Patent Publication Number: US-RE40900-E

Title: Amplifier circuit

Description:
FIELD OF THE INVENTION 
     This invention relates to an amplifier circuit, in particular but not exclusively to an amplifier circuit for providing bandpass amplification at intermediate frequencies in radio receivers. 
     BACKGROUND OF THE INVENTION 
     Amplifiers are widely used in the prior art for amplifying input signals applied thereto to provide amplified output signals. This is particularly important in radio receivers in which radiation received thereat generates corresponding antenna received signals which typically have an amplitude of microvolts. The radio receivers employ amplifiers therein to amplify such received signals to an amplitude in the order of multivolts to volts, for example to drive a loudspeaker. Since it is difficult to prevent amplifiers designed to amplify at radio frequencies from spontaneously oscillating, especially if they comprise cascaded gain providing stages, it is customary to heterodyne the received signals to lower intermediate frequencies whereat it is easier to provide a high degree of amplification and also provide more selective bandpass signal filtration. 
     In prior art radio receivers, it is therefore customary to provide a majority of signal amplification required at intermediate frequencies, namely frequencies lying intermediate between that of the radiation received and audio or video frequencies. For example, a radio receiver receives radiation at a frequency of 500 MHz and generates a corresponding antenna received signal also at 500 MHz. The receiver heterodynes the received signal to generate an intermediate frequency signal in a frequency range around 10.7 MHz which is then amplified and filtered, and finally demodulates the amplified intermediate frequency signal to generate a corresponding audio output signal having signal components in a frequency range of 100 Hz to 5 kHz. 
     Recently, because the radio frequency spectrum is becoming increasingly congested, there is a trend to use an ultra high frequency (UHF) range in contemporary communications systems, namely around 500 MHz; transmission at microwave frequencies, for example 1 GHz to 30 GHz is now also employed. Associated with this is a trend in modern radio receiver design to employ intermediate frequency amplification at several tens of MHz or greater, this is done in order to obtain adequate ghost image rejection associated with using heterodyne processes. 
     In modem  modern mobile phones, most signal amplification is provided in intermediate frequency amplifier circuits incorporated therein. These circuits comprise transmission amplifiers and associated surface acoustic wave (SAW) or ceramic filters to provide a narrow bandpass signal amplification characteristic; the circuits and their associated filters are conventionally referred to collectively as an “intermediate frequency strip”. Such transmission amplifiers consume significant power in operation, for example intermediate frequency amplifier circuits employed in mobile telephones typically consume between several hundred microamperes and several mA of current when operational. 
     In order to provide modern mobile telephones with extended operating time from their associated batteries, new types of battery have been researched and developed which provide enhanced charge storage to weight performance, for example rechargeable metal hydride and lithium batteries. 
     The inventor has appreciated, rather than concentrating on improving battery technology, that reduction in current consumption of intermediate frequency amplifier circuits in radio receivers is desirable to provide extended operating time from batteries. The invention has therefore been made in a endeavour to provide an alternative type of amplifier circuit, for example a circuit especially suitable for use at intermediate frequencies in radio receivers which is capable of requiring less power to operate. 
     It is known in the art, as described in a Japanese patent application no. JP 55137707, to cascade reflection amplifiers in series and interpose filters therebetween to prevent higher harmonic components generated in preceding stages from propagating to successive stages. The filters are not operable to inhibit signal propagation in a reverse direction along the cascaded series of amplifiers to prevent the occurrence of spontaneous oscillation. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided an amplifier circuit for receiving an input signal and providing a corresponding amplified output signal, the circuit characterised in that it comprises:
         (a) a plurality of reflection amplifiers cascaded in series along a signal path and operative to amplify the input signal propagating in a forward direction therealong to provide the output signal; and   (b) connecting means for connecting the reflection amplifiers to form the signal path and for hindering signal propagation in a reverse direction therealong, thereby counteracting spontaneous oscillation from arising within the circuit, the connecting means incorporating filters which are interposed between neighbouring reflection amplifiers along the signal path, and modulating means for modulating the input signal to associated sideband signal components and converting to and from the sideband components along the path, the filters and the modulating means operative to promote signal propagation in the forward direction along the path and hinder signal propagation in the reverse direction therealong.       

