Amplifier with offset correction

Jaffard, Jean-Luc; Fox, Randolph;

A signal processing circuit that includes a differential amplifier and an integrator. The differential amplifier receives at a first input a signal to process and at a second input an offset compensation signal provided by the integrator. The integrator includes a differential transconductance amplifier receiving an output voltage of the signal processing circuit at a first input and a reference voltage at a second input; a capacitor having a first terminal connected to the output of the transconductance amplifier and a second terminal connected to a fixed voltage; and a voltage follower receiving the voltage at the first terminal of the capacitor and providing the offset compensation signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an amplifier including a device for correcting or adjusting the output offset voltage of either the amplifier, or a processing circuit including the amplifier. The invention more particularly relates to an amplifier, such as a magnetic head amplifier, having an active state in which its output is taken into account by further circuits and an inactive state in which its output is ignored.

2. Discussion of the Related Art

When an AC signal must be processed (amplified, filtered, etc.), it is often necessary to eliminate, at one or several stages of the processing circuit, DC components which, if they become too high after successive amplifications, could cause a clipping of the processed signal. The DC components, or offset voltages, are particularly impairing in some applications, such as processing of signals provided by magnetic heads, in which they are very high compared to the effective AC component of the processed signal.

To eliminate a DC component, a capacitor is usually inserted in the path of the processed signal. However, if it is desired to pass low frequencies, as for example in sound processing, the values of the capacitors must be particularly high, which does not allow integration of the capacitors into a circuit. Furthermore, in a processing chain, it is often necessary to eliminate the DC components at several stages and therefore to insert a plurality of capacitors, thus requiring the provision of the same number of additional pins in the integrated circuit when the processing circuit is integrated.

FIG. 1 represents a conventional circuit using a single capacitor to eliminate the DC component, or to adjust the component at a desired value at the output of a processing chain, independently of the offset voltages added to the signal upstream.

In the example of FIG. 1, the processing chain includes an operational amplifier 10 followed by a filter 12 providing the output signal Vout of the processing chain. The elements of the circuit are supplied between a high voltage V+ and a low voltage V-, for example ground. The input signal Vin of the chain is provided at the noninverting input of amplifier 10. Amplifier 10 includes a feedback loop including a resistor R1 connected between its output and its inverting input, and a resistor R2 connected between the inverting input and a correction or compensation voltage Vc. The correction voltage Vc is provided through an integrator 14 which integrates the difference between the output voltage Vout and a reference voltage Vref.

In this configuration, the DC component of the output voltage Vout establishes at value Vref, independently of the elements found upstream the output Vout. Preferably, voltage Vout is selected at the average of the supply voltages V+ and V-, for example by a bridge of resistors R3 and R4 of equal value. Thus, the output voltage can have the same amplitude on both sides of its DC component Vref. An integrator 14 requires an integration capacitor CI that is generally connected between the output and the inverting input of an operational amplifier. If the processed signal is liable to have a low frequency (for example 10 Hz in sound processing), capacitor CI must have a high value to prevent the DC component of signal Vout from oscillating with the low frequency. Therefore, capacitor CI may be physically too large to be integrable, and thus two additional connection pins must be provided on a circuit integrating the processing circuits to connect to two terminals of an external capacitor CI.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an integrated processing circuit with an improved offset voltage compensation loop.

The foregoing object is achieved in one illustrative embodiment of the present invention, in which a signal processing circuit is provided that includes a differential amplifier receiving at a first input a signal to process and at a second input an offset voltage compensation signal provided by an integrator. The integrator includes a differential transconductance amplifier receiving an output voltage of the signal processing circuit at a first input and a reference voltage at a second input; a capacitor having a first terminal connected to the output of the transconductance amplifier and a second terminal connected to a fixed voltage; and a voltage follower receiving the voltage at the first terminal of the capacitor and providing the offset compensation signal.

According to another embodiment of the present invention, the output of the signal processing circuit is taken into account during an active phase and ignored during an inactive phase. The signal processing circuit includes a first switch for connecting the first terminal of the capacitor to the output of the transconductance amplifier during the active phase, and for connecting the first terminal of the capacitor to the output of a reference voltage generator during the inactive phase.

According to a further embodiment of the present invention, the signal processing circuit includes a second switch for connecting the first and second inputs of the transconductance amplifier to the output signal and to the reference voltage, respectively, during the active phase, and for connecting the first and second inputs of the transconductance amplifier to the reference voltage and to the output of the voltage follower, respectively, at the beginning of the inactive phase, the switching of the first switch being delayed.

According to an additional embodiment of the present invention, the transconductance amplifier has two transconductances that can be selected by a control signal such that the highest transconductance is selected at the beginning of each mode transition of the processing circuit.

The foregoing and other objects, features, aspects and advantages of the invention will become apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1, above described, represents a conventional signal processing circuit including an offset compensation loop;

FIG. 2 represents a processing circuit including an embodiment of an offset compensation loop according to the invention;

FIG. 3 represents a signal processing circuit having an active mode and an inactive mode, including an alternative offset compensation loop according to the invention;

FIG. 4 is a timing diagram of various signals illustrating the operation of the offset compensation loop of FIG. 3; and

FIG. 5 represents an exemplary circuit for obtaining signals required by the circuit of FIG. 3.

