GB2536680A

GB2536680A - Linearized negative impedance converter matching circuits

Info

Publication number: GB2536680A
Application number: GB1505063.6A
Authority: GB
Inventors: Hu Sampson; Wan Liang
Original assignee: Smart Antenna Technologies Ltd
Current assignee: Smart Antenna Technologies Ltd
Priority date: 2015-03-25
Filing date: 2015-03-25
Publication date: 2016-09-28
Anticipated expiration: 2035-03-25
Also published as: GB2536680B; GB201505063D0

Abstract

A diode 36 is inserted in the gate biasing circuit of the active transistors 20 (9,10, figure 9) of a negative impedance converter (NIC). The diode rectifies the RF signal so that a higher DC bias current flows through the transistors 20 at higher RF signal levels, thereby reducing distortion at high signal levels while allowing lower power consumption at lower signal levels. The NIC may be used to provide a negative capacitance for cancelling the capacitance of an electrically small antenna. The negative impedance of the NIC may be programmable. A branch (figure 9) comprising resistors and capacitors may be placed across the RF ports of the NIC to enhance matching performance, linearity and stability. The diode 36 may comprise a diode-connected transistor (43,figure13). The NIC transistors may comprise BJTs or FETs.

Description

LINEARIZED NEGATIVE IMPEDANCE CONVERTER MATCHING CIRCUITS

[0001] This invention relates to matching circuits to match antennas to RE sources and, in particular, to matching circuits comprising a negative impedance converter provided with bias circuitry configured to improve linearity performance.

BACKGROUND

[0001] Electrically small antennas can be generally classified as TM (transverse magnetic) and TE (transverse electric) mode antennas. For a TM mode small antenna, which is widely used in wireless communication systems, the input impedance is considerably reactive with a small real part. It is therefore critical to match the antenna to the receiver or transmitter to maximise the total efficiency in the frequency range of interest.

[0002] Normally, an electrically small TM mode antenna can be characterised by or represented as a series connected combination of a resistor, a capacitor and an inductor.

Figures 1 and 2 show that, at low frequencies, the reactance can equally be represented by a series connected capacitor and inductor, with the capacitor playing a dominant role in the reactance. The resistor represents the resistance of the radiating element of the antenna.

[0003] There are two different ways to match a highly reactive antenna of this type. One approach is conventional passive matching, where a large series inductor Lex, is placed between the antenna and the signal port as a necessary component. However, the resistive loss that is introduced by the inductor Lext dramatically degrades the total efficiency. In fact, even with lossless inductors, the match is effective over only extremely small instant bandwidths because the reactive part of the electrically small antenna cannot be neutralised over a broad frequency band with passive components (the real part of the impedance is much smaller than the imaginary part). This is illustrated in Figure 3.

[0004] The other approach uses an NIC (negative impedance converter) to create a negative capacitor, which can then be configured to cancel the reactance of the antenna as much as possible. This is a type of non-Foster impedance matching, and is illustrated in Figure 4.

[0005] There is a relationship between antenna size and the realisable bandwidth as defined by the Chu limit (Chu, L. J.; "Physical limitations of omni-directional antennas"; Journal Applied Physics 19: 1163-1175; December 1948). The Chu limit gives the relationship between the radius of the circle that completely circumscribes an antenna and the Q of the antenna. However, McLean (McLean, J. S.; "A re-examination of the fundamental limits on the radiation Q of electrically small antennas"; IEEE Transactions on Antennas and Propagations; Vol. 44; No. 5; pp. 672-676; May 1996) redefined how the Q of an antenna should be calculated, and this is given in equation 1.1: (ka) where k is the wave number and a is the radius of a sphere that completely circumscribes the antenna as shown in Figure 5.

[0006] McLean's equation is a derivation from the original Chu limits equations. There has also been much research into ways of improving the gain of an antenna through the use of matching networks, but this is also bounded by the Harrington limits (Harrington, R. F.; "Effect of antenna size on gain, bandwidth and efficiency"; Journal of Research of the National Bureau of Standards -D. Radio Propagation; vol. 64D; p. 12; 29"' June 1959) on antennas as given in: G = (ka)2 (1.2) The Chu limit can be related to the antenna bandwidth by rewriting the Q of the antenna as shown in equation 1.3: Vea (ka) (1.3) where fe is the antenna centre frequency at resonance and af is the bandwidth of the antenna.