     This provides the advantage that interposition of the filters between the amplifiers is capable of isolating each amplifier from its neighbouring amplifiers, thereby hindering signal propagation in the reverse direction along the path; incorporation of the modulating means enables the input signal propagating through each amplifier to be converted between a carrier and a sideband signal, thereby enabling it to propagate through the filters in the forward direction along the path. 
     The circuit provides the benefit that is capable of providing signal amplification and consuming less current during operation compared to prior art amplifier circuits. 
     One skilled in the art would not expect it to be practicable to connect a plurality of reflection amplifiers together and obtain stable amplification therefrom because of spontaneous interfering oscillations which would arise during operation. The circuit addresses this problem by incorporating the connecting means which promotes intended signal amplification in the circuit and counteracts signal amplification giving rise to spontaneous oscillation therein. 
     Spontaneous oscillation is defined as self induced oscillation arising along a signal path providing amplification as a consequence of feedback occurring around or within the signal path. 
     Conveniently, the filters are arranged in series along the signal path, the filters alternating between sideband transmissive filters and sideband rejective filters along the path, and the modulating means is arranged to convert the input signal as it propagates along the signal path alternately between a corresponding carrier signal transmissible substantially through the sideband rejective filters only and a corresponding sideband signal transmissible substantially through the sideband transmissive filters only, thereby promoting input signal propagation in the forward direction along the path and hindering signal propagation in the reverse direction therealong. 
     In another aspect, the invention provides a method of amplifying an input signal and providing a corresponding amplified output signal, the method characterised in that it includes the steps of:
         (a) providing a plurality of reflection amplifiers cascaded in series along a signal path, and connecting means for connecting the reflection amplifiers to the signal path and operative to promote signal propagation in a forward direction along the path and counteract signal propagation in a reverse direction therealong, the connecting means incorporating filters which are interposed between neighbouring reflection amplifiers along the signal path, and modulating means for modulating the input signal to associated sideband signal components and converting to and from the sideband components along the path, the filters and the modulating means operative to promote signal propagation in the forward direction along the path and hinder signal propagation in the reverse direction therealong;   (b) receiving the input signal at the signal path;   (c) directing the input signal through the connecting means to one of the reflection amplifiers for amplification therein to provide an amplified signal;   (d) directing the amplified signal to another of the reflection amplifiers for further amplification therein;   (e) repeating step (d) until the amplified signal reaches an output of the signal path; and   (f) outputting the amplified signal as the output signal from the signal path.       

     The method provides the advantage that, during amplification, the signal is selectively directed from amplifier to amplifier in a forward direction along the signal path, thereby counteracting any of the amplifiers reamplifying the input signal and hence preventing any feedback loops being established in which spontaneous oscillation can arise. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the invention will now be described, by way of example only, with reference to the following diagrams in which: 
         FIG. 1  is a schematic of an amplifier circuit in accordance with an embodiment of the invention; 
         FIG. 2  is an illustration of signal transmission characteristics of filters for incorporating into the circuit in  FIG. 1 ; and 
         FIG. 3  is a schematic of a circuit of a reflection amplifier for incorporating into the circuit of FIG.  1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , there is shown an amplifier circuit according to an embodiment of the invention; the circuit is indicated by  600 . It comprises three bandpass filters  610 ,  620 ,  630 , two biphase switches  650 ,  660 , a switching oscillator  670  and two reflection amplifiers  700 ,  710 . Each of the amplifiers  700 ,  710  incorporates a reflection amplifier circuit indicated by  1400  in FIG.  3 . 
