Active RC filter having gain-setting attenuator

• US 8,970,293 B1
• Filed: 10/29/2013
• Issued: 03/03/2015
• Est. Priority Date: 10/29/2013
• Status: Active Grant

• Current Assignee: Motorola Solutions, Inc.
• Original Assignee: Motorola Solutions, Inc.
• Inventors: Salvi, Raul
• Primary Examiner(s): Le, Dinh
• Application Number:
  US14/066,224
• Time in Patent Office: 490 Days
• Field of Search: 327551-554, 327336-337
• US Class Current: 327/553
• CPC Class Codes: H03H 11/126 using a single operational amplifier H03H11/1204 takes precedence; parallel-T filters H03H11/1295
• CPC Class Codes: H03H 11/1291 Current or voltage controlled filters

Abstract

Disclosed herein are embodiments of an active RC filter that has a gain-setting attenuator. An embodiment takes the form of a filter circuit having a filter-circuit input node; a filter-circuit output node; an operational amplifier (op-amp) having first and second inputs and also having an output coupled to the filter-circuit output node; and a passive feedback path extending between the filter-circuit output node and the first op-amp input, the passive feedback path having a gain-setting attenuator segment in series with a signal-filtering segment.

Claims

1. A filter circuit comprising:

a filter-circuit input node;

a filter-circuit output node;

an operational amplifier (op-amp) comprising first and second op-amp inputs, the op-amp further comprising an op-amp output coupled to the filter-circuit output node; and

a passive feedback path extending between the filter-circuit output node and the first op-amp input, the passive feedback path comprising a variable gain-setting attenuator for varying filter circuit gain separate from and in series with a signal-filtering RC network for varying filter circuit bandwidth and quality factor, such that filter circuit gain can be changed independently without impacting filter circuit bandwidth or quality factor.

2. The filter circuit of claim 1, wherein the variable gain-setting attenuator comprises a voltage divider.

3. The filter circuit of claim 2, wherein the voltage divider comprises first and second impedances.

4. The filter circuit of claim 3, wherein the first impedance comprises a first resistor, and wherein the second impedance comprises a second resistor.

5. The filter circuit of claim 4, wherein at least one of the first resistor and the second resistor is a variable resistor.

6. The filter circuit of claim 5, wherein each of the first resistor and the second resistor is a variable resistor.

7. The filter circuit of claim 3, wherein the first impedance comprises a first capacitor, and wherein the second impedance comprises a second capacitor.

8. The filter circuit of claim 7, wherein at least one of the first capacitor and the second capacitor is a variable capacitor.

9. The filter circuit of claim 8, wherein both of the first capacitor and the second capacitor is a variable capacitor.

10. The filter circuit of claim 1, wherein the signal-filtering RC network comprises one of a low-pass signal-filter, a high-pass signal-filter, and a band-pass signal-filter.

11. The filter circuit of claim 1, wherein the signal-filtering RC network comprises a band-stop signal-filter.

12. The filter circuit of claim 1, wherein the signal-filtering RC network comprises an all-pass signal-filter.

13. The filter circuit of claim 1, wherein the filter-circuit input node is a first differential filter-circuit input node, the filter-circuit output node is a first differential filter-circuit output node, the op-amp is a first op-amp, the op-amp output is a first op-amp output, the passive feedback path is a first passive feedback path, the variable gain-setting attenuator is a first variable gain-setting attenuator, and the signal-filtering RC network is a first signal-filter, the filter circuit further comprising:

a differential input comprising the first differential filter-circuit input node and a second differential filter-circuit input node;

a differential output comprising the first differential filter-circuit output node and a second differential filter-circuit output node;

a second op-amp comprising third and fourth op-amp inputs, the second op-amp further comprising a second op-amp output coupled to the second differential filter-circuit output node; and

a second passive feedback path extending between the second differential filter-circuit output node and the third op-amp input, the second passive feedback path comprising a second variable gain-setting attenuator in series with a second signal-filtering RC network.

14. The filter circuit of claim 1, wherein a first node of the variable gain-setting attenuator is coupled to the op-amp output and a second node of the variable gain-setting attenuator is coupled to the signal-filtering RC network.

15. A filter circuit comprising:

a filter-circuit input node;

a filter-circuit output node;

an operational amplifier (op-amp) comprising first and second op-amp inputs, the op-amp further comprising an op-amp output coupled to the filter-circuit output node; and

a passive feedback path extending between the filter-circuit output node and the first op-amp input, the passive feedback path comprising a variable gain-setting attenuator for varying filter circuit gain in series with a signal-filtering RC network for varying filter circuit bandwidth and quality factor, the variable gain-setting attenuator being coupled to the signal-filtering RC network at an intermediate node, the variable gain-setting attenuator extending between the filter-circuit output node and the intermediate node such that filter circuit gain can be changed independently without impacting filter circuit bandwidth or quality factor, the signal-filtering RC network extending between the intermediate node and the first op-amp input, the variable gain-setting attenuator comprising a first variable gain-setting impedance and a second gain-setting impedance, the first variable gain-setting impedance extending between the filter-circuit output node and the intermediate node, the second gain-setting impedance being coupled to the intermediate node, the first variable gain-setting impedance and the second gain-setting impedance cooperating to function as a voltage divider between the filter-circuit output node and the intermediate node.

16. The filter circuit of claim 15, wherein the second gain-setting impedance is a variable impedance.

17. The filter circuit of claim 15, wherein the signal-filtering RC network is selected from the group consisting of a low-pass signal-filter, a high-pass signal-filter, a band-pass signal-filter, a band-stop signal-filter, and an all-pass signal-filter.

18. The filter circuit of claim 15, wherein a first node of the first gain-setting impedance is coupled to the op-amp output and a second node of the first gain-setting impedance is coupled to the intermediate node, and wherein a first node of the second gain-setting impedance is coupled to the intermediate node and a second node of the second gain-setting impedance is coupled to ground.