[0007] Comparing equation 1.1 with equation 1.3, it can be seen that reducing the radius of the sphere which translates to a physical reduction in the antenna size, the antenna bandwidth also reduces. The reduction in size means that the antenna radiation resistance also reduces, and this in turn leads to a reduction the antenna efficiency. From equation 1.2, it is clear that antenna gain is also proportional to the antenna size a.

[0008] These two fundamental limits on the antenna make it difficult to provide a small antenna with a low 0 (wideband). However, more and more devices these days require smaller antennas and there is need for these antennas to still have wide usable bandwidths.

[0009] Passive matching networks help to match antennas, but because they involve resonating the reactive part of the antenna with passive elements, they only give a good match at specific frequencies. Away from the specific frequency, the antenna return loss decreases. This necessitates the use of multiple or reconfigurable matching networks to cover wide frequency bands. However, using non-Foster elements could help provide continuous wideband matching because unlike Foster elements, the slope of the reactance versus frequency of a non-Foster element is always negative as shown in Figure 4.

[0010] With these properties, non-Foster elements are able to cancel out completely the reactance of other elements and antennas because of the difference in slope and direction of rotation on the Smith chart.

[0011] One implementation of non-Foster elements is through the use of NICs (negative impedance converters). NICs were first proposed by Linvill (Linvill, J.G.; "Transistor negative-impedance converters"; Proc. IRE; vol 41, pp 725-729; 1953). The Linvill NIC consists of two transistors connected in a common base configuration. "Common base" or "common gate" refers to a specific input and output setup of a transistor in amplifier applications. In a Linvill type NIC, the RF input terminal is connected to the emitter or source of one transistor, and the RF output terminal to the emitter or source of the other transistor (in fact, since an NIC is normally a bidirectional device, it does not matter which terminal is used as the RF input and which as the RF output). The reactance to be inverted is connected between the two collectors or drains, and the base or gate of each transistor is connected to the collector or drain of the other transistor in the form of a feedback path. The emitters or sources form the two ports of the NIC The circuit schematic of the NIC is shown in Figure 6.

[0012] NICs offer useful features when used for matching antennas to transceiver RF modules. As NICs are active matching circuits, they will consume power, and the amount of power consumed will depend on the maximum power to be transmitted. A more conventional design of NIC is usually based on satisfying the maximum transmitter power bias condition and then using the NIC with the same bias for lower transmitter powers.

This will work, but the amount of battery power consumed at lower transmit powers in a handset, tablet or other mobile device is wasteful. A conventional NIC setup is shown in Figure 7. The NIC is located between the antenna and the RF module (Tx/Rx).

[0013] The parameters that are optimised for NIC are: a) Compression point IP 1 ldB, b) Third order intercept point/IMD3 c) Noise figure d) Antenna efficiency e) Antenna matched bandwidth f) Antenna return loss/VSWR [0014] For the maximum transmitter power case, a high bias current is required to meet the linearity requirement for 3GPP/4G LTE applications -a typical N IC bias current from a 3V battery could be as high as 484mA. This may satisfy the linearity requirement at maximum transmitter power (24dBm for LTE), but for lower transmit powers this is clearly wasteful.

BRIEF SUMMARY OF THE DISCLOSURE

[0015] Viewed from a first aspect, there is provided a negative impedance converter for a matching circuit for matching an impedance of an antenna to an impedance of an RE source or load, the negative impedance converter comprising first and second transistors connected in a cross-over configuration, each transistor having a source or emitter, a drain or collector and a gate or base, and each transistor further having a first biasing circuit connected to its gate or base, wherein the first biasing circuit comprises a first DC biasing signal source and a first diode connected between the first DC biasing signal source and the gate or base.