     The filters  610 ,  630  are identical and employ surface acoustic wave (SAW), bulk acoustic wave (BAW) or ceramic filter components. Each of filters  610 ,  630  provides a bandpass transmission characteristic for signals propagating between its terminals H 1 , H 2 . The characteristic comprises a single transmission peak centred at a frequency f 0  having upper and lower −3 dB cut off frequencies of f 0 +f 1  and f 0 −f 1  respectively. In the circuit  600 , f 0  is 100 MHz and f 1  is 50 KHz. 
     The filter  620  also employs SAW, BAW or ceramic filter components. It provides a double peak transmission characteristic for signals propagating between its terminals H 3 , H 4 . The double peak characteristic comprises two transmission peaks, a first peak centred at a frequency f 0 +f 2  and a second peak centred at a frequency f 0 −f 2 . The first peak has −3 dB upper and lower cut off frequencies of f 0 +f 2 +f 1  and f 0 +f 2 −f 1  respectively. Likewise, the second peak has −3 dB upper and lower cut off frequencies of f 0 −f 2 +f 1  and f 0 −f 2 −f 1  respectively.  FIG. 2  provides a graph indicated by  800  illustrating signal transmission characteristics of the filters  610 ,  620 ,  630 . The graph  800  comprises a first axis  810  representing frequency and a second axis  820  representing relative signal transmission through the filters  610 ,  620 ,  630 . 
     In  FIG. 2 , the single transmission peak of the filters  610 ,  630  is indicated by  850 . Likewise, the first and second transmission peaks of the filter  620  are indicated by  860 ,  870  respectively. Moreover, the filters  610 ,  630  also strongly absorb radiation at frequencies around f 0 −f 2  and f 0 +f 2 , namely around a frequency range of the peaks  860 ,  870 , especially for signals applied to their terminals H 2 . Furthermore, the filter  620  also strongly absorbs radiation around a frequency range of the peak  850 , especially for signals applied to its terminal H 4 . Referring now to  FIG. 1  again, the switching oscillator  670  is operative to generate a binary logic square wave control signal at its output which switches periodically between a logic state 0 and a logic state 1 at the frequency f 2 . The output from the oscillator  670  is connected to input control terminals K of the biphase switches  650 ,  660 . 
     The biphase switches  650 ,  660  are identical and each incorporates three terminals, namely signal terminals J 1 , J 2  and an input terminal K as described above. The switch  650  incorporates an inductor and a varactor, also known in the art as a varicap diode; control signals applied to the terminal K of the switch  650  are operative to control a potential applied to the varactor therein, thereby changing its tuning and affecting a phase shift imparted to signals propagating through the switch  650  between its terminals J 1 , J 2 . When the control signal from the switching oscillator  670  is in the logic state 0, the switches  650 ,  660  are operative to provide 0° phase shift; conversely, when the control signal is in the logic state 1, the switches  650 ,  660  are operative to provide 90° phase shift. Thus, in operation, signals propagating through and subsequently returning from switches  650 ,  660  via their terminals J 1 , J 2  and amplified by associated reflection amplifiers  700 ,  710  are periodically switched in phase between 0° and 180°. 
     Operation of the circuit  600  will now be described with reference to  FIGS. 1 and 2 . The switching oscillator  670  oscillates at the frequency f 2  and generates the control signal at this frequency at its output. The frequency f 2  is selected to be equal to or greater than twice f 1 . The control signal switches the biphase switches  650 ,  660  so that they phase modulate signals passing therethrough at the frequency f 2 . 
