Active RC filters

Abstract

An operational amplifier comprises: a first amplifier stage 4 comprising a first differential pair of transistors 8, 10 arranged to receive and amplify a differential input signal 18, 20 thereby providing a first differential output signal 22, 24; and a second amplifier stage 6 comprising a second differential pair of transistors 26, 28 arranged to receive and amplify the first differential output signal 22, 24 thereby providing a second differential output signal 38, 40.

Claims

1. An operational amplifier comprising: a first amplifier stage comprising a first differential pair of transistors arranged to receive and amplify a differential input signal thereby providing a first differential output signal; and a second amplifier stage comprising a second differential pair of transistors arranged to receive and amplify the first differential output signal thereby providing a second differential output signal wherein a common mode voltage of at least one of said first and second differential output signals is controlled by a replica circuit that reflects a topology of one of said first and second amplifier stages; wherein the replica circuit controls the common mode voltage in a plurality of amplifier stages including said first and second amplifier stages.

2. The operational amplifier as claimed in claim 1 wherein the second amplifier stage comprises one or more components arranged to introduce an additional pole into a transfer function of the operational amplifier.

3. The operational amplifier as claimed in claim 1 wherein the second amplifier stage comprises a reactive portion comprising at least one capacitor.

4. The operational amplifier as claimed in claim 1 wherein either or both of said first or second amplifier stages comprises a long tailed pair transistor configuration.

5. The operational amplifier as claimed in claim 4 wherein said long tailed pair transistor configuration(s) comprise(s) a tail transistor.

6. The operational amplifier as claimed in claim 1 wherein either or both of said first and second amplifier stages is connected to a power supply via a resistor.

7. The operational amplifier as claimed in claim 1 wherein the transistors of the first amplifier stage are different to the transistors of the second amplifier stage.

8. The operational amplifier as claimed in claim 1 wherein the first amplifier stage and the second amplifier stage comprise resistors, and wherein the resistors of the first amplifier stage are different to the resistors of the second amplifier stage.

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

10. The operational amplifier as claimed in claim 1 comprising field effect transistors.

11. A battery powered integrated circuit comprising the operational amplifier as claimed in claim 1.

12. An active RC filter comprising: an operational amplifier that comprises: a first amplifier stage comprising a first differential pair of transistors arranged to receive and amplify a differential input signal thereby providing a first differential output signal; and a second amplifier stage comprising a second differential pair of transistors arranged to receive and amplify the first differential output signal thereby providing a second differential output signal wherein a common mode voltage of at least one of said first and second differential output signals is controlled by a replica circuit that reflects a topology of one of said first and second amplifier stages; wherein the replica circuit controls the common mode voltage in a plurality of amplifier stages including said first and second amplifier stages.

Description

(1) An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a circuit diagram of a first exemplary embodiment of the present invention; and

(3) FIG. 2 is a circuit diagram of a second exemplary embodiment of the present invention.

(4) FIG. 1 shows a circuit diagram of a first exemplary embodiment of an operational amplifier 2 in accordance of the present invention for use in an active RC filter. The operational amplifier 2 comprises a first amplifier stage 4 and a second amplifier stage 6.

(5) The first amplifier stage 4 comprises a long tailed pair transistor configuration, comprising a differential pair of N-channel field effect transistors 8, 10 and a tail transistor 12. The differential pair of transistors 8, 10 are connected via their respective source leads, and then connected to ground 44 via tail transistor 12. Each differential pair transistor 8, 10 is also connected via its drain lead to the positive power supply 42 via a resistor 14, 16.

(6) The second amplifier stage 6 also comprises a long tailed pair transistor configuration, comprising a differential pair of N-channel field effect transistors 26, 28 and a tail transistor 30. The differential pair of transistors 26, 28 are connected via their respective source leads, and then connected to ground 44 via tail transistor 30. Each differential pair transistor 26, 28 is also connected via its drain lead to the positive power supply 42 via a resistor 32, 34. The drain leads of the transistors 26, 28 are connected to one another via a capacitor 36.

(7) When a differential input 18, 20 is applied to the gate leads of each of the differential pair transistors 8, 10, this drives the differential pair to generate an amplified differential output 22, 24 in the manner known per se to a person skilled in the art. This differential output 22, 24 is then fed as an input to the second amplifier stage 6.

(8) The differential input 22, 24 to the second amplifier stage 6 is applied to the gate leads of each of the differential pair transistors 26, 28, which in turn drives the differential pair to generate an amplified differential output 38, 40. The capacitor 36 acts so as to provide the second amplifier stage 6 with a low pass filter transfer function. With increasing frequency, the impedance of the capacitor 36 decreases, which reduces the amplitude of the second differential output signal 38, 40.

(9) The tail transistors 12, 30 act as a constant current source. The source leads of each of the tail transistors 12, 30 are connected to ground 44, and their respective gate leads are connected to a bias voltage 46. This bias voltage 46 is variable so as to alter the current that flows through each of the amplifier stages 4, 6. This current determines the common mode voltage present on the outputs 22, 24, 38, 40 of each of the amplifier stages 4, 6 due to the resistors 14, 16, 32, 34.

(10) The cascaded amplifier stages operate such that a differential signal input is first amplified by the first amplifier stage 4 to produce an amplified differential output signal. The amplified differential output signal possesses the same frequency and general waveform as the differential input signal, but the amplitude is scaled by a predetermined factor or gain. If the differential signal input is too large in amplitude, such that the amplitude when multiplied by the gain would exceed the maximum output amplitude of the first amplifier stage (primarily dictated by the power supply), clipping occurs, wherein any resulting output amplitude that would exceed this limit is instead output at that limit, resulting in the tops of waveforms being cut off. Once the input signal amplitude returns to a level that does not result in an amplified signal exceeding the output amplitude limit, the output signal returns to following the waveform of the differential input signal.

(11) The above-mentioned signal is then input to the second amplifier stage 6 that filters the amplified differential output signal to produce a filtered amplified differential output signal. The second amplifier stage scales the amplified differential output signal from the first amplifier stage by a factor or gain and produces a filtered amplified output signal with an amplitude that depends on the frequency of the signal, such that signals within a particular range have a larger amplitude than signals in a different range of frequencies.

(12) FIG. 2 shows a circuit diagram of a second exemplary embodiment of the present invention implemented within an active RC filter 100. The active RC filter 100 comprises four operational amplifiers 2A, 2B, 2C, 2D as described with reference to FIG. 1 above.

(13) The active RC filter 100 implements a fourth order leapfrog filter that takes a differential input across two input terminals 102, 104 and provides a differential output across two output terminals 106, 108. A series of resistors and capacitors are arranged to provide feedback loops around the operational amplifiers 2A, 2B, 2C, 2D and thus provide the desired filter transfer function. It will be appreciated by a person skilled in the art that this is only one filter topology and many variations and modifications are possible within the scope of the invention.

(14) Thus it will be seen that an operational amplifier particularly suitable for use within an active RC filter comprising cascaded amplifier stages has been described. Although a particular embodiment has been described in detail, many variations and modifications are possible within the scope of the invention.