Patent GB-1312014-A (GB)
Title: Power amplifier including plurality of push-pull amplifier sections coupled by ferrrite matching transformers
IPC: H03F3/26
Filed: 1971-04-19
Abstract: 1312014 Transistor amplifying circuits MOTOROLA Inc 19 April 1971 [24 Feb 1970 (2)] 21774/71 Heading H3T A power amplifier for operation in a frequency range above 25 MHz includes a plurality of push-pull transistor amplifiers with their input and output circuits connected in parallel, being coupled by means of wideband transformers having ferrite cores and closely coupled primary and secondary windings. The transformers isolate the amplifiers from one another for D.C. and hold the signals in phase, and the corresponding windings in one set of transformers are connected in series with one another and with a variable tuning reactance. In one embodiment, operating over the frequency range 150-174 MHz, exciter 10, Fig. 1, feeds via preliminary amplifier 11 into an amplifier transistor 20, the collector of which is connected to negative supply over R.F. choke 21 and damping resistor 22 and feeds into the primary of transformer 24, which is tuned by means of capacitors 25, 26. Transformer 24 has pairs of seriesconnected primary and secondary windings with close coupling, and drives a further stage comprising transistors 30, 31 in push-pull: capacitors 33, 34 balance the drive and R.F. choke 35 provides for base bias. The collector circuit of transistors 30, 31 comprises balancing capacitors 39 and R.F. chokes 37, 38 for D.C. supply; two push-pull output amplifiers are driven by means of transformers 40, 41, each of which has two parallel-connected primary windings, the two pairs being connected in series and series resonated by means of variable capacitor 42. Each transformer 40, 41 has a pair of parallel-connected secondary windings tightly coupled to the primary windings. Transformer 40 drives a push-pull output amplifier comprising transistors 44, 45, capacitors 49, 50 being provided for balancing the drive and R.F. choke 47 and resistor 46 providing D.C. base bias. The collector circuit of transistors 44, 45 includes balancing capacitors 56-59 and R.F. choke 52 for D.C. supply; it feeds into output transformer 54, which has three parallelconnected primary windings. The transformer 41 drives a second amplifier similar to the first output amplifier, comprising transistors 60, 61 operating into output transformer 64. The primaries and secondaries in each of the output transformers 54, 64 are tightly coupled, the secondaries 55, 65 being connected in parallel via capacitors 67,69, and feed via filter 13 into a switch 14 for connection to aerial 15. A second embodiment, operating over the frequency range 25-5 MHz, includes a driver stage comprising transistor 85 the base circuit of which consists of R.F. choke 88; D.C. is applied to the collector over R.F. choke 91 and L.F. choke 90. Negative feedback is applied between collector and base via resistor 95 and capacitor 96, and the collector feeds via seriestuned circuit 98, 99 into the parallel-connected primaries of transformers 100, 101, the primaries and secondaries of which are tightly coupled. Transistor 100 drives a push-pull output amplifier comprising transistors 105, 106 in which base bias is derived over resistor 108 and R.F. choke 109. The collector circuit is balanced by means of capacitors 112, 113, and negative feedback is applied between collector and base of transistor 105 by means of resistor 110 and capacitor 111. D.C. is supplied to the collectors over R.F. choke 116; the collector circuit comprises two parallel-connected primary windings of output transformer 115, the primaries and secondary of which are tightly coupled. Transformer 101 drives a second output amplifier, comprising transistors 120, 121, similar to the first. The secondaries of transformers 115, 125 are connected in series and in series with inductor 129 and variable tuning capacitor 128, and feed aerial 82 via filter 80 and switch 81.