GB1604669A - Electrical amplifier arrangements - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRICAL AMPLIFIER ARRANGEMENTS (71) We SPT COMMUNICATIONS LIM ITED, of Grainger Road, Southend-on-Sea, Essex SS2 500, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to electrical amplifier arrangements.

Conventionally, when an amplifier circuit is required to be capable of providing an output at a plurality of discrete frequencies within a range of frequencies, the circuit comprises one or more tuning elements, such as a variable capacitor or inductor. Each time the circuit is required to operate at a different frequency these elements have to be adjusted and thereby tuned to the new frequency. If the adjustments are carried out manually, they are time-consuming, and if automatic operation is required, for use in coastal radio transmitters, for example, then additional components, such as relays, contactors or switches, and servo-mechanisms, are required. The additional components add cost and weight to the amplifier arrangement, and also complexity, which introduces an increased risk of equipment failure.Furthermore, the above-mentioned additional components usually have a short lifetime compared with the other electrical components of a typical amplifier circuit and associated equipment, thus further decreasing its reliability. Also, the switching of tuning elements reduces their lifetime in comparison with their operation at a fixed frequency, and this is particularly so at high voltages.

In accordance with one aspect of the present invention, there is provided an amplifier comprising a thermionic valve, a choke connected directly or indirectly to the anode of the valve to conduct input signals to the anode, and a wide-band lumpedelement impedance-transforming network coupled to the output side of the valve and connected to a load, the network comprising a plurality of stages arranged as a ladder configuration of series-connected inductances and shunt capacitances.

In accordance with another aspect of the present invention, there is provided an aanpli- fier comprising two thermionic valves whose cathodes are arranged to be at a common reference potential, two wide, band lumped element impedance-transforming networks connected between respective valve anodes and a terminal at said reference potential, and a choke connected directly or indirectly to each valve anode, each network comprising a plurality of stages, arranged as a ladder configuration of series-connected inductances and shunt capacitances.

An amplifier in accordance with the present invention thereby provides an output over a broad band of frequencies. Thus, operation at a plurality of frequencies within the band can take place without the necessity for retuning of circuit elements.

The invention is especially applicable to amb plifier arrangements operating in the low frequency to microwave range of radio frequencies.

The invention finds particular, though not exclusive, application in the power amplifier output stage of an RF transmitter, whereby an infinite number of frequencies in a whole frequency band can be covered without the need for any retuning. This in contrast to known RF transmitters, in which retuning is required throughout the band and then only a small finite number of channels may conveniently be used. Although such transmitters may be manually-operable, as on board ship for example, there are other applications, for example at coastal transmitting stations, which require automatic operation. A long lifetime of components of the coastal transmitter is of particular importance.

Several embodiments of amplifier arrangement in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 shows a single ended output stage of a power amplifier for an RF transmitter; Figure 2 shows a modification of part of the circuit of Figure 1; Figure 3 shows a push-pull output stage of an RF transmitter; Figure 4 shows a modification of part of the circuit of Figure 3; and Figures 5 and 6 show two embodiments of amplifier arrangement having transformer coup ling between the valve and the filter.

Figure 1 shows one form of power amplifier output stage for an RF transmitter that is arranged to operate in the marine frequency waveband of 400-535 kHz, having a power output of about 5 kilowatts. However, it is to be understood that an amplifier arrangement of the present invention may be employed in transmitters covering the entire RF spectrum from low frequencies to microwaves, and operating at any suitable output power. The transmitter employs a thermionic valve VI, which is operated in class AB. Tie valve V1 is connected to the load, indicated by impedance Tri, by a combined matching and tuning network so as to provide a broad band transmission characteristic.The matching and tuning network comprises a lumped element impedance-transforming network having low passband ripple and low insertion loss.

The input impedance to the circuit of Figure 1 is typically 1500 ohms, and the output impedance is typically 50 ohms; the network is arranged to take into account impedance ratios of this magnitude.

Input voltage to the valve V1 is made via a feed choke L, which is aperiodic or critically resonated with its stray capacitance, depending on passband and impedance ratios. The valve V1 is coupled to the matching and tuning network by a coupling capacitor CC in series with the choke L. The coupling capacitor CC is then connected to a plurality of filter stages comprising seriesconnected indicators L1 ... LN, and shunt capacitors CT1, CT2 . . CTN arranged in a ladder configuration.

It will be appreciated that the number of axial cable extending to an antenna, or it may be an intervening antenna tuning unit.

It will be appreciated that the number of stages required for the network filter, and the values of the inductance and capacitance components thereof will be chosen in dependence on the requirements of the transmitter. In particular, important criteria for determining the particular network to be used are the centre frequency of the transmission band employed, the passband required, the amount of deviation (i.e.

ripple) that can be accepted, the required attenuation of the harmonic frequencies, and the ratio of the input to output impedance. For a three stage filter, for example, the attenuation of the second harmonic is at least 50 dB. The network can be arranged such that the transmitter output stage covers a passband of at least half an octave with passband ripple of less than 0.2 dB. A larger number of filter stages than actually required to meet these criteria can be employed, so as to provide an inherent low pass filter characteristic whilst retaining the advantage of having low loaded Q in the stages. Thus, any circulating current and power loss are maintained at low values, which allows the use of low cost inductors and capacitors in the filtering network.

