Amplifier circuit

An amplifier circuit includes three amplifier units connected in series. The first amplifier unit includes an input connector for inputting signals and a first transistor amplifier module connected to the input connector. The second amplifier unit includes a notch-filter circuit, a main filter circuit and a second transistor amplifier module. The notch-filter circuit allows only signals in a predetermined frequency to be transmitted from the first transistor amplifier module to the second transistor amplifier module and amplified. The main filter circuit filters signals in frequencies different from the predetermined frequency. The third amplifier unit includes a third transistor amplifier module and an output connector for outputting amplified signals.

BACKGROUND

1. Technical Field

The present disclosure relates to amplifier circuits, and particularly to an amplifier circuit capable of rejecting image signals.

2. Description of Related Art

In wireless communication, when wireless signals are received by communication terminals, the communication terminals often need to convert frequencies of the received signals into working signals capable of being transmitted and processed in the communication terminals. However, the frequency conversion operations may generate image signals interfering with wireless communication. Thus, the communication terminals generally require filters used therein for filtering the image signals.

After the frequency conversion operations, the working signals are generally inputted into amplifier circuits to be amplified, and then are transmitted to other functional modules (e.g., decoders or speakers) of the communication terminals to be further processed. Thus, in most communication terminals, the filters for filtering image signals are integrated with the amplifier circuits, such that the image signals are filtered before the working signals are further processed. For example, surface acoustic wave (SAW) filters are often integrated with amplifier circuits of wireless communication terminals to filter image signals. However, conventional filters integrated with the amplifier circuits, such as the SAW filters, are usually expensive, which increases cost of the communication terminals.

Therefore, there is room for improvement within the art.

DETAILED DESCRIPTION

Referring to the figure, an amplifier circuit100according to an exemplary embodiment is provided. The amplifier circuit100is used in a wireless communication terminal (not shown) such as a mobile phone, a radiogram or an interphone to amplify desired working signals and filter image signals generated in frequency conversion operations. The amplifier circuit100includes a first amplifier unit10, a second amplifier unit20and a third amplifier unit30connected in series.

The first amplifier unit10includes an input connector12, an input filter circuit14and a first transistor amplifier module16. The input connector12is electronically connected to other circuits of the communication terminal to input signals. The input filter circuit14includes a capacitor C1and an inductor L1. The capacitor C1has one end connected to the feed connector12and another end connected to one end of the inductor L1. Another end of the inductor L1is connected to the first transistor amplifier module16.

The first transistor amplifier module16includes a power supply V1, a first transistor M1, a resistor R1and an inductor L2. The first transistor M1is a Complementary Metal Oxide Semiconductor (CMOS) transistor. The inductor L1has one end connected to the gate of the first transistor M1. The resistor R1has one end connected to the power supply V1and another end connected to the gate of the first transistor M1. Thus, the power supply V1can provide a working electric potential to the first transistor M1through the resistor R1. The inductor L2has one end connected to the source of the first transistor M1and another end grounded.

The drain of the first transistor M1is connected to the second amplifier unit20to input signals. The second amplifier unit20includes a second transistor amplifier module22, a main filter circuit24, a subsidiary filter circuit26and a notch-filter28. The second transistor amplifier module22includes a power supply V2, a second transistor M2, a resistor R2and a capacitor C2. The second transistor M2is a CMOS transistor. The resistor R2has one end connected to the power supply V2and another end connected to the gate of the second transistor M2. Thus, the power supply V2can provide a working electric potential to the second transistor M2through the resistor R2. The main filter circuit24includes a capacitor C3and an inductor L3, which are connected in parallel between the drain of the first transistor M1and the source of the second transistor M2. The subsidiary filter circuit26includes a capacitor C4and an inductor L4connected in series, wherein the capacitor C4has one end connected to the source of the second transistor M2and another end connected to the inductor L4, and the inductor L4has one end connected to the capacitor C4and another end grounded. Thus, the main filter circuit24and the source of the second transistor M2are both connected to the ground via the subsidiary filter circuit26.

