Amplifiers – With semiconductor amplifying device – Including differential amplifier
Patent
1998-01-29
2000-07-04
Mottola, Steven J.
Amplifiers
With semiconductor amplifying device
Including differential amplifier
327359, H03F 345
Patent
active
060844728
ABSTRACT:
A biasing scheme for a multi-tanh amplifier improves the dynamic range of the amplifier by utilizing emitter degeneration resistors to reduce uncorrelated noise contributed by current source transistors used to bias the multi-tanh core. The current source transistors form part of a current mirror which can be coupled to a linear-in-dB cell through another current mirror to provide linear-in-dB gain control. An optimal version of the biasing scheme for a multi-tanh triplet minimizes noise at high gain while maximizing linearity and input signal range at low gain by varying both the absolute and relative magnitudes of the bias currents for the triplet core, thereby varying the shape of the transconductance function. The variable bias currents are provided by a multiple output current mirror in which the emitter of the center mirror transistor is connected directly to power supply ground, while the outer mirror transistors include degeneration resistors in their emitter paths. The bias current to the center pair is varied exponentially in response to linear changes in a control signal so as to provide linear-in-dB gain control. The bias currents to the outer differential pairs are increased to a lesser extent than the center pair due to the back emf across the degeneration resistors, and thus, the outer pairs are effectively debiased at high gain, thereby eliminating their contribution to the noise of the amplifier. At low gain, the back emf across the degeneration resistors is negligible, so the linearity of the triplet is maximized to handle large input signals.
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Gilbert, Barrie; "Design Considerations for BJT Active Mixers Rev. 3.0"; Jun. 18, 1995; pp. 62-76.
Analog Devices Inc.
Mottola Steven J.
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