GB2099634A

GB2099634A - Impedance matching circuits

Info

Publication number: GB2099634A
Application number: GB8115699A
Authority: GB
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1981-05-21
Filing date: 1981-05-21
Publication date: 1982-12-08
Also published as: GB2099634B

Abstract

An impedance matching circuit is provided for enabling an amplifier to feed a load, e.g. an antenna or a higher power amplifier, in a particularly efficient manner. It is often desired to match the output impedance of an amplifier to that of the load to which it is coupled, but this can result in the loss of power in an impedance matching circuit. In the present invention, two amplifiers 11, 12 receive the same input signal and after amplification the amplified signals are combined to provide a single output signal. The output impedance of one amplifier is equal to the reciprocal of the output impedance of the other amplifier. An output source impedance RS is coupled to the two amplifiers and it has a value equal to the nominal impedance of a load RL. However, when the impedance of the load equals that of the source impedance substantially no power is dissipated in the source impedance. Many loads have an impedance which varies in frequency and it is only at those frequencies for which the load impedance does not equal the source impedance that power is dissipated in the source impedance itself. <IMAGE>

Description

SPECIFICATION Impedance matching circuits This invention relates to impedance matching circuits and is, for example, applicable to broad band radio frequency amplifiers which drive radiating antennas or which drive higher power amplifiers. It is often desirable that the output source impedance of an amplifier is matched to its load, since if this is so, small variations in the load impedance about its nominal value causes minimal variations in power in the load thus enabling a reasonably flat frequency response to be achieved into a practical load which may be slightly mismatched at certain frequencies.

Known matching circuits are often wasteful of power, and in certain applications the power loss can be unacceptably large. The present invention seeks to provide an improved impedance matching circuit.

According to this invention, an impedance matching circuit for use with an external impedance of predetermined nominal value, includes two coherent signal paths connected in shunt with each other, and a dummy load having an impedance Rs, and wherein the impedance Rv of one signal path and the impedance R, of the other signal path are related to the impedance Rs is of the dummy load by RvxR=4Rs2.

The invention is further described by way of example with reference to the accompanying drawings, in which Figure 1 shows a known impedance matching circuit, Figure 2 is a diagram of an equivalent circuit, Figures 3, 4 and 5 illustrate the present invention in diagrammatic form, Figure 6 illustrates an impedance matching circuit in accordance with the present invention, and Figure 7 is an explanatory diagram.

Figure 1 shows the basic circuit elements of a transistor amplifier circuit from which the usually provided bias components have been omitted for the sake of clarity, and Figure 2 shows its equivalent circuit. For example, the load would be constituted by a broadcast antenna and the input signal applied to the amplifier would represent the radio frequency signal which is to be transmitted.

Underthe normal operating conditions, radio frequency power is dissipated in the source impedance Rs which acts as a dummy load of the amplifier and this equals the power dissipated in the load itself. Thus half of the available radio frequency power which is generated by the transistor amplifier is wasted in the impedance Rs of the dummy load. This reduces the efficiency by a factor of two, but a matched output impedance is achieved. An alternative technique which has been used to achieve a matched output impedance is that described in our U.K. patent number 1,088,251. This eliminates the power dissipated in the source resistance by the simultaneous use of both voltage and current feedback techniques, but some power is dissipated in the feedback resistors, and it suffers from reduced gain due to the increased feedback.

The principle of the present invention is illustrated in Figure 3 in which the input signal is fed in parallel to two amplifiers 1 and 2 having output source impedances, which are complementary, i.e. the output impedance of one is related to the reciprocal of the other. The outputs of the two amplifiers 1 and 2 are combined in a combiner circuit 3 and the resulting signals are fed to the two impendances R1 and R2. The load can be constituted by either impedance R, or R2 as determined by the relative phases of the voltage and current sources.

The gain of the two amplifiers is so proportioned that each delivers the same output power under matched conditions. Thus V2 V --i2R, from which -=R=R1=R2 R Although the two amplifiers 1 and 2 can be driven in phase or out of phase (delivering power to R1 or R2 respectively) it is preferred to drive them out of phase as the circuit then operates as a push-pull circuit enabling second harmonic cancellation to be achieved. The equivalent circuit which indicates these current and voltage sources, is illustrated in Figure 4 in which the voltage source represents one amplifier, and in which the current source represents the other amplifiers. A diagrammatic implementation of this circuit is shown in Figure 5 in which a hybrid transformer is used as the combiner circuit 3.In this case the load impedance is R, and the effective source impedance of the combined amplifiers is represented by the internal dummy load resistance 4Rs.

