Amplifier circuit

Abstract

An amplifier circuit for amplifying an input signal to generate an amplifier output signal incorporates a cascaded series of reflection amplifiers arranged along a signal path and operative to amplify signals propagating in a forward direction along the signal path. The circuit is operative to counteract signal propagation in a reverse direction along the signal path, thereby hindering spontaneous oscillation from arising within the circuit. Incorporation of reflection amplifiers into the circuit enables it to provide high gain, for example 50 dB, while consuming low currents, for example, tens of microamperes. The circuit is especially suitable for use at intermediate frequencies in radio receivers such as mobile telephones.

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 amplifiers amplifies the signal S_m1, to provide a second amplified modulated signal S2 S_m2. The signal S_m2 propagates from the port T3 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 J1.

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 H3 of the filter 620 to the terminal H4 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 H4 of the filter 620 to the terminal J1 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 J1 to its terminal J2 to emerge therefrom as a fourth signal S_m4. Because the switch 660 provides phase modulation at the frequency f₂, 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 J2 of the switch 660 to a port T3 of the amplifier 710 wherein it is reflectively amplified to provide an amplified signal S_m5. The signal S_m5 propagates from the port T3 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 J1 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 H4 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 H1 to its terminal H2 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 T3 which is connected to a gate electrode 1420g 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 1420d 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 1420s 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 1420g receives an incoming signal 6 applied through the port T3. The incoming signal causes a signal current corresponding to the incoming signal to flow between the source electrode 1420g and the drain electrode 1420d. 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 1420g which is an amplified version of the incoming signal. The incoming signal is reflected at the gate electrode 1420g where it is combined with the outgoing signal which propagates out through the port T3.

On account of the circuit 1400 receiving the incoming signal and returning the combined signal via one terminal, namely the port T3, 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.

Claims

1. An amplifier circuit for receiving an input signal and providing a corresponding amplified output signal, the circuit comprising:

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 neighboring 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 associated sideband signal components along the path, the filters and the modulating means being operative to promote signal propagation in the forward direction along the path and hinder signal propagation in the reverse direction therealong.

2. The circuit according to claim 1, wherein 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.

3. The circuit according to claim 1, wherein the modulating means comprises a plurality of phase switches, each phase switch having a respective reflection amplifier associated therewith.

4. The circuit according to claim 3, wherein the phase switches are interposed between the filters and their associated amplifiers.

5. The circuit according to claim 1, wherein the modulating means is operable at a rate having an associated frequency which is half a frequency separation of transmission peaks of the sideband transmission characteristics of the si and sideband transmissive filters.

6. The circuit according to claim 3, wherein the phase switches each incorporate a tuned circuit comprising an inductor and a modulated varactor.

7. An intermediate frequency strip incorporating an amplifier circuit for receiving an input signal and providing a corresponding amplified output signal, the circuit comprising:

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 neighboring 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 associated sideband signal components along the path, the filters and the modulating means being operative to promote signal propagation in the forward direction along the path and hinder signal propagation in the reverse direction therealong.

8. An intermediate frequency receiver incorporating an amplifier circuit for receiving an input signal and providing a corresponding amplified output signal, the circuit comprising:

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 neighboring 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 associated sideband signal components along the path, the filters and the modulating means being operative to promote signal propagation in the forward direction along the path and hinder signal propagation in the reverse direction therealong.

9. A mobile telephone incorporating an amplifier circuit for receiving an input signal and providing a corresponding amplified output signal, the circuit comprising:

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 neighboring 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 associated sideband signal components along the path, the filters and the modulating means being operative to promote signal propagation in the forward direction along the path and hinder signal propagation in the reverse direction therealong.

10. A method of amplifying an input signal and providing a corresponding amplified output signal, the method comprising 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 neighboring reflection amplifiers along the signal path, and modulating means for modulating the input signal to associated sideband signal components and converting to and from associated sideband signal components along the path, the filters and the modulating means being 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 in the forward direction 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.

11. An amplifier circuit for receiving an input signal and providing a corresponding amplified output signal, the circuit comprising:

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 one or more filters which are interposed between neighboring 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 associated sideband signal components along the path, the one or more filters and the modulating means being operative to promote signal propagation in the forward direction along the path and hinder signal propagation in the reverse direction therealong.

12. The circuit according to claim 11, wherein the one or more filters are arranged in series along the signal path, the one or more 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 transmission 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.

13. The circuit according to claim 11, wherein the modulating means comprises a plurality of phase switches, each phase switch having a respective reflection amplifier associated therewith.

