Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – Logarithmic
Reexamination Certificate
1999-07-16
2001-07-24
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
Logarithmic
C327S351000, C327S362000
Reexamination Certificate
active
06265928
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates, in general, to logarithmic amplifiers and, in particular, to a method and apparatus for the reduction of unwanted interference parameters at the output of a logarithmic amplifier.
BACKGROUND OF THE INVENTION:
Without limiting the scope of the invention, its background is described in connection with logarithmic amplifiers used in a power detector.
In a wireless communication system, for example, a Global System for Mobile (GSM) system using a Time Division Multiple Access (TDMA) signaling format that includes a framed structure comprising eight time slots, a mobile station communicates with a base station by transmitting and receiving information in one or more of the time slots that comprise a channel. Each channel is assigned to a different user, with mobile-to-base transmission (uplink) on one frequency band and base-to-mobile (downlink) on a second frequency band.
In order to preserve the integrity of the transmitted and received information and to reduce adjacent channel interference, the system operates according to a standardized format that defines the requirements of transmission and reception. A system transmitting and receiving information often produces unwanted interference. This unwanted interference affects the integrity of the transmitted and received information. For example, a power control loop uses negative feedback to adjust the operating point of a power amplifier so that the power amplifier operates in a specified range. However, unwanted interference parameters inherent in the operation of the power control loop may cause an inaccurate representation of the information to be controlled in the feedback loop resulting in inaccurate adjustment of the power amplifier's operating point.
The feedback control loop controls the operation of the power amplifier by using an RF linear detector to sample the output signal and compare the output signal with a reference signal, where the reference signal is proportional to the required output. The RF linear detector output is used as an error signal to adjust the power amplifier's operating point to correct any unwanted deviations detected at the output. Unwanted interference parameters of the RF linear detector could affect the signals in the loop and may result in an incorrect adjustment of the power amplifier.
Reference is now made to
FIG. 1
, wherein a prior art logarithmic amplifier used in RF linear detectors is illustrated and denoted generally as
10
. Logarithmic amplifier
10
includes an operational amplifier
12
and a diode
14
that operates in the small signal region. A small signal input I
1
is connected to the inverting input of operational amplifier
12
, and the non-inverting input is coupled to ground through resistor R
4
. Bias voltage V
s
is coupled to the inverting input and the anode of diode
14
, through a current limiting resistor R
b
, and produces a bias current I
b
that biases diode
14
. The output of operational amplifier
12
is coupled to the cathode of diode
14
through resistor R
0
. Output V
o
of logarithmic amplifier
10
is taken from the output of operational amplifier
12
.
Ideally, output V
o
should be a true representation of the logarithmic value of I
1
; however, there are parameters of the logarithmic amplifier
10
which produce variations in output V
o
. A saturation current I
s
(T), in diode
14
, is a function of temperature and causes variations of the output V
o
when operating at different temperatures (T). Bias current I
b
, generated by V
s
, also is an unwanted parameter at output V
o
that affects the linearity by introducing an additional constant voltage at output V
o
. The effects of these interference parameters on the output V
o
can be seen from equation 1 below, which represents the output V
o
of the logarithmic amplifier
10
of FIG.
1
.
V
o
=
-
(
I
1
+
I
b
)
·
R
o
-
n
·
k
·
ln
(
I
1
+
I
b
I
s
(
T
)
)
Equation
1
As may be seen from Equation 1, an improved apparatus to effectively remove interference parameters from the output of a logarithmic amplifier could improve the accuracy and performance of the logarithmic amplifier.
SUMMARY OF THE INVENTION:
The present invention presents an improved apparatus for reducing interference parameters at the output of a logarithmic amplifier. This allows a more accurate logarithmic representation of the input signal at the output.
In an embodiment, the invention comprises a precision controlled logarithmic amplifier comprising a logarithmic amplifier having a signal input for receiving an input signal and a signal output providing an output voltage that is a logarithmic representation of the input signal. The output voltage is affected by a first bias current and a first saturation current generated within the logarithmic amplifier. A precision-control circuit is coupled to the logarithmic amplifier. The precision-control circuit is configured to produce a second bias current and a second saturation current. The second bias current and the second saturation current act to reduce the effects of the first bias current and the first saturation current, respectively, on the output voltage of the logarithmic amplifier.
REFERENCES:
patent: 4430626 (1984-02-01), Adams
patent: 4906836 (1990-03-01), Yamashita et al.
patent: 5012140 (1991-04-01), Bateman
patent: 5578958 (1996-11-01), Yasuda
patent: 5699004 (1997-12-01), Picciotto
patent: 6066976 (2000-05-01), Cho
Neitiniemi Jukka-Pekka
Tran Kim Anh
Wey Chia-Sam
Dinh Paul
Hayes Thomas B.
Nokia Telecommunications Oy
Rivers Brian T.
Wells Kenneth B.
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