Although the invention is of general application, it can be implemented in a very simple manner when the output impedances associated with one amplifier is zero and that associated with the other is infinitely large, i.e. one amplifier presents a short circuit output and the other presents an open circuit output. It is under these circumstances that one amplifier can be represented as a voltage source and the other as a current source.

Figure 6 shows a practical implementation of this circuit in diagrammatic form, from which bias components have been omitted for clarity. A radio frequency input signal is received at terminals 6 and 7 and is coupled to two secondary windings 9 and 10 of a wide band transformer 8. The output from winding 9 is applied to a first transistor amplifying element 11, and the output of the winding 10 is applied to a second transistor amplifying element 12. Voltage feedback as represented by the path containing impedance Rf is applied to the element 11 and this effectively converts its operation to that of a voltage source.

Conversely, current feedback is applied to the other amplifying element 12 by means of a transformer 13 connected in a suitable feedback path.

The two secondary windings 9 and 10 of the transformer 8 are poled as shown (by the dot notation) so that the two transistor currents are driven in anti-phase-i.e. with a 1 800 phase difference-so that the two transistors are driven in a push-pull operation. The output point of the amplifying elements 11 is coupled directly to one end of the primary winding to a further wide band transformer 14, the other end of which is connected via the transformer 13 to the other amplifying element 12. The secondary winding of the transformer 1 4 drives the load impedance RL.

The primary winding of the transformer 14 consists of two input windings which are wound with a polarity indicated by the dot notation, and the mid-point of the primary winding is coupled via a dummy load impedance Rs to a reference potentiai 1 5.

When an input signal is applied to the transformer 8, via terminals 6, 7,the amplified output power is dissipated wholly in the load impedance RL, and no power is dissipated in the dummy load impedance Rs providing that the load impedance RL value is properly matched, i.e.

R,=4Rs. To achieve this it is also necessary that the effective gains of the two transistors with the feedback applied are correctly proportioned so that no power is dissipated in the dummy load Rs when the load RL is at its nominal value. However, when the load impedance is mismatched, power is dissipated in the dummy toad impedance Rs, and the amount of dissipated power is related to the degree of mismatch.

As previously mentioned, the gain of the two amplifiers is such that both deliver equal power when the load impedance has its nominal value.

This, taken together with the condition that no power is dissipated in the dummy load R5 when R,=4Rs, provides that the ratio of the voltage of the voltage source 11 to the current of the current source 12 is equal to 2Rs:gRL.

Although in Figure 6, the dummy load Rs actually has a true resistance of value Rs, it would be possible to use a dummy load of different value and to couple it via an extra transformer of appropriate turns ratio so that it exhibited an effective value Rs as required. The actual value of R, can tasso be charged by altering the primary/ secondary turns ratio of transformer 14.

It will be appreciated that the impedance matching circuit shown in Figure 6 can be so proportioned as to behave as a matched source without loss of power in any internal source resistance, thus giving improved frequency response when the circuit forms part of a practical radio frequency amplifying system. In order to achieve a given output power level, the total power developed by the two active amplifier devices is only half of that which would be required in a conventional amplifier of the kind shown in Figure 1. This enables reduced distortion levels to be achieved. It is obvious that a greatly improved efficiency is obtained when the load has its correct nominal value, since all of the power generated by the amplifier is transferred to the load.In practice, if the load is a radiating antenna, it is unlikeiy to be perfectly matched over the whole of its operating band, as the impedance of an antenna is somewhat frequency dependent in a manner which is not wholly predictable; In addition, the push-pull mode of operation shown in Figure 6 enables cancellation of even harmonics to be obtained although if this is not an important consideration the two transistor amplifying elements 11 and 12 could be driven in-phase. Furthermore, the form of voltage and current feedback illustrated in Figure 6 enables increased gain to be achieved as compared with certain known circuits; for example, as compared with those described in our earlier U.K. patent, number 1,088,251.

