Rail splitting virtual ground generator for single supply systems

• US 5,291,121 A
• Filed: 03/24/1992
• Issued: 03/01/1994
• Est. Priority Date: 09/12/1991
• Status: Expired due to Term

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Neale, Todd M., Whitney, Brad P., Granahan, Mark E.
• Primary Examiner(s): STERRETT, JEFFREY L
• Application Number:
  US07/856,856
• Time in Patent Office: 707 Days
• Field of Search: 323/311, 323/312, 323/313, 323/314, 323/907
• US Class Current: 323/313
• CPC Class Codes: G05F 1/56 using semiconductor devices in series with the load as final control devices G05F1/461 takes precedence
• CPC Class Codes: G05F 3/22 wherein the transistors are of the bipolar type only G05F3/26, G05F3/30 take precedence
• CPC Class Codes: Y10S 323/907 Temperature compensation of semiconductor

Abstract

A method and apparatus for a circuit physically realizing a virtual ground function producing a virtual ground voltage halfway between the supply and common voltages and having improved accuracy and stability is described. A stable bias current source is coupled to a precision resistor voltage divider network, which is used as a voltage reference generator to produce a virtual ground voltage of a precise value. This reference voltage is coupled to an operational amplifier configured in a unity gain configuration. The circuit thus created offers numerous advantages over the virtual ground circuits in use in the prior art. An integrated circuit implementing this circuit is described, and alternative packaging embodiments are disclosed. Other embodiments are also disclosed.

Claims

1. A rail splitting virtual ground generator circuit, comprising:

first and second resistors, coupled to an voltage reference node and further coupled between first and second supply voltages, operable such that a predetermined reference voltage is available at the voltage reference node;

a bias current source coupled to said first supply voltage;

an operational amplifier coupled to said reference voltage in a unity gain configuration, having an output coupled to an output terminal, and powered by said bias current source and said second supply voltage;

a noise reduction input coupled to said reference voltage; and

operable to provide a stable voltage at the output terminal which is equal to half the difference between said first and second supply voltages over a wide range of supply voltage and temperature conditions.

2. The rail splitting virtual ground generator circuit of claim 1, and further comprising:

a first capacitor coupled between said voltage reference node and said second supply voltage, operable for reducing signal noise at the voltage output.

3. The rail splitting virtual ground generator circuit of claim 1, wherein said bias current source further comprises transistors coupled for providing a reference current of a predetermined value, and further comprising transistors coupled as compensation circuitry, operable so that current variations with in the bias source due to ambient temperature variations do not change the reference current value.

4. The rail splitting virtual ground generator circuit of claim 3 wherein said compensation circuitry comprises BET transistors.

5. The rail splitting virtual ground generator circuit of claim 3 wherein said compensation circuitry comprises isolated vertical bipolar transistors.

6. The rail splitting virtual ground generator circuit of claim 1, wherein said operational amplifier further comprises one differential input coupled to said reference voltage and a second differential input coupled to said voltage output.

7. The rail splitting virtual ground generator circuit of claim 1, wherein said operational amplifier further comprises an output stage coupled to said output terminal and further coupled to a boost stage, said boost stage operable to provide additional current sinking or sourcing capability when the current flowing through the output stage exceeds a predetermined threshold.

8. The rail splitting virtual ground generator circuit of claim 7 wherein said operational amplifier output stage further comprises a sensing transistor coupled to said boost stage, and operable to turn on said boost stage when the current flowing through the conduction path of said sensing transistor exceeds a certain threshold.

9. The rail splitting virtual ground generator circuit of claim 8, wherein said boost stage of said operational amplifier comprises a first and second bipolar transistor, each having its base coupled to said sensing transistor, and each having its conduction path coupled between said voltage output and one of said voltage supplies, operable to provide increased current flow through the conduction paths of said first or second transistor when the current flowing at the voltage output exceeds said predetermined threshold.

10. The rail splitting virtual ground generator circuit of claim 7, wherein said output stage is coupled to a plurality of gain stages, coupled together and further coupled to said voltage reference node, operable to provide a large gain in signal amplitude from said voltage reference to said output stage.

11. The rail splitting virtual ground generator circuit of claim 1, wherein said operational amplifier is comprised of isolated vertical bipolar transistors coupled to form differential input stage, a plurality of gain stages coupled together and to the differential input stage, and an output stage coupled to said voltage output.

12. The rail splitting virtual ground generator circuit of claim 1, wherein said operational amplifier further comprises a plurality of resistors coupled with programmable fuses, operable to allow selective programmation of the offset voltage of said operational amplifier, to allow minimization said offset voltage.