[0016] Viewed from a second aspect, there is provided a negative impedance converter for a matching circuit for matching an impedance of an antenna to an impedance of an RE source or load, the negative impedance converter comprising first and second transistors connected in a cross-over configuration, each transistor having a source or emitter, a drain or collector and a gate or base, and each transistor further having a first biasing circuit connected to its gate or base, wherein the first biasing circuit comprises a first DC biasing signal source and a third transistor connected between the first DC biasing signal source and the gate or base.

[0017] The first diode or third transistor helps to promote RE linearization.

[0018] Additionally, there may be provided a second diode connected in series with the first diode or the third transistor, or a further transistor connected in series with the first diode or the third transistor.

[0019] The second diode or further transistor helps to compensate for voltage changes in the first diode or third transistor due to physical environmental changes, such as temperature.

[0020] It will be understood that the NIC may comprise field effect transistors, in which case the transistors will have a source, a drain and a gate. Alternatively, the NIC may comprise bipolar junction transistors, in which case the transistors will have an emitter, a collector and a base.

[0021] The NIC has an RF input port connected to the source or emitter of the first transistor, and an RF output port connected to the source or emitter of the second transistor. The collector or drain of the first transistor is connected to the base or gate of the second transistor, and the collector or drain of the second transistor is connected to the base or gate of the first transistor. A predetermined impedance is provided between the respective collectors or drains on the one hand, and the respective bases or gates on the other hand, of the first and second transistors. The predetermined impedance determines the negative impedance that the NIC applies to an RF signal passing between the input port and the output port. The predetermined impedance is preferably adjustable, for example by employing a variable capacitor and/or a variable inductor. The predetermined impedance may be adjustable by way of a control input from a digital controller, such as a microprocessor, field programmable gate array, PIC or the like.

[0022] The base or gate is provided with a conductive base or gate connection to allow an RF signal to be connected to the base or gate. The base or gate connection is connected to the first biasing circuit between the respective first and second diodes, third and fourth transistors, or first diode and third transistor. The first biasing circuit applies a first biasing signal to the base or gate, with the RF signal being superposed on the first biasing signal. Where the transistors are bipolar junction transistors, the first biasing signal will be a biasing current. Where the transistors are field effect transistors, the first biasing signal will be a biasing voltage.

[0023] The first biasing circuit further comprises a source of biasing signal, such as a DC source, and may further include one or more resistors, capacitors and/or inductors so as to allow the first biasing signal to be conditioned as required.

[0024] The diodes or transistors or diode and transistor are connected in series with each other with the same polarity, and are connected between the source of biasing signal and ground. Resistors may be provided in the first biasing circuit to act as a potential divider, thereby allowing the first biasing circuit to apply the required first biasing signal to the base or gate. The diodes or transistors are also configured to help apply the desired bias. A capacitor may be connected in parallel with one of the diodes or transistors, advantageously the diode or transistor closest to the source of biasing signal, and an additional capacitor may be connected between the input of this diode or transistor and ground, so as to enable RF power linearization.

[0025] When the functional transistors of the NIC are field effect transistors (FETs), the first and second diodes (or third transistor and further transistor, or first diode and further transistor), in combination with resistors in the first biasing circuit, are used to provide a DC biasing voltage to the gate of the functional FET of the NIC. This gate biasing voltage controls the biasing current from the drain to the source of the functional FET of the NIC. When the functional transistors of the NIC are bipolar junction transistors (BJTs), the first and second diodes (or third transistor and further transistor, or first diode and further transistor), in combination with resistors in the first biasing circuit, are used to provide a DC biasing current to the base of the functional BJT of the NIC. This gate biasing current controls the biasing current from the collector to the emitter of the functional BJT of the NIC. The function of the second diode (or the further transistor) is to compensate the DC voltage or current change on the first diode (or the third transistor) that may arise due to changes in physical conditions (for example an external or internal temperature change). The first diode (or the third transistor), together with a parallel capacitor and a grounded capacitor, serves to provide NIC RF linearization. When the input RF signal power is injected at the source or emitter terminal of the NIC functional transistor, part of the signal is coupled to the gate or base terminal of the NIC functional transistor, and some of this part is coupled to ground by the grounded capacitor. Meanwhile, the first diode (or the third transistor) rectifies the coupled RF signal to decrease its equivalent resistance. As a result, the DC voltage at the gate of the NIC functional transistor (or the DC current at the base of the NIC functional transistor) is increased and the biasing current from drain to source (or from collector to emitter) of the NIC functional transistor is increased, which improves the linearity of the NIC. The grounded and parallel capacitors can be used to control the strength of RF signal coupling and rectification, and an inductor between the two diodes (or two transistors, or one diode and one transistor) can be used to choke the RF signal and further to improve RF power linearization.