In Figure 6, the output impedance provided by one amplifying element is substantially zero and that provided by the other amplifying element is substantially infinitely great. This represents a special case of the more general relationship in which the two impedances are complementary, i.e. one is related to the reciprocal of the other by a constant.Figure 7 shows a generalised equivalent circuit which is derived from Figure 5 and in which the two generators are replaced by the equivalent impedances Rv and Rl. In general terms, the output impedances of the circuit shown in Figure 7 is given by: Rs(RV+Rj)+Rv R Zout= (1) Rs+1/4 (RV+R,) In practice the output impedance Zaut is equal to the nominal value of a load impedance R, For normal balanced hybrid conditions, Rv=Rj=2Rs and RvxRj=4Rs2 (2) The matched condition applies when the effective source resistance equals load resistance Substituting these values in (1) gives Zout=4Rs (3) Consider the case where Rv=kx2Rs and R1=1/kx2R9 (4) where k can be any value between 0 and 1 and may be Complex, i.e. k can have real and imaginary parts.

Combining these gives RVXRi=4Rs2 (5) Comparing this relationship with equation 2, it can be seen that the impedances Rv and Ri relative to their normalised value of 2Rs are complementary to each other. Thus in a practical 50Q system, ratios R,,"Rj could be 50/50Q, or 25/1 ooh or 0 and coQ. Combining the relationship of equation 4 and equation 1 , it can be shown, after simplification, that the output impedance Zout is still given by Zut=4Rs (6) and this is the same as equation 3.

Hence, providing the impedances Rv and R1 satisfy the relationships (4) and (5) (i.e.

normalised complementary) the output impedance of the overall circuit is independent of their nominal values. Thus Rv and R1 could be of equal value, complex (resistive and reactive), or even short circuit/open circuit, and still maintain a matched output impedance, which is the purpose of the circuit.

Although the general relationship is valid, the limiting conditions represented by the circuit implementation of Figure 6 are particularly convenient, since it is relatively easy to achieve short circuit and open circuits for impedances Rv and Ri respectively by means of appropriate feedback loops.

Claims

1. An impedance matching circuit for use with an external impedance of predetermined nominal value, including two signal paths connected in shunt with each other, and a dummy load having an impedance Rs, and wherein the impedance Rv of one signal path and the impedance Rl of the other signal path are relayed to the impedance R5 of the dummy load by RVXRl=4Rs2.

2. A circuit as claimed in claim 1 and wherein the impedance Rv is substantially zero, and the impedance R1 is infinitely great, so that one of said signal paths represents a short circuit and the other of said circuits represents an open circuit.

3. A circuit as claimed in claim 1 or 2 and wherein each signal path includes an amplifier having output impedances Rv and R1 respectively.

4. A circuit as claimed in claim 3 and wherein current feedback is applied to one amplifier so that it acts as a current source, and voltage feedback is applied to the other amplifier so that it acts as a voltage source.

5. A circuit as claimed in claim 4 and wherein the two amplifiers are connected in a push-pull configuration to minimise even harmonic distortion.

6. A circuit as claimed in claim 3, 4 or 5 and wherein the two amplifiers are arranged to operate so that no power is dissipated in the dummy load Rs when an external load RL having a value given by R,=4Rs is connected to the impedance matching circuit.

7. A circuit as claimed in Claim 3, 4, 5 or 6 and wherein equal power is provided by both amplifiers when an external load R, having a value given by R,=4Rs is connected to the impedance matching circuit.

8. A circuit as claimed in claim 7 and wherein the ratio of the voltage of the voltage source to the current of the current source is equal to the ratio 2Rs:wR, when RL=4Rs.

9. An impedance matching circuit substantially as illustrated in and described with reference to Figures 3, 4, 5, 6 or 7 of the accompanying drawings.

New Claims Filed on 23.4.82 New Claims:

10. A circuit as claimed in claim 1 and wherein a transformer is operative to connect both of the two said signal paths to the dummy load.

11. A circuit as claimed inany of claims 3 to 8 and wherein the outputs of the two amplifiers are connected together by means of a common transformer winding having a centre tap to which the dummy load is connected.

12. A circuit as claimed in claim 11 a load R, is connected to a secondary transformer winding which is inductively coupled to said common transformer winding.