13. A method for creating a virtual ground reference voltage for use in a single supply system, comprising the steps of:

providing a pair of precision resistors coupled between a first and second voltage supply;

providing a temperature stable bias current source coupled to said first voltage supply;

providing an operational amplifier having increased current drive capability, powered by said bias current source and said second voltage supply;

coupling a predetermined reference voltage produced by the said pair of precision resistors one input terminal of said operational amplifier;

coupling the output of said operational amplifier to a voltage output terminal, and further coupling said voltage output terminal to a second input terminal of said operation amplifier; and

operating said operational amplifier in a unity gain configuration so as to provide a voltage at the voltage output terminal that is one half the difference in said first and second supply voltages.

14. The method of claim 13, and further comprising the step of:

coupling a capacitor between said reference voltage and said second voltage supply to reduce the signal noise at the output terminal.

15. The method of claim 14, wherein said step of providing an operational amplifier comprises providing an operational amplifier fabricated from isolated vertical bipolar transistors.

16. The method of claim 15, wherein said step of providing an operational amplifier further comprises providing an operational amplifier having a boost circuit comprising a first and second isolated vertical bipolar transistor each having their conduction paths coupled between said output voltage terminal and one of said first and second voltage supplies.

17. An integrated circuit implementing a virtual ground reference voltage circuit, comprising:

first and second voltage supply terminals;

a voltage output terminal;

first and second resistors, coupled to an voltage reference node and further coupled between said first and second supply voltages, operable such that a predetermined reference voltage is available at the voltage reference node;

a bias current source coupled to said first supply voltage;

an operational amplifier coupled to said reference voltage in a unity gain configuration, having an output coupled to said output terminal, and powered by said bias current source and said second supply voltage;

a noise reduction input coupled to said reference voltage; and

operable to provide a stable voltage at the output terminal which is equal to half the difference between said first and second supply voltages over a wide range of supply voltage and temperature conditions.

18. The integrated circuit of claim 17, and further comprising:

a noise reduction terminal coupled to said reference voltage, operable to selectively allow coupling of a capacitor between said noise reduction terminal and said second voltage supply for reducing signal noise at said output voltage terminal.

19. A rail splitting virtual ground generator circuit, comprising:

a resistor voltage divider coupled between first and second supply voltages operable to provide a predetermined reference voltage;

a bias current source coupled to said first supply voltage;

an amplifier coupled to said reference voltage and having an output coupled to an output terminal, said amplifier being powered by said bias current source; and

operable to provide a stable voltage at the output terminal which is equal to half the difference of said first and second supply voltages.

20. The rail splitting virtual ground generator circuit of claim 19, and further comprising a noise reduction input coupled to said reference voltage.

21. The rail splitting virtual ground generator circuit of claim 20, and further comprising a first capacitor coupled between said voltage reference node and said second supply voltage, operable for reducing signal noise at the output terminal.

22. The rail splitting virtual ground generator circuit of claim 19, wherein said bias current source further comprises transistors coupled for providing a reference current of a predetermined value, and further comprising transistors coupled as compensation circuitry, operable so that current variations within the bias current source due to ambient temperature variations do not change the reference current value.

23. The rail splitting virtual ground generator circuit of claim 19, wherein said amplifier comprises an operational amplifier coupled in a unity gain configuration.

24. The rail splitting virtual ground generator circuit of claim 23, wherein said operational amplifier further comprises an output stage coupled to a boost stage, said boost stage operable to provide additional current sourcing or sinking capability when the current flowing through said output stage exceeds a certain threshold.

25. The rail splitting virtual ground generator circuit of claim 24, wherein said operational amplifier further comprises a sensing transistor coupled to said boost stage and operable to turn on said boost stage when the current flowing through the conduction path of said sensing transistor exceeds a certain threshold.

26. A method for creating a rail splitting virtual ground generator circuit, comprising the steps of:

providing a resistor voltage divider coupled between first and second supply voltages operable to provide a reference voltage;

providing a bias current source coupled to said first supply voltage;

providing an amplifier coupled to said reference voltage and having an output coupled to an output terminal, said amplifier being powered by said bias current source; and

operating said amplifier and bias current source to provide a stable voltage at the output terminal which is equal to half the difference of said first and second supply voltages.

27. The method of claim 26, and further comprising the step of coupling a capacitance between said reference voltage and said second voltage supply to reduce the signal noise at the output terminal.

28. The method of claim 26, wherein said step of providing an amplifier coupled to said reference voltage comprises providing an operational amplifier coupled in a unity gain configuration and having increased current drive capability.

29. The method of claim 28, wherein said step of providing an operational amplifier further comprises the steps of:

providing a plurality of isolated bipolar transistors coupled to form a differential input stage,

providing a plurality of gain stages coupled together and to the differential input stage; and

providing an output stage coupled to the voltage output and to said plurality of gain stages.

30. The method of claim 29, wherein said step of providing a bias current source comprises selecting a bias current circuit which has a stable output which remains constant over a wide range of temperature and voltage conditions.