[0026] The first biasing circuit helps to improve the efficiency and hence power handling capability of the negative impedance converter. In particular, third order intermodulation distortion, IMD3, is much reduced at higher input powers.

[0027] A second biasing circuit, comprising a second DC biasing source, may be connected by way of an inductor to the collector or drain of the NIC functional transistor, with the emitter or source being connected to ground by way of an inductor. The inductors serve to block RF signals. The second biasing circuit applies a second biasing signal across the collector or drain and the emitter or source of the transistor, the second biasing signal controlling a biasing voltage across the drain and the source or the collector and the emitter of the NIC functional transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: Figure 1 shows an electrically small antenna connected to a 50 ohm signal port; Figure 2 shows the antenna of Figure 1 represented as an equivalent series connected resistor, capacitor and inductor; Figure 3 shows the arrangement of Figure 2 provided with a passive impedance matching network, together with a plot of reactance against angular frequency; Figure 4 shows the arrangement of Figure 2 provided with a non-Foster matching network comprising a negative capacitance, together with a plot of reactance against angular frequency; Figure 5 illustrates an antenna circumscribed by a sphere of radius a; Figure 6 is a schematic of a conventional Linvill type negative impedance converter (NIC); Figure 7 shows a conventional NIC arrangement for matching an antenna to a 20 transceiver; Figure 8 shows an NIC-based matching circuit for an electrically small antenna; Figure 9 shows the NIC of Figure 8 in more detail; Figure 10 shows a conventional biasing circuit for one transistor in an NIC; Figure 11 shows a first embodiment; Figure 12 shows a second embodiment; Figure 13 shows a third embodiment; and Figure 14 shows a plot of gain vs. power for the circuits of Figures 10, 11 and 12.

DETAILED DESCRIPTION

[0029] Figure 8 illustrates an embodiment comprising an NIC-based matching circuit for an electrically small antenna. The circuit comprises an output termination 1, a two-port antenna model 2, a neutralization inductor 3, an NIC block 4, a capacitor 5 for impedance transformation and an RF source 6.

[0030] Figure 9 shows the NIC block 4 in more detail, the NIC including an input port 7 connected to the emitter or source port P3 of a first transistor sub-circuit 9, and an output port 8 connected to the emitter or source port P3 of a second transistor sub-circuit 10. The transistor sub-circuits 9, 10 are connected in a cross-over configuration, with the base or gate port P1 of the first transistor sub-circuit 9 connected to the collector or drain port P2 of the second transistor sub-circuit 10, and the collector or drain port P2 of the first transistor sub-circuit 9 connected to the base or gate port P1 of the second transistor sub-circuit 10.

A capacitor 100 and a lossy inductor 101 are connected between the collector or drain ports P2 of the transistor sub-circuits 9, 10, the capacitor 100 and inductor 101 defining the negative impedance that is presented by the NIC block 4 between its input port 7 and output port 8. By applying a negative impedance to the RF signal passing from input port 7 to output port 8, the NIC block 4 can match the RF signal to the antenna.

[0031] The NIC block 4 further comprises parallel-connected passive components in the form of resistors 102, 103 and capacitors 104, 105 which are used to adjust the impedance of the NIC block 4, thereby to enhance matching performance, linearity and stability.

[0032] Impedance tuning of the NIC block 4 can be controlled by some external device such as a microprocessor (not shown).

