc.
SUMMARY OF THE INVENTION
According to the present invention, a circuit and method of transforming a single-ended signal to differential signals exhibiting good phase balance independent of signal frequency is disclosed.
According to one aspect of the present invention a circuit is disclosed which receives and single input signal and uses the signal to generate a pair of differential signals. The circuit includes a differential signal phase balance circuit that analyzes the phase of the differential signals and provides compensation based on the phase analysis in order to cause the differential signals to more closely be 180 degrees out of phase with one another independent of signal frequency. The present invention may be employed in various types of single-to-differential circuit applications, for example, buffers and amplifiers.
According to another aspect of the present invention, a single-to-differential LNA is disclosed which exhibits good phase balance independent of signal frequency. According to one exemplary aspect of the present invention, the LNA includes two coupled cascode type LNA amplifiers wherein an AC ground conventionally associated with a bias input is removed and a control node associated with the amplifier consequently is allowed to vary due to parasitic-type coupling effects. The control node voltage variations are a function of the phase balance of the differential signals and cause the timing at which various circuit functions occur to change. Such changes result in the phase of the differential signals to more closely be 180 degrees out of phase with one another, thus providing good phase balance.
According to yet another aspect of the present invention, a method of transforming a single-ended signal into a pair of differential signals exhibiting good phase balance independent of frequency is disclosed. The method includes generating the differential signals using the received single-ended signal and analyzing the phase of the differential
Bellaouar Abdellatif
Litmanen Petteri M.
Brady III W. James
Moore J. Dennis
Mottola Steven J.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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