Impedance matched variable gain low noise amplifier using shunt feed-back

• US 20080068076A1
• Filed: 11/20/2007
• Published: 03/20/2008
• Est. Priority Date: 12/13/2004
• Status: Active Grant

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Broadcom Corporation (Broadcom, Inc.)
• Inventors: Manstretta, Danilo
• Granted Patent: US 7,724,085 B2
• Time in Patent Office: Days
• US Class Current: 330/85
• CPC Class Codes: H03F 1/34 Negative-feedback-circuit arrangements with or without positive feedback H03F1/02 - H03F1/30, H03F1/38 - H03F1/50, H03F3/50 take precedence ; for rejection of common mode signals H03F3/45479
• CPC Class Codes: H03F 1/56 Modifications of input or output impedances, not otherwise provided for
• CPC Class Codes: H03F 3/45475 using IC blocks as the active amplifying circuit

Abstract

The variable gain amplifier includes a forward path that provides the amplifier variable gain, and a feedback path. The feedback path uses a switch that is turned on at low gain levels. The switch taps into the feedback resistor, shunting it to signal-ground and eliminating the feedback mechanism. This ensures that the input impedance seen at the input port does not grow excessively, using part of the feedback resistor as a passive termination at low gain levels. In this way variable gain ranges in excess of 30 dB can be achieved.

Claims

1. An amplifier, comprising:

a forward amplifier having an input and an output;

a feedback path coupled across said input and output, including,

a feedback amplifier coupled to an impedance; and

wherein a signal output of said feedback amplifier is shunted to ground for low gain applications.

2. The amplifier of claim 1, wherein said impedance includes a first component impedance and a second component impedance, further comprising a switch inserted between said first component impedance and said second component impedance, said switch providing a connection to virtual ground when closed.

3. The amplifier of claim 2, wherein said feedback path is differential, and said switch is closed for low gain applications, and shorts a positive component to a negative component of said differential feedback path for low gain applications.

4. The amplifier of claim 3, wherein said switch is open for high gain applications, enabling a feedback signal corresponding to said feedback path to reach said input.

5. A variable gain differential amplifier, comprising:

a differential forward amplifier having a differential input and a differential output; and

a differential feedback path coupled across said differential input and said differential output;

wherein said differential feedback path is shorted together for low gain applications.

6. The variable gain differential amplifier of claim 5, wherein said differential feedback path includes;

a differential feedback amplifier;

a first differential resistor coupled to an output of said differential feedback amplifier; and

means for shorting a first terminal of said first differential resistor to a second terminal of said first differential resistor.

7. The variable gain differential amplifier of claim 6, further comprising:

a second differential resistor coupled said first differential resistor and to said differential input.

8. The differential amplifier of claim 6, wherein said means for shorting is implemented for low gain applications of said differential amplifier.

9. The differential amplifier of claim 6, wherein said means for shorting includes a switch coupled across differential terminals of said first differential resistor.

10. The differential amplifier of claim 9, wherein said switch is closed for low gain applications, so as to short-out a feedback signal associated with said feedback path for low gain applications.

11. The differential amplifier of claim 10, wherein said feedback signal is terminated in said first differential resistor.

12. The differential amplifier of claim 10, wherein said switch includes a control terminal that is activated for low gain applications.

13. The differential amplifier of claim 6, wherein said first differential resistor has a pair of component resistors, said switch coupled across corresponding terminals of said pair of component resistors.

14. The differential amplifier of claim 6, wherein said differential forward amplifier provides a variable gain, wherein said means for shorting includes a switch that is closed for low gain applications of said differential amplifier, wherein said switch is opened for other applications of said differential amplifier.

15. A method of variably amplifying a differential input signal, comprising:

receiving a differential input signal at an input terminal;

variably amplifying the differential input signal to produce a differential amplified signal at an output;

feeding back a portion of the differential amplified signal through a feedback path that is coupled to the input terminal to produce a differential feedback signal; and

shorting together components of the differential feedback signal for low gain applications.