Push pull buffer with noise cancelling symmetry

• US 5,513,389 A
• Filed: 07/28/1995
• Issued: 04/30/1996
• Est. Priority Date: 08/27/1992
• Status: Expired due to Fees

• Current Assignee: CTS Corporation
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Connell, Lawrence E., Reeser, Glen O.
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): VO, NGUYEN THANH
• Application Number:
  US08/508,552
• Time in Patent Office: 277 Days
• Field of Search: 455/76, 455/89, 455/259, 455/258, 455/260, 455/318, 455/319, 455/316, 455/317, 455/264, 455/332, 455/341, 455/183.1, 455/255, 455/310, 455/295, 455/296, 455/311, 455/343, 330/263, 330/266, 330/272, 330/264, 330/267, 330/268, 330/273, 330/274, 330/277, 331/8, 331/18, 331/25, 331/117 R, 331/117 FE, 331/177 V, 331/36 C, 331/44, 331/108 A, 331/108 C, 331/108 D
• US Class Current: 455/311
• CPC Class Codes: H03F 1/3211 in differential amplifiers
• CPC Class Codes: H03F 3/3001 with field-effect transistors
• CPC Class Codes: H03F 3/3016 with symmetrical driving of the end stage

Abstract

A buffer (200) having output devices (101 and 102) exhibiting complementary symmetry and configured as voltage-followers in push pull is biased for class AB operation by a bias network including two substantially equal resistors (201 and 202) arranged and configured to substantially reduce noise attributable to the bias circuit.

Claims

1. An apparatus comprising:

a class AB amplifier circuit;

a bias circuit operably coupled to the class AB amplifier circuit, wherein the bias circuit contains exactly two substantially equal resistors arranged and configured to substantially reduce noise attributable to the bias circuit,

wherein the bias circuit includes two active elements coupled between a voltage source and ground, with the substantially equal resistors configured in series with the two active elements,

and wherein a first one of the two substantially equal resistors couples the two active elements to the voltage source, and a second one of the two substantially equal resistors couples the two active elements to ground,

wherein the bias circuit does not include any other circuit elements in parallel with the two substantially equal resistors that would preclude noise attributable to the bias circuit from being substantially reduced,

and wherein the noise is due in part to thermal noise and in part to flicker noise.

2. The apparatus of claim 1, wherein both of the two active elements are diode-connected transistors.

3. The apparatus of claim 2, wherein the transistors are MOSFETs.

4. The apparatus of claim 2, wherein each of the substantially equal resistors has a resistance greater than 10K.

5. The apparatus of claim 2, wherein each of the substantially equal resistors has a resistance substantially equal to 25K.

6. A push pull buffer comprising:

an output stage including;

a pair of complementary MOSFETs configured as voltage followers and connected together in push pull with an output between the pair;

a bias network including;

a pair of diode-connected complementary MOSFETs arranged in series;

a pair of matched resistors forming a first matched resistor and a second matched resistor, with the first matched resistor coupling one side of the pair of diode-connected MOSFETs to a voltage source, and the second matched resistor coupling the other side of the pair of diode-connected MOSFETs to a ground;

a pair of coupling resistors operably interconnecting the bias network and the output stage to bias the complementary MOSFETs of the output stage for class AB operation;

such that noise voltages generated between the pair of matched resistors are substantially equally divided between the complementary MOSFETs of the output stage, resulting in essentially no net change in voltage at the output,

wherein the bias circuit does not include any other circuit elements in parallel with the matched resistors that would preclude the noise voltages from being substantially equally divided between the complementary MOSFETs of the output stage resulting in essentially no net change in voltage at the output,

and wherein the noise voltages are due in part to thermal noise, and in part to flicker noise.