| 摘要 |
Improved linearity in a line driver amplifier is realized by employing one or more amplifier sections each including an adaptive gain amplifier connected in parallel with a distributed gain amplifier. In an embodiment of the invention, first and second amplifier sections are connected in circuit relationship to an input and an output of the line driver amplifier to form a symmetrical amplifier configuration. In a specific embodiment of the invention, the adaptive gain amplifiers in each amplifier section are class AB type amplifiers and the distributed gain amplifiers in each amplifier section are class B amplifiers. Specifically, each of the distributed gain class B amplifiers each include a plurality of output transistors and a corresponding plurality of auxiliary amplifiers. The output transistor and auxiliary amplifier pairs are connected in parallel. Each of the auxiliary amplifiers includes a built in voltage offset, beginning with an auxiliary amplifier having the smallest voltage offset to the auxiliary amplifier having the largest offset. The individual auxiliary amplifiers maintain their corresponding output transistors in an OFF state so long as the overall amplifier input signal has a magnitude less than the auxiliary amplifier offset voltage level. When the input signal magnitude level is equal to or greater than the offset level of an auxiliary amplifier, that auxiliary amplifier turns its corresponding output transistor to an ON state. Thus, for lower magnitude input signals fewer than all of the output transistors are in an ON state, while for maximum magnitude input signals all of the output transistors are in an ON state. Use of the auxiliary amplifier-output transistor pairs allows use of smaller size transistors, each of which has a lower output current than an equivalent single output transistor. Since all of the output transistors in each amplifier stage are connected in parallel, the amplifier output current is the sum of the currents passing through the ON output transistors. Moreover, the adaptive gain class AB amplifier's gain varies as a function of input signal such that for larger input signals there is more gain and, consequently, less distortion. By employing the adaptive gain amplifiers and distributed gain amplifiers the line driver amplifier open loop gain characteristic is much smoother resulting in increased linearity.
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