[0033] Figure 10 shows one of the transistor sub-circuits 9, 10 of Figure 9 in more detail, shown here configured with known biasing circuitry so as to illustrate present embodiments more clearly. The sub-circuit comprises a transistor 20 having an emitter or source 21, a collector or drain 22 and a base or gate 23. The base or gate 23 of the transistor 20 is connected to port P1 by way of a DC block 34. A first DC biasing signal is applied to the gate 23 by a DC source 32 and an inductor 33. The emitter or source 21 is connected to port P3 by way of a DC block 30, and the collector or drain 22 is connected to port P2 by way of a DC block 29. A second DC biasing signal is applied between the collector or drain 22 and the emitter or source 21 by way of DC source 26. Inductors 27 and 28 are provided to block RF signals. The DC source 32 controls the bias current and the DC source 26 controls the bias voltage across the collector or drain 22 and the emitter or source 21 of the transistor 20. The DC blocks 29, 30, 34 are provided to isolate the ports P2, P3, P1 from the biasing signals.

[0034] To achieve high linearity and simultaneous transmit and receive, the functional transistors in the NIC are preferably biased in a Class-A (linear) bias condition.

[0035] Figure 11 shows a first embodiment, comprising a transistor sub-circuit 9, 10 based around a transistor 20. The sub-circuit comprises a transistor 20 having an emitter or source 21, a collector or drain 22 and a base or gate 23. The base or gate 23 of the transistor 20 is connected to port P1 by way of a DC block 34. A first DC biasing signal is applied to the gate 23 by a DC source 32. The first biasing signal passes through a first resistor 35, a first diode 36, an inductor 37, a second diode 38 and a second resistor 39 to ground. A first capacitor 40 connects the input to the first diode 36 to ground, and a second capacitor 41 is connected in parallel with the first diode 36. The first and second diodes 36, 38 have the same polarity. Resistors 35 and 39 are configured as a potential divider and can be adjusted so as to vary the first biasing signal as required, together with the diodes 36, 38. The first diode 36 and the capacitors 40, 41 help to promote RF power linearization. The emitter or source 21 is connected to port P3 by way of a DC block 30, and the collector or drain 22 is connected to port P2 by way of a DC block 29. A second DC biasing signal is applied between the drain 22 and the source 21 by way of DC source 26. Inductors 27 and 28 are provided to block RE signals. The DC source 32 controls the bias current and the DC source 26 controls the bias voltage. The DC blocks 29, 30, 34 are provided to isolate the ports P2, P3, P1 from the biasing signals.

[0036] Figure 12 shows an alternative implementation of the Figure 11 embodiment, in which the second diode 38 is replaced with a further transistor 42.

[0037] A further alternative implementation of the Figure 11 embodiment is shown in Figure 13, where both the first and second diodes 36, 38 are replaced with a third transistor 43 and a further transistor 44.

[0038] The three biasing circuits shown in Figures 10 (prior art), 11 (two diodes) and 12 (one diode, one transistor) were tested with the two main RE transistors in the NIC 4 biased at 40mA and 2.5V. Figure 14 shows a plot of gain against power (in) for the different biasing circuits, with the plot for the Figure 10 circuit identified at 45, the plot for the Figure 11 embodiment identified at 46, and the plot for the Figure 12 embodiment identified at 47.

[0039] Table 1 below gives the values for IMD3 (in dBc) for the different biasing circuits of Figures 10, 11 and 12.

Table 1

821MHz 851MHz 881MHz Pin (dBm) 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 Fig 10 -50 -38 -23 -18 -17 -52 -39 -24 -20 -18 -57 -44 -27 -23 -22 Fig 11 -44 -41 -42 -45 -43 -38 -38 -41 -45 -37 -36 -37 -40 -45 -37 Fig 12 -77 -55 -42 -41 -40 -71 -43 -36 -42 -35 -60 -39 -34 -36 -33 [0040] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0041] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0042] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

