Patent GB-1312357-A (GB)
Title: Power amplifier
IPC: H03F3/68
Filed: 1970-07-27
Abstract: 1312357 Amplifier circuits BENDIX CORP 27 July 1970 [26 Aug 1969] 36271/70 Heading H3T A power amplifying arrangement for driving a load 52, whose impedance is subject to variations, comprises a power amplifier including at least one stage 44, 46, 48 and means 36, 38, 42 driving and controlling an alternating current input signal to the stage so that it operates in a linear mode for a first level of the input signal to faithfully reproduce the input signal and in a saturation mode where the stage is operated into a saturated condition for a second higher level of the input signal such that saturation occurs for a portion of the cycle of the input signal which is less than the entire cycle and that the output power of the fundamental component of the output signal is maximized. In the transmitter arrangement shown in Fig. 5 a variable voltage 34 varies the output frequency of a voltage controlled oscillator 36. Variable control source 40 varies the amplitude of the output from amplitude modulator 38 and a driver amplifier 42 feeds three amplifiers 44, 46, 48 which drive a load such as an electroacoustic transducer 52. The output from the amplifiers 44, 46, 48 is a faithful reproduction of the input signal over the driving range from O to Vps, Fig. 5a, but beyond this the output waveform becomes clipped such that the amplifiers operate in the forced saturation region the amount of forced saturation being defined by the clipping angle #c. In a modification of the above including a single class B power amplifier (28, Fig. 4, not shown), for a sonar transmitter, timing signals are supplied from a timer (10) such as a counter or master oscillator, to a variable frequency oscillator (12), a wave shaper (14) and a pulse width generator (16). The output carrier frequency of the oscillator (12) may be switched by (18) and the modulating input to the modulator (20) may be selected by a switch (24) from output signals from the wave shaper (14). The pulse width generator (16) supplies an input to the wave shaper (14) which may be used to control the duration of each transmitted pulse at the selected carrier frequency. The power amplifier (26) is driven in the linear region (Fig. 4a, not shown) and in the forced saturation region. In a further embodiment (Fig. 1, not shown) a class B push-pull transistor power amplifier (10) may be driven in the linear and forced saturation mode by adjusting the input signal amplitude via a potentiometer (16). The amplifier may be used for underwater telephony and echo ranging. Alternative arrangements for class B pushpull transistor amplifiers which may be operated in the linear and forced saturation mode are disclosed (Figs. 6, 7, and 8, not shown). The amplifiers described in (Fig. 8, not shown) and in Fig. 9 may be used in the Fig. 5 arrangement and and provide Class B full bridge operation such that during each half cycle diagonally arranged transistors conduct. A bias regulator amplifier 108 maintains the voltage level at the centre tap of the output transformer 116 at ¢ Vps. The amplifier may be protected against damage due to short circuit at or upstream of the driver by connecting oppositely poled Zener diodes (136, 138), (Fig. 10, not shown), in series between the output and input of the driver amplifier so that they operate to limit the driver output clipping each half cycle at the Zener break down level. The amplifier may also be useful in radar applications.