The low loaded Q circuitry also leads to a reduction in the physical size of the amplifier, and the low insertion loss results in a higher operating efficiency than is the case with a conventional tunable network.

The filter network can be applied to transmitters covering the whole RF spectrum from low frequencies to microwave frequencies, and particularly up to 30 MHz, in lumped or semilumped element form, the upper frequency limit being determined by circuit stray capacitance and inductance.

The circuit of Figure 1 may be modified in accordance with the particular requirements, so that, for example, critical resonance of the choke L can be obtained by an additional cap acitance added to supplement the stray capacitance of the choke. Furthermore, the resonance point may be made dependent on the passband and the ratio of the input/output impedance required for the amplifier, and also on the required passband ripple.

It will he appreciated that the characteristics of the feed choke have to be determined in accordance with its relationship to the anode of the valve V1.

Figure 3 shows an alternative power amplifier circuit for a radio frequency transmitter. In this case, push-pull amplifiers are used to provide a balanced configuration. It will be appreciated that the arrangement comprises the circuitry of Figure 1 together with its mirror image about the earthed conductor, and primed references are used in Figure 3 for the mirror circuitry. The outputs of the two halves of the network are combined in the common antenna load R1', which is connected to the final stage inductors Ln, Ln' by balanced coaxial cables.

Figure 2 shows an alternative load arrangement for connection to the final stage filter LN, CTN of Figure 1. A wide band transformer T1 is employed to cope with varying load impedance from then nominal design impedance of the amplifier arrangement. The transformer T1 can also be used for reducing the effective impedance ratio from the anode of the valve Vitro the load, thus bringing about a simplification of the filter or extending the capability of the filter with regard to the impedance ratio it can handle. As shown, a load R1" is tapped from the output winding on the transmormer T1. As discussed above, the load may be an antenna or an antenna-tuning unit.

As a further modification, the wide band transformer T1 may be inserted at the input end of the filter arrangement, that is to say, between, on the one hand, the junction of capacitor CC and inductor Ll and, on the other hand, the earth conductor.

Figure 4 shows a modification of Figure 3, in a way corresponding to the modification of the arrangement of Figure 1 by Figure 2, so as to use a wide band transformer Tri'. With this modification, it will be appreciated that the combining of the output from the two filter stages will take place in the transformer Ti' before being passed on to the antenna or antenna tuning unit comprising the load Tri"'.

In further embodiments of the invention, a pair of push-pull amplifiers are coupled by a transformer into the LC filter network of Figure 1, and such arrangements find particular application where the load is an unbalanced coaxial transmission cable. Circuit diagrams for two such embodiments are shown in Figures 5 and 6.

In the amplifier arrangement of Figure 5 a push-pull operation is obtained from two thermionic valves V1 and V1' which have their anodes interconnected by a balanced primary winding W1 of a transformer T2. The input signal to the anodes of valves Vl and Vl' is supplied through a choke L connected to the mid point of the transformer primary winding W1.

An unbalanced secondary winding W2 of the transformer T2 is connected across a capacitor C5, which forms the capacitance element of the first stage of a single-ended filter network as described with reference to Figure 1. The filter network of Figure 5 transforms an essentially resistive load R.

Figure 6 shows an amplifier arrangement that is a modification of that of Figure 5. Referring to Figure 6, input signals are fed separately to feed chokes connected to the anodes of thermionic valves V1 and Vi' respectively. The valve anodes are connected by respective capacitors C6 and C6' to a balanced primary winding W3 of a transformer T3, which has an unbalanced secondary winding W4 connected to a filter network as described with reference to Figure 6. The use of capacitors C6 and C6' prevents any direct current entering the transformer primary winding W3.

It will be appreciated that an amplifier arrangement of the invention, when applied to a ratio transmitter, may be used at a different stage of the transmitter from that described above, for example in a lower power stage provide a wide band input to the succeeding stage, whose input impedance then serves as the load of the amplifier arrangement.

Although the thermionic valves of the embodiments described are arranged to operate in class AB, it will be appreciated that the invention can be embodied in amplifier circuit arrangements where the thermionic valve is used in any class of operation.

WHAT WE CLAIM IS: 1. An amplifier comprising a thermionic valve, a choke connected directly or indirectly to the anode of the valve to conduct input signals to the anode, and a wide-band lumpedelement impedance-transforming network coupled to the output side of the valve and connected to a load, the network comprising a plurality of stages arranged as a ladder configuration of series-connected inductances and shunt capacitances.