CLAIMS: 1. A negative impedance converter for a matching circuit for matching an impedance of an antenna to an impedance of an RF source or load, the negative impedance converter comprising first and second transistors connected in a cross-over configuration, each transistor having a source or emitter, a drain or collector and a gate or base, and each transistor further having a first biasing circuit connected to its gate or base, wherein the first biasing circuit comprises a first DC biasing signal source and a first diode connected between the first DC biasing signal source and the gate or base.
2. A negative impedance converter for a matching circuit for matching an impedance of an antenna to an impedance of an RF source or load, the negative impedance converter comprising first and second transistors connected in a cross-over configuration, each transistor having a source or emitter, a drain or collector and a gate or base, and each transistor further having a first biasing circuit connected to its gate or base, wherein the first biasing circuit comprises a first DC biasing signal source and a third transistor connected between the first DC biasing signal source and the gate or base.
3. A negative impedance converter as claimed in claim 1 or 2, further comprising a second diode connected in series with the first diode or the third transistor.
4. A negative impedance converter as claimed in claim 1 or 2, further comprising a further transistor connected in series with the first diode or the third transistor.
5. A negative impedance converter as claimed in any preceding claim, wherein the gate or base of each of the first and second transistors is provided with a conductive gate or base connection to allow an RF signal to be connected to the gate or base.
6. A negative impedance converter as claimed in claim 5, wherein the gate or base connection of each of the first and second transistors is connected to the respective first biasing circuit between the respective first and second diodes, third transistor and further transistor, or first diode and further transistor.
7. A negative impedance converter as claimed in any preceding claim, wherein the first biasing circuit of each of the first and second transistors further comprises one or more resistors, capacitors and/or inductors so as to allow the first DC biasing signal to be conditioned as required.
8. A negative impedance converter as claimed in any preceding claim, wherein for each of the first and second transistors, the first and second diodes or third transistor and further transistor or first diode and further transistor are connected in series with each other with the same polarity, and are connected between the first DC biasing signal source and ground.
9. A negative impedance converter as claimed in claim 8, wherein for each of the first and second transistors, resistors are provided in the first biasing circuit to act as a potential divider, thereby allowing the first biasing circuit to apply the required first DC biasing signal to the base or gate.
10. A negative impedance converter as claimed in any preceding claim, wherein for each of the first and second transistors, a first capacitor is connected in parallel with the first diode or third transistor.
11. A negative impedance converter as claimed in claim 10, wherein for each of the first and second transistors, an additional capacitor is connected between an input of the first diode or third transistor and ground, so as to enable RF power linearization.13. A negative impedance converter as claimed in any preceding claim, wherein for each of the first and second transistors, an inductor is connected between the first and second diodes, or between the third transistor and the further transistor or between the first diode and the further transistor.14. A negative impedance converter as claimed in any preceding claim, wherein for each of the first and second transistors, there is provided a second biasing circuit connected across the collector or drain and the emitter or source of the transistor.15. A negative impedance converter as claimed in claim 14, wherein the second biasing circuit of each of the first and second transistors further comprises a second DC biasing signal source.16. A negative impedance converter as claimed in claim 14 or 15, wherein for each of the first and second transistors, the second biasing circuit is connected by way of an inductor to the collector or drain of the transistor, and wherein the emitter or source is connected to ground by way of a further inductor.17. A negative impedance converter as claimed in any preceding claim, further comprising an RF input port connected to the emitter or source of the first transistor, and an RF output port connected to the emitter or source of the second transistor.18. A negative impedance converter as claimed in claim 17, further comprising at least one capacitor connected in parallel between the RF input port and RF output port.19. A negative impedance converter as claimed in claim 18, further comprising at least one resistor connected in parallel with the at least one capacitor.20. A negative impedance converter as claimed in any preceding claim, wherein the collector or drain of the first transistor is connected to the base or gate of the second transistor, and the collector or drain of the second transistor is connected to the base or gate of the first transistor.21. A negative impedance converter as claimed in claim 20, wherein a predetermined impedance is provided between the respective collectors or drains on the one hand, and the respective bases or gates on the other hand, of the first and second transistors.22. A negative impedance converter as claimed in claim 21, wherein the predetermined impedance determines the negative impedance that the NIC applies to an RF signal passing between the input port and the output port.23. A negative impedance converter as claimed in claim 22, wherein the predetermined impedance is adjustable.24. A negative impedance converter as claimed in claim 23, wherein the predetermined impedance comprises a variable capacitor and/or a variable inductor.25. A negative impedance converter as claimed in claim 23 or 24, wherein the predetermined impedance is configured to be adjustable by way of a control input from a digital controller.26. A negative impedance converter substantially as hereinbefore described with reference to or as shown in Figures 8, 9 and 11 to 14 of the accompanying drawings.