The present invention relates to an amplifier circuit, and more specifically, to a preamplifier circuit typically used in an optical receiver, such as a barcode scanner, for pre-amplifying a photocurrent generated by a photo detector such as a photodiode.
A two-stage preamplifier circuit used in an optical receiver such as a barcode reader or scanner generally comprises three blocks of circuits as illustrated in
In Block 1, the photodiode PD receives a light signal (shown as double arrows) and generates the photodiode current 11 as an input signal to a first stage amplifier OP1. A feedback resistance (implemented as a resistor R1) is connected in parallel to the amplifier OP1 so as to realize a high gain of the amplifier OP1. To generate a reliable output 15, the signal-to-noise (SN) of the amplifier circuit in Block 1 must be relatively large, for example, more than 20 dB.
The feedback resistance is a major source of undesirable noise, and it is well-known that increasing the value R of the feedback resistance is necessary to improve the signal to noise ration. However, in the design of the preamplifier circuit as illustrated in
Therefore, the feedback resistance is often not optimum as to a small input signal because the resistance has to be low enough to ensure that the amplifier will not saturate when the input signal is high. The problem is particularly pronounced when the input signal varies over a large dynamic range. For example, the input signal is much larger when a barcode reader or scanner is close to the barcode or a bright light (such as sunlight) enters the barcode reader, than when the scanner reads the barcode at a distance. Thus, the relatively small value of the feedback resistance decided according to the possible maximum input signal limits the barcode reading distance because of the need for a sufficient signal-to-noise (SN) ratio.
It is therefore an object of the present invention to provide a preamplifier circuit which has optimum signal-to-noise (SN) ratio for an input signal variable in a wide range.
Another object of the present invention is to provide a preamplifier circuit which has acceptable signal-to-noise (SN) when the input signal is small, and which also does not saturate when the input signal is large.
A further object of the present invention is to provide a method for amplifying an input signal by a preamplifier circuit with an optimum or acceptable signal-to-noise (SN).
According to an aspect of the present invention, a preamplifier circuit is provided for amplifying an input signal. A feedback resistance is connected to the amplifier in parallel, wherein a value of said feedback resistance is controllably variable, preferably according to level of input signal expected or measured. Thus, the value of the feedback resistance (and therefore the gain) can be decreased to avoid saturation when the input signal is large, but can remain a relatively large value to keep a sufficient signal-to-noise (SN) ratio when the input signal is small.
In a preferred embodiment, the feedback resistance comprises a first resistor and a second resistor, and the second resistor is connected in parallel to the first resistor through a normally-off switch which is turned on only when the output signal reaches a predetermined threshold. Thus, when the input signal is small, the feedback resistance remains at the larger value to obtain a high signal-to-noise (SN); however, when the input signal is large, the feedback resistance is changed to a smaller value so as to decrease the gain of the amplifier, preventing the amplifier from operating in a saturated mode. It is noted that when the input signal is large, the SN is still kept at an acceptable level because of the large input signal, even though the noise increases because of the smaller value of the feedback resistance.
In another preferred embodiment, the feedback resistance is implemented as a single variable resistor, the value of which is controllably altered such that the output signal is substantially equal to a predetermined maximum value. The variance of resistance value may be incrementally stepped or continuous.
According to another aspect of the present invention, a method for amplifying an input signal is provided which comprises the steps of receiving the input signal at an amplifier to generate a first output signal, comparing the first output signal with a reference to decide whether it meets a predetermined requirement, and if not, altering a value of a feedback resistance and receiving the input signal at the amplifier again to generate a new output signal. Thus, the value of the feedback resistance can be adjusted according to the output signal magnitude observed, so as to realize an optimum performance of the amplifier circuit.
In a preferred embodiment, the value of the feedback resistance is changed from a larger value to a smaller value when the first output signal reaches or is larger than the reference. Preferably, the feedback resistance comprises a first resistor and a second resistor connected to the first resistor in parallel through a switch, and the changing of the value of the feedback resistance is done by turning on the switch so as to connect the second resistor to the first resistor.
In another preferred embodiment, the value of the feedback resistance is changed such that the new output signal will be substantially equal to the reference.
In another preferred embodiment, one or more scans of the signal to be read are first performed to calibrate the system, more specifically, by determining whether to switch in a second resistor, or how to vary a variable resistor. Then, one or more actual scans are performed to read the symbol.