14. The circuit according to claim 13, wherein the phase switches are interposed between the one or more filters and their associated amplifiers.

15. The circuit according to claim 13, wherein the phase switches each incorporate a tuned circuit comprising an inductor and a modulated varactor.

16. The circuit according to claim 11, wherein the modulating means is operable at a rate having an associated frequency which is half a frequency separation of transmission peaks of the sideband transmission characteristics of the sideband transmissive filters.

17. The circuit according to claim 11 further comprising a first filter preceding the first reflection amplifier in the series along the signal path.

18. The circuit according to claim 17 further comprising a last filter succeeding the last reflection amplifier in the series along the signal path.

19. The circuit according to claim 18, wherein the first filter, the one or more filters which are interposed between neighboring reflection amplifiers along the signal path, and the last filter are arranged in series along the signal path, all of said 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.

20. The circuit according to claim 19, wherein the modulating means comprises a plurality of phase switches, each phase switch having a respective amplifier associated therewith.

21. The circuit according to claim 20, wherein the phase switches are interposed between the first filter, the one or more filters which are interposed between neighboring reflection amplifiers along the signal path, and the last filter and their associated amplifiers.

22. The circuit according to claim 19, wherein the first filter, the one or more filters which are interposed between neighboring reflection amplifiers along the signal path, and the last filter are selected from the group consisting of SAW filters, ceramic filters, tuned inductance/capacitance filters, and bulk acoustic wave filters.

23. The circuit according to claim 11, wherein the plurality of reflection amplifiers are connected to a bias controller arranged to control transistor currents within the amplifiers.

24. The circuit according to claim 19, wherein the phase switches each incorporate a tuned circuit comprising an inductor and a modulated varactor.

25. An intermediate frequency strip incorporating an amplifier circuit for receiving an input signal and providing a corresponding amplified output signal, the circuit comprising:

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 one or more filters which are interposed between neighboring 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 associated sideband signal components along the path, the one or more filters and the modulating means being operative to promote signal propagation in the forward direction along the path and hinder signal propagation in the reverse direction therealong.

26. The circuit according to claim 25, the one or more filters are arranged in series along the signal path, the one or more 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.

27. The circuit according to claim 25, wherein the modulating means comprises a plurality of phase switches, each phase switch having a respective amplifier associated therewith.

28. The circuit according to claim 27 wherein the phase switches are interposed between the one or more filters and their associated amplifiers.

29. The circuit according to claim 25, wherein the modulating means is operable at a rate having an associated frequency which is half a frequency separation of transmission peaks of the sideband transmission characteristics of the sideband transmissive filters.

30. The circuit according to claim 25 further comprising a first filter preceding the first reflection amplifier in the series along the signal path.

31. The circuit according to claim 30 further comprising a last filter succeeding the last reflection amplifier in the series along the signal path.

32. The circuit according to claim 31, wherein the first filter, the one or more filters which are interposed between neighboring reflection amplifiers along the signal path, and the last filter are arranged in series along the signal path, all of said 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.

33. The circuit according to claim 32, wherein the modulating means comprises a plurality of phase switches, each phase switch having a respective amplifier associated therewith.

34. The circuit according to claim 33 wherein the phase switches are interposed between the first filter, the one or more filters which are interposed between neighboring reflection amplifiers along the signal path, and the last filter and their associated amplifiers.

35. The circuit according to claim 32, wherein the first filter, the one or more filters which are interposed between neighboring reflection amplifiers along the signal path, and the last filter are selected from the group consisting of SAW filters, ceramic filters, tuned inductance/capacitance filters, and bulk acoustic wave filters.

36. The circuit according to claim 25, wherein the plurality of reflection amplifiers are connected to a bias controller arranged to control transistor currents within the amplifiers.

37. An intermediate frequency receiver incorporating an amplifier circuit for receiving an input signal and providing a corresponding amplified output signal, the circuit comprising:

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 one or more filters which are interposed between neighboring 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 associated sideband signal components along the path, the one or more filters and the modulating means being operative to promote signal propagation in the forward direction along the path and hinder signal propagation in the reverse direction therealong.

38. The circuit according to claim 37, the one or more filters are arranged in series along the signal path, the one or more 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.

39. The circuit according to claim 37, wherein the modulating means comprises a plurality of phase switches, each phase switch having a respective amplifier associated therewith.

40. The circuit according to claim 39, wherein the phase switches are interposed between the one or more filters and their associated amplifiers.

41. The circuit according to claim 39, wherein the phase switches each incorporate a tuned circuit comprising an inductor and a modulated varactor.