     The filter  610  receives an input signal S in  at its terminal H 1  input. The signal S in  is for example, generated in a preceding stage (not shown) which heterodynes a received signal to generate the signal S in  as an intermediate frequency signal including signal components in a frequency range of f 0 −f 1  to f 0 +f 1 . The signal S in  is transmitted through the filter  610  from the terminal H 1  to the terminal H 2  thereof because its signal components are within the frequency range of the peak  850  of the filter  610 . When the signal S in  propagates from the terminal H 2 , it is unable to pass through the filter  620  because it is not transmissive at frequencies of the signal components; the signal S in  thus propagates from the terminal H 2  to the terminal J 1  of the switch  650  and becomes phase modulated therein to emerge at the terminal J 2  as a first modulated signal S m1 . The modulated signal S m1  propagates to a port T 3  of the amplifier  700  which reflectively amplifiers  amplifies the signal S m1 , to provide a second amplified modulated signal S 2   S m2 . The signal S m2  propagates from the port T 3  of the amplifier  700  back through the switch  650  whereat it is further phase modulated to provide a third modulated signal S m3  which is output at the terminal J 1 . 
     The signal S m3  is phase modulated and comprises two sidebands including signal components in the frequency range of peaks  860 ,  870 . The sidebands in the signal S m3  are prevented from propagating back through the filter  610  because it is non-transmissive at the frequencies of these sidebands. The signal S m3  thus propagates from the terminal H 3  of the filter  620  to the terminal H 4  thereof because the sidebands are within the frequency range of the peaks  860 ,  870  of the filter  620 . 
     The signal S m3  propagates from the terminal H 4  of the filter  620  to the terminal J 1  of the switch  660 . The filter  630  is unable to transmit the signal S m3  because it is not transmissive at the frequency ranges of the sidebands of the signal. The signal S m3  thus propagates through the switch  660  from its terminal J 1  to its terminal J 2  to emerge therefrom as a fourth signal S m4 . Because the switch  660  provides phase modulation at the frequency f 2 , the sidebands in the signal S m3  am heterodyned to generate a signal component in the signal S m4  in a frequency range of the peak  850 . The signal S m4  propagates from the terminal J 2  of the switch  660  to a port T 3  of the amplifier  710  wherein it is reflectively amplified to provide an amplified signal S m5 . The signal S m5  propagates from the port T 3  of the amplifier  710  back through the switch  660  whereat it is further phase modulated to emerge as a sixth signal S m6  at the terminal J 1  of the switch  660 . The signal S m6  includes, from the signal S m5 , signal components in the frequency range of the peak  850  after amplification thereof. 
     Because the filter  620  is untransmissive to signals including signal components within the frequency range of the peak  850 , especially at its H 4  terminal, the signal S m6  is prevented from being transmitted back through the filter  620 . The signal S m6  thus propagates through the filter  630  from its terminal H 1  to its terminal H 2  to propagate therefrom as the signal S out . The signal S out  incorporates signal components present in the signal S in  which have been amplified by the circuit  600 . 
     In broad overview, the circuit  600  alternately converts from stage to stage the signal S in  to be amplified from carrier frequency, namely within the frequency range of the peak  850 , to sidebands, namely within the frequency range of the peaks  860 ,  870 . Thus, the switches  650 ,  660  in combination with the filters  610 ,  620 ,  630  are effective at counteracting signal propagation back in a reverse direction along a path from the output S out  to the input S in ; this isolates the amplifiers  700 ,  710  thereby enabling greater signal amplification to be achieved in the circuit  600  without spontaneous oscillations arising. Hence, the circuit  600  is capable of providing high signal amplification approaching 50 dB for low current consumption in the order of a few tens of microamperes on account of employing reflection amplifiers. 
     If the reflection amplifiers  700 ,  710  were merely cascaded together without the switches  650 ,  660  and the filters  610 ,  620 ,  630 , severe spontaneous oscillation problems would be encountered which would hinder intended input signal amplification from being achieved. 
     The circuit  600  can be modified to include more amplification stages, each stage incorporating a reflection amplifier and being isolated from its neighbouring stages by a filter like the filter  610  in a first direction along the signal path, and by a filter like the filter  620  in a second direction along the signal path, said directions being mutually opposite. This enables higher gain to be achieved on account of incorporating more amplifier stages than illustrated in FIG.  1 . 