The above and other features and advantages will become clearer by reading the following detailed descriptions of the preferred embodiments of the present invention with reference to the accompanying drawings, in which:
a is a first embodiment of the preamplifier circuit according to the present invention;
b is a block chart illustrating the operation of the preamplifier circuit in
a is a second embodiment of the preamplifier circuit according to the present invention; and
b is a block chart illustrating the operation of the preamplifier circuit in
The preferred embodiments of the preamplifier circuit according to the present invention are shown in
a illustrates a first preferred embodiment of the two-stage preamplifier circuit according to the present invention, which comprises four blocks of circuits. The circuits in Blocks 1 and 3 are the same as those in the prior art preamplifier circuit as shown in
The circuit in Block 1 is different from that shown in
Block 4 is a control circuit for controlling the switch SW to be turned on or off. More specifically, an output 14 of the second stage amplifier OP2 in Block 3, which is generated by amplifying the output 15 of amplifier OP1, is provided to a comparator 12 in the control circuit (Block 4). The comparator 12 compares the output 14 with a reference signal 13 and provides a result to the controller 16, which in response turns on or off the switch SW of resistor R2.
The reference signal 13 has a predetermined value at which the output 14 of the amplifier OP2 is saturated. When the result from the comparator 12 indicates that the output 14 reaches, or is larger than, the value of the reference signal 13, the controller 16 turns on the switch SW to connect the resistor R2 in parallel with the resistor R1 to lower the value of the feedback resistance, thus decreasing the gain of the amplifier OP1. Consequently, the output 15, which is an input to the amplifier OP 2, is also decreased, and the output 14 of amplifier OP2 is therefore lowered to below the saturation level.
The operation of the two-stage preamplifier circuit shown in
Thus, when the input signal 11 is relatively small, such as when the scanner is at a distance from the barcode, the output 14 is below the saturation level. With the switch SW normally off, the feedback resistance only comprises the first resistor R1 and therefore assumes a larger value. This results in a higher signal-to-noise (SN).
When the scanner is close to the barcode, a large photocurrent is generated as the input signal 11 to the amplifier OP1. If it is too large for the amplifiers to process, the switch SW is automatically turned on to connect the second resistor R2 with the resistor R1 in parallel to decrease the feedback resistance and consequently thus the gain of the amplifier OP1 so that the output 14 of amplifier OP2 is lowered to below the saturation level. Even though the decrease in the feedback resistance will increase the noise, such a situation only occurs when the input signal is high, and hence, an acceptable signal noise ratio is achieved.
Typically, although not necessarily, a first scan cycle is used to adjust the resistance, and subsequent scan cycles are used to actually read the data. Thus, after a first scan, the additional resistance is switched in or out of the circuit as may be necessary.
Preferably, the value of the reference signal 13 is equal to a predetermined acceptable maximum value of the output 15 of the amplifier OP1, which is below the saturation level of amplifier OP1 and will not result in a saturated output 14 at the next stage amplifier OP2. The comparator 12 decides whether the output 15 of the amplifier OP1 is substantially equal to the reference signal 13, and the result is provided to the controller 16. The controller 16 may be a microprocessor which comprises an algorithm for determining a new value for the variable resistor R3 with which the output 15 will be equal to the reference signal 13 at the same input signal 11. After the new value is determined, the controller 16 sends a control signal to the variable resistor R3 to alter it to the newly determined value.
The operation of the preamplifier circuit shown in
With this embodiment, the variable resistor R3 may always be adjusted to a value that will generate a predetermined maximum value of the output 15. More specifically, if the output 15 is larger than the acceptable maximum value (e.g., when it is saturated), the resistor R3 and therefore the gain of the amplifier OP1 will be decreased so that the output 15 will be lowered to the predetermined maximum value which is below the saturation level. If the output 15 is smaller than the predetermined maximum value, the variable resistor R3 will be altered to a larger value determined by the controller 16 so that the gain of the amplifier OP1 will be increased to generate an output 15 of the predetermined maximum value, and the signal-to-noise (SN) will be increased. Thus, for each specific input signal, the variable resistor R3 may always be altered to assume an acceptable maximum value in view of the output 15, and therefore results in a substantially the highest signal-to-noise (SN) for every specific input signal.
The above has described in detail the preferred embodiments of the present invention. However, it shall be understood that, without departing the gist of the present invention, numerous variations, modifications and adaptations are readily available to a person of ordinary skill in the art. For example, one or both of the resistors in the embodiment in