Amplifiers – With semiconductor amplifying device – Including differential amplifier
Patent
1995-06-07
1996-11-05
Mottola, Steven
Amplifiers
With semiconductor amplifying device
Including differential amplifier
327346, H03F 345, H03G 330
Patent
active
055721664
ABSTRACT:
A gain control circuit provides linear-in-decibel gain control for an RF signal variable gain amplifier. The gain control circuit utilizes the transconductance characteristics of bipolar transistors to generate a logarithmic relationship between a gain control current and an amplifier bias current. The gain control circuit comprises essentially a current mirror having two transistors with a resistor coupled between the associated base terminals of the two transistors. A third transistor and a resistor are also provided to absorb the gain control current. The gain control current is applied to the base of a first one of the two transistors and a voltage is thereby established across the resistor. This voltage subtracts from the base-to-emitter voltage of the second transistor thereby producing a corresponding exponential reduction in the current through the second transistor. This current is provided to a gm stage, whose gain is linearly proportional to this current. Thus, a linear change in the gain control current produces an exponential change in the gain of the gm stage. Accordingly, a linear-in-dB variable gain amplifier is achieved.
REFERENCES:
patent: 4621238 (1986-11-01), Fenk
patent: 4990803 (1991-02-01), Gilbert
patent: 5442311 (1995-08-01), Trafton
Analog Devices Inc.
Mottola Steven
LandOfFree
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