AMPLIFIER WITH OUTPUT TRANSFORMER

Abstract: Amplifiers are known and are for example used in Public Address Systems to send audio signals over long distances. To improve the amplifier the invention proposes a amplifier of the push-pull type with a first and a second switch and an output transformer with a primary winding coupled to the switches and a secondary winding coupled to the output of the push-pull amplifier and which output transformer comprises a further secondary winding coupled to a comparator as a feedback loop.

Description: Amplifier with output transformer The invention relates to an amplifier with an output transformer. The invention further relates to an output stage for use in such an amplifier. The invention further relates to a public address system comprising such an amplifier. Amplifiers are known in the art and are for example, used as so-called PA amplifiers (Public Address amplifiers) in a Public Address System to send audio signals over long distances, for several different uses such as fire indicating installations, call installations etc. Such a system receives at an input an audio signal and the PA amplifier supplies via an output transformer an audio signal with for example, an output voltage of 100 V rms to speakers with an input transformer over long distances. The known amplifier use a so-called H-bridge or Half H-bridge with respectively four or two FETs. A disadvantage of these known amplifiers is that the efficiency is less than 70%. A further disadvantage of these known amplifiers is that the output behaviour in relation to noise is not good. An object of the invention is to provide an amplifier not having the disadvantages of the prior art amplifiers. To this end an amplifier according to the invention comprises the features of claim 1. The amplifier according to the invention is of the so-called push-pull type. Further the amplifier comprises an output transformer which is required in a lot of Public Address Systems because the user requires a galvanic separation. An extra secondary winding is used as a feedback connection. In this way a push-pull amplifier is obtained having a much higher efficiency than the prior art amplifiers and using a feedback loop for obtaining a stable amplifier. Embodiments of the invention are described in the dependent claims. These and other aspects of the invention will be apparent from and elucidated with reference to the examples described hereinafter, herein shows The Figure an example of a push-pull amplifier according to the invention. The Figure shows block schematically an example of a push-pull amplifier PPA according to the invention. Push-pull amplifiers are for example, used as so-called PA amplifiers (Public Address amplifiers) in a Public Address System to send audio signals over long distances, for several different uses such as fire indicating installations, call installations etc. Such a system receives at an input an audio signal and the PA amplifier supplies via an output transformer an audio signal with for example, an output voltage of 100 Vr m s to loudspeakers each having an input transformer over long distances. At an input I the push-pull amplifier PPA receives an audio signal ias after filtering in an input filter IF, the signal is supplied to an input of a subtractor SUB. The output of the subtractor is coupled via a control circuit to a comparator COM which receives at the other input a signal from a triangle oscillator OSC. The comparator supplies at it's output a pulse width modulated signal pwms. The comparator is coupled with it's output to an output stage OS comprising an output driver OD for supplying drive signals to switches SI and S2. The switches comprise besides the "real" switches anti-parallel each a diode Dl, respectively D2. Each switch is in this example coupled to a winding L of a low pass filter LDF. The low pass filter further comprises in this example coupled to each winding a capacitor Cl. The connection points of each winding L and capacitor C is coupled to an output transformer OT, whereby each connection point is coupled to one side of the primary winding PW. The middle of the primary winding is coupled to a positive voltage V+. The secondary winding SW of the output transformer OT is coupled via a band block filter BBF to the first and second output 01, respectively 02 of the push-pull amplifier PPA, for supplying an output audio signal to loudspeakers (not shown). The output transformer OT further comprises a feedback secondary winding FSW for supplying a feedback signal fs via a feedback control FC to the other input of the subtractor SUB. The output stage comprises in this example the output driver OD, the switches SI and S2, the low pass filter LPF, the output transformer OT, and the band block filter BBF. The low pass filter LPF coupled between the switches SI, S2 and the output transformer OT improves the efficiency of the push-pull amplifier even further. The band block filter BBF between the output transformer OT and the outputs of the push-pull amplifier 01 , 02 improves the output behaviour of the push-pull amplifier. The input filter IF of the push-pull amplifier improves the characteristics of the push-pull amplifier. It is to be noticed here that a push-pull amplifier according to the invention has been described above on the basis of one example but that the man skilled in the art is well aware of a lot of other implementations within the scope of the invention. For example the low pass filter can be amended. Further the band block filter can be changed from the shown example but resulting in the same or corresponding results. The push-pull amplifier according to the invention can also be used in other applications than the Public Address System. The output stage OT can also be used in combination with other push-pull amplifiers than the one described above.

Claims: CLAIMS: 1. Push-pull amplifier comprising an input for receiving an input signal, an output for supplying an output signal, a comparator for converting the input signal into a pulse with modulated signal and a push-pull output stage comprising a first and a second switch and an output transformer with a primary winding coupled to the switches and a secondary winding coupled to the output of the push-pull amplifier and which output transformer comprises a further secondary winding coupled to the comparator as a feedback loop. 2. Push-pull amplifier as claimed in claim 1 characterized in that the output transformer is coupled via a band block filter to the output of the push-pull amplifier. 3. Push-pull amplifier as claimed in claim 1 characterized in that the push-pull amplifier comprises a low pass filter between the switches and the output transformer. 4. Output stage for use in a push-pull amplifier as claimed in claim 1. 5. Public address system comprising loudspeakers each with an input transformer and further comprising a push-pull amplifier as claimed in claim 1.

Inventors: MAASKANT ROB, NUIJTEN CORNELIS P, MAASKANT, ROB, NUIJTEN, CORNELIS, P.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2001
Main classification: H03F3
IPC codes: H03F3