The notch-filter circuit28includes two capacitors C51, C52and two inductors L51, L52. The two capacitors C51, C52are connected in series between the gate of the second transistor M2and one end of the capacitor C2. The inductor L51has one end connected between the capacitors C51, C52and another end grounded. The inductor L52has one end connected between the gate of the second transistor M2and the capacitor C51, and another end connected between the capacitor C52and the capacitor C2. Another end of the capacitor C2is connected to the drain of the first transistor M1.

The drain of the second transistor M2is connected to the third amplifier unit30to input signals. The third amplifier unit30includes a third transistor amplifier module32, an output filter circuit34and an output connector36. The third transistor amplifier module32includes a power supply V3, a third transistor M3, a resistor R3, a capacitor C6and an inductor L6. The third transistor M3is a CMOS transistor. The resistor R3has one end connected to the power supply V3and another end connected to the gate of the third transistor M3. Thus, the power supply V3can provide a working electric potential to the third transistor M3through the resistor R3. The source of the third transistor M3is connected to the drain of the second transistor M2. The capacitor C6has one end connected to the gate of the third transistor M3and another end grounded. The inductor L6has one end connected to the power supply V3and another end connected to the drain of the third transistor M3.

The output filter circuit34includes three capacitors C7, C8, C9. The capacitor C7has one end connected between the inductor L6and the drain of the third transistor M3and another end connected to the capacitor C8. The capacitor C8has one end connected to the capacitor C7and another end connected to the output connector36. The capacitor C9has one end connected between the capacitors C7, C8and another end grounded.

The amplifier circuit100is installed in a wireless communication terminal, e.g., a mobile phone, a radiogram or an interphone. The capacitances of the capacitors C3, C4, C51, C52and the inductances of the inductors L3, L4, L51, L52are regulated to set the resonating frequencies of the input filter circuit24, the main filter circuit26and the notch-filter circuit28. An open circuit resonating frequency of the main filter circuit24and a resonating frequency of the notch-filter circuit28are set to be equal to a same predetermined frequency of the working signals used by the communication terminal. A short circuit resonating frequency of the subsidiary filter circuit26is set to be equal to a frequency of unneeded signals, e.g., image signals generated in the communication terminal.

In use, the communication terminal receives wireless communication signals. The frequency of the received signals is converted, and the signals are then transformed into working signals to be further processed. In the frequency conversion operation, image signals may be generated. The working signals and the image signals both enter the amplifier circuit100and are amplified by the first amplifier unit10. The input filter circuit10filters noise signals and the inductor L2cooperatives with the first transistor M1to form a source feedback to improve amplifying quality.

The amplified working signals and image signals are both transmitted to the second amplifier unit20from the drain of the first transistor M1. The capacitor C2can further filter noise signals. According to above-mentioned frequency characteristics of the main filter circuit24, the third filter26and the notch-filter circuit28, the working signals are not able to pass through the main filter circuit24, and the notch-filter circuit28allows only the working signals to pass through. Thus, the desired working signals pass through the notch-filter circuit28and are inputted into the gate of the second transistor M2to be amplified, and the unneeded image signals pass through the main filter circuit24and the subsidiary filter circuit24to be grounded. In this way, the image signals are filtered.

The working signals are then transmitted from the drain of the second transistor M2to the source of the third transistor M3. Thus, the first transistor amplifier module16and the second transistor amplifier module22cooperate with the third transistor amplifier module32to form a cascade amplifier, which improves quality of the amplified working signals outputted from the output connector36. The output filter circuit40can filter noises in the outputted working signals.

The amplifier circuit100can be widely used in wireless communication terminals. After a communication terminal receives wireless communication signals and converts the frequency of the wireless signals into working signals, the amplifier circuit100can both amplify the working signals and filter the image signals generated in the frequency conversion operation. Furthermore, the amplifier circuit100can also provide high power plus to the amplified working signals. Understandably, the amplifier circuit100can also filter other unneeded signals mixed with the working signals, if only the frequencies of the unneeded signals are different from the frequency of the working signals.