     The filters  610 ,  620 ,  630  can be one or more of SAW filters, ceramic filters or tuned inductance/capacitance filters. For high frequency operation, bulk acoustic wave filters can also be employed. 
     The amplifiers  700 ,  710  can be connected to a bias controller arranged to control transistor currents within the amplifiers  700 ,  710  thereby enabling dynamic control of their gain, for example where automatic gain control (AGC) is required to cater for a relatively large dynamic range of signals applied at S in . 
     The amplifier circuit  600  incorporates a cascaded series of reflection amplifiers connected to form a signal path along which input signal amplification occurs. The reflection amplifiers are connected by switched devices, for example the switches  650 ,  660  and the filters  610 ,  620 ,  630 , to facilitate signal propagation in a forward direction along the path for amplification and counteract signal propagation in a reverse direction along the path which can give rise to spontaneous oscillation. This enables higher amplification gains to be achieved for a lower current consumption which is less than required for prior art transmission amplifiers providing comparable gain. 
     The reflection amplifier circuit  1400  will now be further described with reference to FIG.  3 . The circuit  1400  is included within a dotted line  1410  and comprises a silicon or gallium arsenide (GaAs) transistor indicated by  1420 , a capacitor  1430  and a resistor  1440  forming a termination network for the transistor  1420 , a feedback capacitor  1450 , an inductor  1460  and a resistor  1470  forming a bias network, and a current source  1480 . The circuit  1400  includes an input/output port T 3  which is connected to a gate electrode  1420 g of the transistor  1420  and to a first terminal of the capacitor  1450 . 
     The circuit  1400  is connected to a power supply  1500  for supplying the circuit  1400  with power. The supply  1500  is connected to a drain electrode  1420 d of the transistor  1420  and also to a first terminal of the capacitor  1430 ; a second terminal of the capacitor  1430  is connected to a signal ground. The capacitor  1450  provides a second terminal which is connected to a source electrode  1420 s of the transistor  1420 , to the resistor  1440  which is grounded, and through the inductor  1460  and the resistor  1470  in series to the source  1480 , which is connected to the signal ground. 
     In operation of the circuit  1400 , the gate electrode  1420 g receives an incoming signal  6  applied through the port T 3 . The incoming signal causes a signal current corresponding to the incoming signal to flow between the source electrode  1420 g and the drain electrode  1420 d. The signal current is coupled through gate-drain and gate-source capacitances of the transistor  1420  and also through the capacitor  1450 , thereby generating an outgoing signal at the gate electrode  1420 g which is an amplified version of the incoming signal. The incoming signal is reflected at the gate electrode  1420 g where it is combined with the outgoing signal which propagates out through the port T 3 . 
     On account of the circuit  1400  receiving the incoming signal and returning the combined signal via one terminal, namely the port T 3 , it behaves as a reflecting negative resistance. The circuit  1400  and its associated components shown within the dotted line  1410  are capable of providing a high power gain approaching +30 dB for a drain/source current through the transistor  1420  in the order of a few tens of microamperes. Such a high power gain is not achievable from a transmission amplifier operating on such a low supply current. 
     When incorporated into a mobile telephone as part of its intermediate frequency strip, the amplifier circuit  600  incorporating a plurality of the circuits  1400  is capable of providing an order of magnitude reduction in telephone current consumption associated with amplifying signals therein at intermediate frequencies compared to prior art. This is of considerable benefit which provides extended duration of telephone operation from power supplied from rechargeable batteries for example. 
     It will be appreciated by those skilled in the art that variations can be made to the circuit  600  without departing from the scope of the invention. Thus, alternative switching devices, or equivalent devices, can be used with reflection amplifiers provided they exhibit similar characteristics to the switches and filters in the circuit  600 , namely for counteracting spurious oscillation from arising. 
     The circuit  600  can be incorporated into radio receivers, for example mobile telephones, to function as intermediate frequency strips therein. Moreover, when provided with a demodulator to convert signals output from the circuit  600 , the circuit is capable of operating as an IF receiver.