US 5,572,166 A · Linear-in-decibel variable gain amplifier

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.

Claims30

Dependent claims

A linear-in-dB gain amplifier comprising:

an amplifier circuit having a gain that is proportional to an amplifier bias current;
and
a gain control circuit having an input terminal for receiving a gain control current and an output terminal coupled to the amplifier circuit for providing the amplifier bias current, the amplifier bias current varying exponentially in response to linear changes in the gain control current thereby producing an exactly linear-in-decibel relationship between the gain control current and the amplifier circuit gain over the entire range of the gain control current.

A linear-in-dB gain amplifier system comprising:

an amplifier circuit having a gain that is proportional to an amplifier bias current;
and
a gain control circuit including:
a first transistor having a control terminal for receiving a gain control current and being biased to an operating point,
a second transistor having a control terminal and an input terminal coupled to the amplifier circuit for providing the amplifier bias current thereto, and
a resistor coupled between the control terminals of the first and second transistors so that the gain control current passes through the resistor,
the voltage across the resistor due to a linear change in the gain control current producing an exponential change in the amplifier bias current produced by the second transistor thereby producing a linear-in-decibel relationship between the gain control current and the amplifier circuit gain.

4 more elements

A linear-in-dB gain amplifier comprising:

a differential pair of transistors having a pair of input terminals for receiving an input signal, a pair of output terminals for providing an output signal, and a common node for receiving an amplifier bias current;
a first transistor having a control terminal for receiving a gain control current, an input terminal for receiving a reference current, and an output terminal coupled to a first supply voltage terminal;
a second transistor having a control terminal, an input terminal coupled to the common node of the differential pair for providing the amplifier bias current, and an output terminal coupled to the first supply voltage terminal;
a first resistor coupled between the control terminals of the first and second transistors,
a third transistor having a control terminal coupled to the input terminal of the first transistor, an input terminal coupled to a second supply voltage terminal, and an output terminal to the control terminal of the second transistor;
a second resistor coupled between output terminal of the third transistor and the first supply voltage terminal;
the amplifier bias current varying exponentially in response to linear changes in the gain control current thereby producing a linear-in-decibel relationship between the gain control current and the amplifier circuit gain.

4 more elements

Dependencies are inferred from the claim text.

Linear-in-decibel variable gain amplifier

Life of this patent

Linear-in-decibel variable gain amplifier

Life of this patent

Abstract

Claims30