Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2001-10-05
2003-10-21
Young, Brian (Department: 2819)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S12400D, C330S275000, C330S278000
Reexamination Certificate
active
06636116
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to the art of amplifier circuits and more particularly to a fully differential current feedback amplifier.
BACKGROUND OF THE INVENTION
In broadband and wireless communications systems, information is transferred between sources and destinations through various communications media. For example, in wireless systems, analog signals carrying information are transmitted to and received from the ambient via an antenna. When such a communications signal is received, it is initially processed in an analog signal chain between the receiving antenna and a digital processing system. An analog to digital (A/D) converter digitizes the processed analog signal, which is then subjected to signal processing to obtain the received signal information. The analog signal chain operates to condition, filter, and amplify the received analog signal, and to provide frequency conversion as necessary in order to reduce noise and to provide a proper input signal to the A/D converter. Recently, the frequencies of signals within such circuits are increasing in order to provide improved speed, conforming to communication protocols or standards, etc. Consequently, analog signal chains and particularly amplifiers thereof in modern communications systems are required to operate at ever higher frequencies with larger dynamic range, decreased distortion, and lower power supply voltages.
In a typical communications system, an antenna signal is provided to a low noise amplifier (LNA), and from there into a mixer that takes the received signal from the transmission frequency down to an intermediate frequency. More than one such frequency down-conversion may take place, for example, via subsequent mixer stages. Various filters, such as band-pass filter stages, are typically provided between the antenna, amplifiers, and mixers, in order to reduce or remove noise and signals that are outside the frequency band of interest. Once the resulting analog signal has been converted to a manageable frequency, it is amplified once again and provided to an A/D converter. In order to perform a proper A/D conversion so as to extract all the information of interest from the signal, it is desirable to provide the A/D with a very large signal. Thus, one or more amplifiers are found in the typical analog signal chain to amplify the relatively weak signal at the antenna and/or at intermediate stages, wherein the A/D converter receives a signal which is slow enough to digitize, and also amplified so that the A/D can digitize all the fine signal details.
Initially, a relatively weak single-ended signal is obtained from the receiving antenna. Differential signals have many advantages over single-ended signals in such systems, including increased dynamic range for a given supply voltage, suppression of even order harmonic distortion, and enhanced common mode noise rejection. Thus, at some point in the analog signal chain, the single-ended signal is converted into a differential signal. The earlier that the conversion from single-ended to differential takes place in the signal chain, the earlier the benefits of differential signals are realized in the communications system. A portion of a conventional heterodyne type receiver system
10
is illustrated in prior art
FIG. 1
, where a heterodyne receiver translates the desired radio frequency (RF) signal to one or more intermediate frequencies before demodulation. The exemplary receiver system
10
includes several active and passive function blocks and each contributes to the system's overall signal gain and noise figure (NF). The receiver system
10
of
FIG. 1
includes an antenna
12
, a duplexer
14
, an amplifier
16
, one or more filters
18
a
and
18
b
, a mixer
20
driven by a local oscillator (LO)
21
, a second amplifier
24
, and an A/D converter
26
.
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, wherein 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
also includes a first amplifier
16
, typically a low-noise amplifier (LNA), which increases the amplitude of the signal received from the antenna
12
allowing 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.
Band-pass type filters
18
a
and
18
b
form one or more networks which allow a range of RF frequencies to pass therethrough, while blocking RF signals outside of their designed passband. The filter
18
a
located before the LNA
16
is called a preselect filter and the post-amplifier filter
18
b
is often called an image-reject filter. The preselect filter
18
a
prevents signals far outside of the desired passband from saturating the front end of the amplifier
16
so as to reduce intermodulation distortion products related to those signals at far away frequencies, while the image-reject filter
18
b
rejects spurious response type signals. The mixer
20
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 a signal from the local oscillator
21
, which mixes with the RF signal. Thereafter, a second amplifier
24
further amplifies the analog signal from the mixer
20
and provides an input to the A/D converter
26
.
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. Where the mixer
20
is designed to receive a differential signal input and the antenna
12
generates a single-ended 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. Alternatively, the single-ended to differential conversion can be performed in either the first LNA
16
or in the second amplifier
24
supplying a differential input to the A/D converter
26
.
Conversions or translations of single-ended signals to differential signals have sometimes been accomplished using differential voltage feedback amplifiers or current feedback amplifiers, each having particular shortcomings. In the first case, the amplifiers
16
and/or
26
in
FIG. 1
may be such a voltage feedback type amplifier. However, voltage feedback amplifiers suffer from various drawbacks in high-speed applications. For instance, the bandwidth in differential voltage feedback amplifiers is dependent upon gain, thus limiting design flexibility for gain adjustment where a large bandwidth is contemplated. For example, with a voltage feedback amplifier, as the amplifier closed loop gain is increased, the speed of the signals it is able to support decreases. Thus, conventional voltage
Brady W. James
Nguyen John B
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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