42. The circuit according to claim 37, wherein the modulating means is operable at a rate having an associated frequency which is half a frequency separation of transmission peaks of the sideband transmission characteristics of the sideband transmissive filters.

43. The circuit according to claim 37 further comprising a first filter preceding the first reflection amplifier in the series along the signal path.

44. The circuit according to claim 43 further comprising a last filter succeeding the last reflection amplifier in the series along the signal path.

45. The circuit according to claim 44, wherein the first filter, the one or more filters which are interposed between neighboring reflection amplifiers along the signal path, and the last filter are arranged in series along the signal path, all of said 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.

46. The circuit according to claim 45, wherein the modulating means comprises a plurality of phase switches, each phase switch having a respective amplifier associated therewith.

47. The circuit according to claim 46, wherein the phase switches are interposed between the first filter, the one or more filters which are interposed between neighboring reflection amplifiers along the signal path, and the last filter and their associated amplifiers.

48. The circuit according to claim 45, wherein the first filter, the one or more filters which are interposed between neighboring reflection amplifiers along the signal path, and the last filter are selected from the group consisting of SAW filters, ceramic filters, tuned inductance/capacitance filters, and bulk acoustic wave filters.

49. The circuit according to claim 37, wherein the plurality of reflection amplifiers are connected to a bias controller arranged to control transistor currents within the amplifiers.

50. A mobile telephone incorporating an amplifier circuit for receiving an input signal and providing a corresponding amplified output signal, the circuit comprising:

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 one or more filters which are interposed between neighboring 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 associated sideband signal components along the path, the one or more filters and the modulating means being operative to promote signal propagation in the forward direction along the path and hinder signal propagation in the reverse direction therealong.

51. The circuit according to claim 50, the one or more filters are arranged in series along the signal path, the one or more 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.

52. The circuit according to claim 50, wherein the modulating means comprises a plurality of phase switches, each phase switch having a respective amplifier associated therewith.

53. The circuit according to claim 52, wherein the phase switches are interposed between the one or more filters and their associated amplifiers.

54. The circuit according to claim 52, wherein the phase switches each incorporate a tuned circuit comprising an inductor and a modulated varactor.

55. The circuit according to claim 50, wherein the modulating means is operable at a rate having an associated frequency which is half a frequency separation of transmission peaks of the sideband transmission characteristics of the sideband transmissive filters.

56. The circuit according to claim 50 further comprising a first filter preceding the first reflection amplifier in the series along the signal path.

57. The circuit according to claim 56 further comprising a last filter succeeding the last reflection amplifier in the series along the signal path.

58. The circuit according to claim 57, wherein the first filter, the one or more filters which are interposed between neighboring reflection amplifiers along the signal path, and the last filter are arranged in series along the signal path, all of said 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.

59. The circuit according to claim 58, wherein the modulating means comprises a plurality of phase switches, each phase switch having a respective amplifier associated therewith.

60. The circuit according to claim 59, wherein the phase switches are interposed between the first filter, the one or more filters which are interposed between neighboring reflection amplifiers along the signal path, and the last filter and their associated amplifiers.

61. The circuit according to claim 57, wherein the first filter, the one or more filters which are interposed between neighboring reflection amplifiers along the signal path, and the last filter are selected from the group consisting of SAW filters, ceramic filters, tuned inductance/capacitance filters, and bulk acoustic wave filters.

62. The circuit according to claim 50, wherein the plurality of reflection amplifiers are connected to a bias controller arranged to control transistor currents within the amplifiers.

63. A method of amplifying an input signal and providing a corresponding amplified output signal, the method comprising the steps of:

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 one or more filters which are interposed between neighboring 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 associated sideband signal components along the path, the one or more filters and the modulating means being operative to promote signal propagation in the forward direction along the path and hinder signal propagation in the reverse direction therealong.

64. The method according to claim 63 further comprising providing a first filter preceding the first reflection amplifier in the series along the signal path.

65. The method according to claim 64 further comprising providing a last filter succeeding the last reflection amplifier in the series along the signal path.

66. The method according to claim 65, wherein the modulating means comprises a plurality of phase switches, each phase switch having a respective reflection amplifier associated therewith.

67. The method according to claim 66, wherein directing the input signal through the connecting means to one of the reflection amplifiers comprises directing the input signal through the first filter and through a first of the plurality of phase switches.

68. The method according to claim 67, wherein directing the amplified signal in the forward direction to another of the reflection amplifiers comprises directing the amplified signal back through the first of the plurality of phase switches, through one of the one or more filters, and through a second of the plurality of phase switches.