2. An amplifier according to Claim 1, in which the choke is connected to a tapping point of the primary winding of a transformer, the winding being coupled between the anode of said thermionic valve and the anode of a further thermionic valve, and wherein the secondary winding of the transformer is coupled in parallel with the first capacitance of the wide-band network.

3. An amplifier according to Claim 1, com- prising a further thermionic valve with choke, the anodes of the thermionic valves being coupled together by the primary winding of a transformer, the secondary winding of which is coupled in parallel with the first capacitance of the wide-band network.

4. An amplifier comprising two thermionic valves whose cathodes are arranged to be at a common reference potential, two wide-band lumped-element impedance-transforming networks connected between respective valve anodes and a terminal at said reference potenial, and a choke connected directly or indirectly to each valve anode, each network comprising a plurality of stages arranged as a ladder configuration of series-connected inductances and shunt capacitances.

5. An amplifier according to Claim 1 or 5, wherein the anode of the or each valve is coupled to the or each network by a capacitor.

6. An amplifier according to any preceding claim, wherein the or each network is connected directly to a load of the amplifier.

7. An amplifier according to any of Claims 1 to 5, comprising a transformer arranged to couple the or each network to a load of the amplifier.

8. An amplifier substantially as hereinbefore described with reference to any one of the Figures of the accompanying drawings.

9. A radio transmitter compirsing an amplifier as claimed in any preceding claim.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. stages will take place in the transformer Ti' before being passed on to the antenna or antenna tuning unit comprising the load Tri"'. In further embodiments of the invention, a pair of push-pull amplifiers are coupled by a transformer into the LC filter network of Figure 1, and such arrangements find particular application where the load is an unbalanced coaxial transmission cable. Circuit diagrams for two such embodiments are shown in Figures 5 and 6. In the amplifier arrangement of Figure 5 a push-pull operation is obtained from two thermionic valves V1 and V1' which have their anodes interconnected by a balanced primary winding W1 of a transformer T2. The input signal to the anodes of valves Vl and Vl' is supplied through a choke L connected to the mid point of the transformer primary winding W1. An unbalanced secondary winding W2 of the transformer T2 is connected across a capacitor C5, which forms the capacitance element of the first stage of a single-ended filter network as described with reference to Figure 1. The filter network of Figure 5 transforms an essentially resistive load R. Figure 6 shows an amplifier arrangement that is a modification of that of Figure 5. Referring to Figure 6, input signals are fed separately to feed chokes connected to the anodes of thermionic valves V1 and Vi' respectively. The valve anodes are connected by respective capacitors C6 and C6' to a balanced primary winding W3 of a transformer T3, which has an unbalanced secondary winding W4 connected to a filter network as described with reference to Figure 6. The use of capacitors C6 and C6' prevents any direct current entering the transformer primary winding W3. It will be appreciated that an amplifier arrangement of the invention, when applied to a ratio transmitter, may be used at a different stage of the transmitter from that described above, for example in a lower power stage provide a wide band input to the succeeding stage, whose input impedance then serves as the load of the amplifier arrangement. Although the thermionic valves of the embodiments described are arranged to operate in class AB, it will be appreciated that the invention can be embodied in amplifier circuit arrangements where the thermionic valve is used in any class of operation. WHAT WE CLAIM IS:

1. An amplifier comprising a thermionic valve, a choke connected directly or indirectly to the anode of the valve to conduct input signals to the anode, and a wide-band lumpedelement impedance-transforming network coupled to the output side of the valve and connected to a load, the network comprising a plurality of stages arranged as a ladder configuration of series-connected inductances and shunt capacitances.

2. An amplifier according to Claim 1, in which the choke is connected to a tapping point of the primary winding of a transformer, the winding being coupled between the anode of said thermionic valve and the anode of a further thermionic valve, and wherein the secondary winding of the transformer is coupled in parallel with the first capacitance of the wide-band network.

3. An amplifier according to Claim 1, com- prising a further thermionic valve with choke, the anodes of the thermionic valves being coupled together by the primary winding of a transformer, the secondary winding of which is coupled in parallel with the first capacitance of the wide-band network.

4. An amplifier comprising two thermionic valves whose cathodes are arranged to be at a common reference potential, two wide-band lumped-element impedance-transforming networks connected between respective valve anodes and a terminal at said reference potenial, and a choke connected directly or indirectly to each valve anode, each network comprising a plurality of stages arranged as a ladder configuration of series-connected inductances and shunt capacitances.

5. An amplifier according to Claim 1 or 5, wherein the anode of the or each valve is coupled to the or each network by a capacitor.

6. An amplifier according to any preceding claim, wherein the or each network is connected directly to a load of the amplifier.

7. An amplifier according to any of Claims 1 to 5, comprising a transformer arranged to couple the or each network to a load of the amplifier.

8. An amplifier substantially as hereinbefore described with reference to any one of the Figures of the accompanying drawings.

9. A radio transmitter compirsing an amplifier as claimed in any preceding claim.