Amplifiers – With plural amplifier channels – Redundant amplifier circuits
Patent
1996-09-12
1998-10-27
Mottola, Steven
Amplifiers
With plural amplifier channels
Redundant amplifier circuits
330 84, H03F 368
Patent
active
058282670
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to an amplifier which generates an output voltage which is proportional to an input voltage and can be tapped between the outputs of two push-pull output stages and has peak-to-peak values of as much as twice the supply voltage.
b) Description of the Related Art
In order to modulate the light intensities of lasers, voltages of several hundred volts are required to drive a substantially capacitive load of 100 pF. When lasers are used in video technology to illuminate a television screen, an amplifier driving the modulator must have a wide bandwidth of up to 30 MHz in addition to the required high output voltage.
Conventional amplifiers using discrete semiconductor components are known in various circuit arrangements. For instance, semiconductor amplifiers are commonly used as cascode amplifiers, transimpedance amplifiers and differential amplifiers. However, these amplifiers are limited with respect to the output voltage and bandwidth which can be achieved.
On the other hand, tube amplifiers having a large bandwidth and also sufficiently high output voltages are available. However, the technical cost is also proportionately high. In addition, high-quality amplifiers also require considerable space to accommodate the tubes and cooling means for the latter, so that this type of amplifier is not suitable for home video equipment with laser sources. In particular, the space requirement in color video equipment in which three lasers of different colors must be driven in order to mix the colors to be shown is not practical.
Thus, for a commercial color video device it is only practical to use semiconductor amplifiers. However, currently available transistors with breakdown voltages greater than 300 V cannot be used to drive the modulators directly, since the parasitic capacitances limit the limiting frequency to approximately 15 MHz as is indicated, for example, in the book "Transistoren-Kurztabelle" by Hans Guinther Steidle, Franzis-Verlag, Munich, 1989, and similar tables. An additional problem is caused by the required rupture capacity, since the impedance at 30 MHz and 100 pF is less than 100 .OMEGA..
On the other hand, there are enough known transistor types with limiting frequencies of more than 100 MHz for breakdown voltage greater than 150 V and less than 300 V, such as the 2SA639S or 2SA 913, both of which have limiting frequencies of 130 MHz. The latter transistor can even handle power losses of 15 W. However, because of the required output voltage of greater than 150 V, the modulators cannot be driven directly by these transistors.
A problem of a similar nature occurred with air-core or transformerless low-frequency amplifiers in the sixties, when transistors were available with high output but with breakdown voltages of only up to 30 V. This was remedied by switching the load between two push-pull output stages, one of which was driven in opposite phase to the other. In this way, it was possible to achieve double the voltage swing given by the supply voltage.
Such amplifiers with two push-pull output stages in which the load is operated between the outputs are known, e.g., from DE-AS 20 61 943 and DE-OS 24 31 485. DE-AS 20 61 943 describes a differential amplifier in which the negative branch of the first push-pull output stage is connected with the positive branch of the second push-pull output stage and vice versa. DE-OS 24 31 485 shows circuits in a number of embodiment examples in which the polarity reversal of one push-pull output stage is produced for the other push-pull output stage by means of an inverting amplifier.
For high frequencies, however, the use of such amplifiers is limited, since the capacitance at the point where the input of the amplifier is connected is increased and the limiting frequency is accordingly reduced owing to the parallel connection of the amplifier needed to reverse polarity. This problem could be bypassed by driving at suitably low impedance at this point. However, this unnecessarily i
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Mottola Steven
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