Automatic gain control circuit for analog signals

Details Automatic gain control circuit for analog signals Automatic gain control circuit for analog signals

2001-11-08

2003-06-10

Shingleton, Michael B (Department: 2817)

Amplifiers

With control of power supply or bias voltage

With control of input electrode or gain control electrode bias

C330S086000, C330S140000, C330S144000, C330S282000, C330S284000

Reexamination Certificate

active

06577191

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit for processing analog signals, and particularly to an automatic gain control circuit for analog signals.
2. Description of Related Art
The analog-to-digital circuit is usually used in all kinds of equipment such as computers, gauges and industrial controlling apparatus. It can transform analog signals (ex. temperature, pressure etc.) to digital signals that are easily received and further processed by the digital processor.
FIG. 1
(PRIOR ART) is a block diagram of a conventional analog-to-digital circuit with a single multiplication factor. The circuit comprises a signal detector
10
, a pre-processing circuit
11
, a signal amplifying/attenuating circuit
12
, a sample/hold circuit
13
, an analog-to-digital converter (ADC)
14
and a microprocessor
15
. The signal detector
10
detects an analog input signal Vin. The pre-processing circuit
11
attenuates the analog input signal Vin to an analog signal Vin′ with an amplitude that is acceptable for the system by an attenuation factor 1/a. Then, the signal amplifying/attenuating circuit
12
amplifies or attenuates the analog signal Vin′ to an analog signal Vin″ by an amplification gain b. The sample/hold circuit
13
samples and holds the analog signal Vin″ and the ADC
14
transforms the analog signal to a digital signal. Finally, the digital signal is processed by the microprocessor
15
.
The microprocessor
15
regularly triggers the sample/hold circuit
13
to sample the analog signal Vin″ by control lines. Besides, the microprocessor
15
notifies the ADC
14
to transform the sampled signal. After finishing transforming, the ADC
14
notifies the microprocessor
15
to access the digital signal that has been transformed and to perform the numerical operation.
However, the microprocessor
15
should consider the combined multiplication constants b/a that is determined by the attenuation factor 1/a of the pre-processing circuit
11
and the amplification gain b of the signal amplifying/attenuating circuit
12
when the microprocessor
15
performs the numerical operation. The resulted signal amplitude is b/a times of that of the original signal. Therefore, a circuit designer must store a restoring parameter a/b into the calculating program run by the microprocessor
15
to ensure that the numeral result conforms to the input signal Vin.
The above-described circuit architecture and signal-processing scheme are suited for the transforming procedure using one multiplication constant. For a signal-processing scheme requiring a transforming procedure using a plurality of multiplication constants, processing circuits that have switch devices for enabling different multiplication constants are required. As shown in
FIG. 2
(Prior Art), the circuit comprises a signal detector
20
, a pre-processing circuit using multiple multiplication constants
21
, an amplifying/attenuating circuit using multiple multiplication constants
22
, a sample/hold circuit
23
, an ADC
24
and a microprocessor
25
. The pre-processing circuit using multiple multiplication constant
21
employs three attenuation factors, such as 1/a1, 1/a2 and 1/a3, and the amplifying/attenuating circuit using multiple multiplication constants
22
also employs three amplification gains, such as b1, b2 and b3. There are nine combined multiplication constants bn/am, where m=1~3 and n=1~3. The circuit designer must store the nine restoring parameters am/bn, where m=1~3 and n=1~3, into the calculating program run by the microprocessor
25
. In these restoring parameters, the maximum is a3/b3 and the minimum is a1/b1. It can ensure that the numeral result conforms to the original analog input signal Vin. There are two conventional schemes for the microprocessor
25
to select an appropriate restoring parameter. The first scheme is to add a judging program in the calculating program run by the microprocessor
25
and to select the appropriate restoring parameter am/bn in accordance with the result form the deciding program. The second scheme is to select the appropriate restoring parameter manually.
In the first scheme for the selection of the appropriate restoring parameter, as shown in
FIG. 2
, the restoring parameter is initially set to the maximum a3/b3. Besides, the switch device of the pre-processing circuit
21
is switched to select the attenuation factor 1/a3 and the switch device of the amplifying/attenuating circuit
22
is switched to select the amplification gain b3, respectively. When the calculation/decision result of the microprocessor
25
reveals the overflow status, the microprocessor
25
will calculate and determine the system status by using the next maximal parameter a3/b2. In addition, the microprocessor
25
triggers the switch device of the pre-processing circuit
21
to select the attenuation factor 1/a3 and the switch device of the amplifying/attenuating circuit
22
to select the amplification gain b2, respectively. If the result is still the overflow status, the microprocessor
25
will continue calculating and determining based on the next maximal constant a3/b1. The microprocessor
25
would not stop the calculating and determining procedure until the calculating result conforms to the input signal Vin.
In the second scheme for the selection of the appropriate restoring parameter, a manual switch for changing the multiplication constant
26
is set into the control circuitry, as shown in the
FIG. 3
(Prior Art). The structure of the control circuit is similar to that shown in the FIG.
2
. The calculating program of the microprocessor
25
also includes a judging program, but does not automatically trigger the switch devices of the pre-processing circuit
21
and the amplifying/attenuating circuit
22
, respectively. Depending on whether the calculating result reveals the overflow status or not, the user manually change the manual switch for selecting the multiplication constants
26
to the appropriate location. The manual switch for changing the multiplication constants
26
will not stop changing until the calculating result conforms to the input signal Vin.
The second scheme cannot be widely applied in the industry since it requires manually changing the manual switch for changing the multiplication constants. On the other hand, although the first scheme employs an automatic procedure for adjusting and changing multiplication constants, the microprocessor requires much time in each trial to find out the correct result. Apparently, these two schemes are not suitable for the high-speed system.
SUMMARY OF THE INVENTION
Accordingly, the purpose of the present invention is to provide an AGC circuit for analog signals, which produces gain-control signals S
1
and S
2
for controlling the gain of the signal amplification or attenuation, thereby automatically adjusting the amplification and attenuation of the input signals. In addition, the microprocessor selects one appropriate restoring parameters according to these gain-control signals to ensure that the result can conform to the input signal. When the AGC circuit of the present invention is applied to an analog-to-digital circuit, it can cooperate with the microprocessor to achieve the automatic gain switching function of the input analog signals by the microprocessor. In addition, in the conversion of the input signals, it can provide a quick response for the automatic adjustment of the gain.
For the above purpose, there is a preferred embodiment of an automatic gain control circuit for automatically adjusting an amplitude of an analog input signal, comprising: a buffering circuit for buffering the analog input signal and generating a first signal; an amplifying/attenuating circuit for receiving the first signal form the buffer circuit and attenuating/amplifying an amplitude of the first signal to be an output signal within an operational amplitude range by a gain value of a predetermined gain range; a pre-processing gain-adjusting

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