DIFFERENTIAL AMPLIFIERS

Abstract

A differential amplifier comprises: a long tailed pair transistor configuration comprising a differential pair of transistors and a tail transistor; and a replica circuit configured to vary a feedback current in the replica circuit to match a replica voltage to a reference voltage, wherein varying the feedback current in the replica circuit 4 provides a bias voltage to the tail transistor in the long tailed pair which controls a tail current through the tail transistor to determine a common mode voltage in the long tailed pair.

Claims

1. A differential amplifier comprising: a long tailed pair transistor configuration comprising a differential pair of transistors and a tail transistor; and a replica circuit configured to vary a feedback current in the replica circuit to match a replica voltage to a reference voltage, wherein varying the feedback current in the replica circuit provides a bias voltage to the tail transistor in the long tailed pair which controls a tail current through the tail transistor to determine a common mode voltage in the long tailed pair.

2. The differential amplifier as claimed in claim 1 configured as a fully differential amplifier.

3. The differential amplifier as claimed in claim 1 wherein the replica circuit controls the common mode voltage in the differential pair and at least one additional differential pair.

4. The differential amplifier as claimed in 1 claim wherein the replica circuit includes a feedback loop comprising half a differential pair.

5. The differential amplifier as claimed in claim 4 wherein the feedback loop further comprises an amplifier.

6. The differential amplifier as claimed in claim 1 wherein at least one of the differential pair of transistors is connected to a power supply voltage via a resistor.

7. The differential amplifier as claimed in claim 1 wherein the replica circuit is scaled to use less current than the long tailed pair current.

8. The differential amplifier as claimed in claim 7 wherein the replica circuit is scaled to use less than 50% of the long tailed pair current.

9. The differential amplifier as claimed claim 1 comprising field effect transistors.

10. A battery powered integrated circuit comprising a differential amplifier, said differential amplifier comprising: a long tailed pair transistor configuration comprising a differential pair of transistors and a tail transistor; and a replica circuit configured to vary a feedback current in the replica circuit to match a replica voltage to a reference voltage, wherein varying the feedback current in the replica circuit provides a bias voltage to the tail transistor in the long tailed pair which controls a tail current through the tail transistor to determine a common mode voltage in the long tailed pair.

11. The battery powered integrated circuit as claimed in claim 10 wherein the differential amplifier is configured as a fully differential amplifier.

12. The battery powered integrated circuit as claimed in claim 10 wherein the replica circuit controls the common mode voltage in the differential pair and at least one additional differential pair.

13. The battery powered integrated circuit as claimed in claim 10 wherein the replica circuit includes a feedback loop comprising half a differential pair.

14. The battery powered integrated circuit as claimed in claim 13 wherein the feedback loop further comprises an amplifier.

15. The battery powered integrated circuit as claimed in claim 10 wherein at least one of the differential pair of transistors is connected to a power supply voltage via a resistor.

16. The battery powered integrated circuit as claimed in claim 10 wherein the replica circuit is scaled to use less current than the long tailed pair current.

17. The battery powered integrated circuit as claimed in claim 16 wherein the replica circuit is scaled to use less than 50% of the long tailed pair current.

18. The battery powered integrated circuit as claimed in claim 10 comprising field effect transistors.

Description

[0021] An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0022] FIG. 1 is a circuit diagram of an exemplary embodiment of the present invention.

[0023] FIG. 1 shows a circuit diagram of an exemplary embodiment of the present invention. A differential amplifier comprises a long tailed pair 2 and a replica circuit 4. The replica circuit 4 provides an output that is connected to the tail transistor 10 of the long tailed pair 2, as will be described in further detail below.

[0024] The long tailed pair 2 comprises a differential pair of N-channel field effect transistors 6, 8 that are arranged such that their sources are connected to one another and subsequently to ground 14 via a tail transistor 10. The drain of each respective transistor is then connected to a positive power supply 12 via respective resistors 16, 18. The gates of each of the N-channel FETs 6, 8 are then connected to the positive signal input 20 and negative signal input 22 respectively. This differential input drives the long tailed pair in a manner known per se to those skilled in the art.

[0025] The replica circuit 4 comprises a high gain amplifier 28 arranged with a feedback loop that resembles one half of a long tailed pair, containing a transistor 30 that performs in a similar manner to the differential pair transistors 6, 8. The high gain amplifier 28 is connected to the positive power supply 12 and ground 14, and is provided with a reference voltage 36 on its negative input. This reference voltage 36 is the voltage to which the common mode voltage of the long tailed pair 2 will be driven.

[0026] The output of the high gain amplifier 28 drives a replica tail transistor 32 within the feedback loop, as well as the tail transistor 10 of the long tailed pair 2 as will be described further below. The replica tail transistor 32 varies the amount of current flowing around the feedback loop, which due to the resistor 38, varies a replica voltage 34. The high gain amplifier 28 amplifies the difference between replica voltage 34 and reference voltage 36, so any difference between them will cause the replica tail transistor 32 to act such as to oppose any difference between the voltages, driving the replica voltage 34 to match the reference voltage 36.

[0027] The output from the high gain amplifier 28 is also connected to the tail transistor 10 as mentioned above. This ensures that operations performed on the replica tail transistor 32 are reflected as similar operations performed on the tail transistor 10, thus controlling the current flowing through the long tailed pair 2. Due to the fixed resistors 16, 18, this controlled current is analogous to controlling the common mode voltage present on the output 24, 26 of the long tailed pair 2, and the common mode voltage is driven to match the replica voltage 34 and reference voltage 36.

[0028] The transistors 30, 32 and resistor 38 within the replica circuit 4 are scaled down to use'¼ of the long tailed pair current. This reduces the power consumption of the replica circuit 4 while maintaining the ability to control the common mode voltage in the long tailed pair 2.

[0029] Additional differential pairs 40 can be driven by the same replica circuit 4 by connecting them to additional tail transistors 42, 44, 46. The output of the high gain amplifier 28 is connected to the gates of further tail transistors 42, 44, 46, providing the same common mode voltage at their respective outputs as provided to the output of long tailed pair 2.

[0030] Thus it will be seen that a differential amplifier with feedforward common mode signal control has been described. Although a particular embodiment has been described in detail, many variations and modifications are possible within the scope of the invention.

[0031] The replica circuit provides a real time model of the common mode voltage present on the output of the long tailed pair(s). The high gain amplifier within the feedback loop compares the replica of the common mode voltage to the reference voltage and alters its output voltage to drive the former toward the value of the latter, i.e. the output voltage of the high gain amplifier depends on the difference between the replica voltage and the reference voltage. This then causes the tail transistor(s) to vary the current that can flow therethrough. This controlled current then drives the actual common mode voltage present on the output of the long tailed pair(s) toward the value of the reference voltage. In practice, this causes the voltage around which output signals are centred to shift higher or lower to the value of the reference voltage.