Feed-forward compensation scheme for feedback circuits

• US 6,222,418 B1
• Filed: 02/29/2000
• Issued: 04/24/2001
• Est. Priority Date: 02/29/2000
• Status: Expired due to Term

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Lucent Technologies, Inc. (Nokia Corporation)
• Inventors: Gopinathan, Venugopal, Prodanov, Vladimir I.
• Primary Examiner(s): Pascal, Robert
• Assistant Examiner(s): Choe, Henry
• Application Number:
  US09/515,533
• Time in Patent Office: 420 Days
• Field of Search: 330/98, 330/99, 330/100, 330/124 R, 330/151, 330/292
• US Class Current: 330/292
• CPC Class Codes: H03F 1/083 in transistor amplifiers H03F1/10 - H03F1/22 take precedence

Abstract

A feed-forward compensated negative feedback circuit comprises an operational amplifier having an inverting and a non-inverting input and an output. A feedback element is connected between the output of the operational amplifier and its inverting input to form a negative feedback loop. The inverting input of the op-amp is driven with a first transconductance amplifier which produces an output current proportional to an input voltage. A feed-forward transconductance amplifier receives the input voltage and produces an inverted output current proportional to the input voltage. A feed-forward current is injected at the output of the operational amplifier. By providing at the output of the op-amp the current it would be required to carry over the feedback loop, a voltage differential at the op-amp inputs is avoided, thus eliminating parasitic current flows across the parasitic input capacitance and thereby improving the circuits overall performance. In a second embodiment of the invention, a unity-gain buffer is included in the feedback loop to produce a unidirectional path. To reduce the power requirements of the buffer, a feed-forward current is injected at a point between the feedback impedance element and the unity-gain feedback buffer such that the buffer does not need to source any current through the impedance element.

Claims

1. A feed-forward compensated negative feedback circuit comprising:

an operational amplifier with conductance gm and having an inverting and a non-inverting input and an output;

a non-capacitive impedance element connected between the output of the operational amplifier and the inverting input of the operational amplifier;

a first transconductance amplifier with conductance Gm and having an inverting and non-inverting input receiving an input signal and having an output connected to the inverting input of the operational amplifier; and

a second transconductance amplifier with conductance substantially equal to Gm and having an inverting and non-inverting input receiving said input signal and having an inverted output connected to the output of the operational amplifier.

2. The circuit of claim 1, wherein said first and second transconductance amplifiers are substantially identical.

3. The circuit of claim 1, wherein gm is substantially greater than Gm.

4. The circuit of claim 1, wherein the operational amplifier, and the first and second transconductance amplifiers are implemented in MOS.

5. A feed-forward compensated negative feedback circuit comprising:

an operational amplifier with conductance gm and having an inverting and a non-inverting input and an output providing an output signal;

a unity-gain buffer connected to the output of the operational amplifier;

an impedance element connected between an output of the unity-gain buffer and the inverting input of the operational amplifier;

a first transconductance amplifier with conductance Gm and having an input receiving an input signal and having an output connected to the inverting input of the operational amplifier; and

a second transconductance amplifier with conductance substantially equal to Gm, receiving said input signal, and having an inverted output connected between the unity-gain buffer and the impedance element.

6. The circuit of claim 5, wherein said first and second transconductance amplifiers are substantially identical.

7. The circuit of claim 5, wherein the impedance element comprises a capacitor.

8. The circuit of claim 5, wherein gm is substantially greater than Gm.

9. The circuit of claim 5, wherein the operational amplifier, unity gain buffer, and the first and second transconductance amplifiers are implemented in MOS.