DETAILED DESCRIPTION

FIG. 2 represents many of the same elements as in FIG. 1, labeled with same reference numerals. According to one embodiment of the invention, a particular structure of integrator 14 is selected. The integrator 16 includes a transconductance amplifier 16 receiving at its non-inverting input the output voltage Vout and receiving at its inverting input the reference voltage Vref. The transconductance amplifier 16 provides at a terminal A of an integration capacitor CI a current substantially proportional to the difference between its input voltages, Vout and Vref. The second terminal of capacitor CI is connected to a fixed voltage that is available outside the circuit if the latter is integrated, as for example the low supply voltage V-.

With this configuration, the greater the difference between voltages Vout and Vref, the faster capacitor CI is charged, which corresponds to an integration of this difference.

The voltage at terminal A of capacitor CI corresponds to the correction voltage Vc to be applied to resistor R2. However, to prevent the output current of the transconductance amplifier 16 from being loaded by resistor R2, and therefore not to impair the integration function, a voltage follower 18 is placed between resistor R2 and terminal A. In one embodiment, the transconductance amplifier 16 and the voltage follower 18 work together as a compensation amplifier that provides a compensation loop as described in more detail below. In such an embodiment, one input terminal of the compensation amplifier is the noninverting input of the transconductance amplifier 16, a second input terminal of the compensation amplifier is the inverting input of the transconductance amplifier 16, and a third input terminal of the compensation amplifier is the input of the voltage follower 18. The output of the compensation amplifier is the output of the voltage follower 18.

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The result obtained with a compensation loop according to the invention is the same as that of the loop of FIG. 1. In contrast, one terminal of capacitor CI is not connected inside the integrated circuit; it is connected to a supply voltage (V- in the given example) that is available externally. Accordingly, only a single pin is needed to implement the integration capacitor CI in the integrated circuit.

Among the processing circuits that require offset compensation, there are circuits having two modes of operation, that is, an active mode during which the output of the processing circuit is effectively taken into account and an inactive mode during which the output of the processing circuit is ignored. This is in particular the case for magnetic head playback amplifiers in which, during a playback phase, the signal provided by the magnetic head is amplified and, during a record phase, the playback amplifier is not used.

In such a circuit, the present invention further uses the integration capacitor CI to filter, while the processing circuit is inactive, a reference voltage needed by further processing circuits which are then active.

FIG. 3 represents a circuit designed for obtaining this result according to one embodiment of the invention. When a magnetic head playback amplifier is used, the reference voltage Vref, to which the DC component of the output voltage Vout is to be adjusted, also serves as a reference voltage for circuits (not shown) designed to carry out recording. This reference voltage, obtained through a bridge of resistors R3, R4, followed by a voltage follower 20, is very noisy and should be filtered.

Conventionally, filtering is obtained by connecting the node between resistors R3 and R4 to one of the supply voltages through a high value capacitor that must obviously be connected externally to the integrated circuit.

An embodiment of the present invention provides to connect, through a switch S1, the terminal A of the integration capacitor CI to the node between resistors R3 and R4 during the inactive phase of the processing circuit. Thus, capacitor CI is used as an integration capacitor during the active phase and as a filtering capacitor for filtering the reference voltage during the inactive phase. Accordingly, a filtering capacitor and a pin to connect it to the integrated circuit are spared. This embodiment can be easily realized because one of the terminals of capacitor CI is constantly connected to the same node, V-.

Switch S1 can be controlled by a signal selecting the mode of the processing circuit, for example a signal PB/REC selecting the playback mode or the record mode in a tape recorder.

However, in one particular embodiment, during the active phase of the processing circuit, capacitor CI is loaded at a compensation voltage Vc that differs from the reference voltage Vref. This difference increases with the offset introduced by the processing circuit. Thus, if terminal A of capacitor CI is abruptly switched so as to be coupled to the resistor bridge R3, R4, the reference voltage Vref abruptly varies and progressively recovers its initial state. This abrupt variation would cause, in a tape recorder, the recording of a spurious pulse on the tape.

To avoid this drawback, one embodiment of the present invention provides a switch S2 which, during the inactive phase, connects the non-inverting input of the transconductance amplifier 16 to the reference voltage Vref and connects the inverting input to the output of the voltage follower 18. During the active phase, switch S2 establishes the connections shown in FIG. 2.

FIG. 4 represents, in the example of a tape recorder, the waveforms of the control signals of switches S1 and S2 as a function of a signal PB/REC for selecting the mode of the processing circuit and of a signal MUTE that is conventionally used in a tape recorder to attenuate the parasitic effects when switching modes and powering on. Signal MUTE acts, for example, by forcing to 0 the outputs of the amplifiers likely to generate spurious pulses.

At time t.sub.0, the circuit is powered. Signal MUTE is active in order to attenuate the parasitic effects caused by the powering on. Signal PB/REC is at 1 and selects the active mode of the circuit. The control signals S1 and S2 of switches S1 and S2 are at 1; switches S1 and S2 are in the positions represented in FIG. 3.

At time t.sub.1, signal MUTE switches to 0, causing the circuit to normally operate in the active mode.

At time t.sub.2, the processing circuit is set to its inactive mode by the switching to 0 of signal PB/REC. Signal MUTE is activated to attenuate the parasitic effects caused by the switching. Switch S2 is switched but switch S1 is not yet activated. The transconductance amplifier 16, receiving the reference voltage Vref and the voltage Vc on terminal A of capacitor CI, provides a current to capacitor CI to allow the lather to be charged to voltage Vref.

At time t.sub.3, signal MUTE is deactivated. Capacitor CI had sufficient time to charge to value Vref. Switch S2 is set to its initial state. Switch S1 is switched to connect capacitor CI to the resistor bridge R3, R4. At time t.sub.3, the voltage across the terminals of capacitor CI and at the output of the resistor bridge R3 and R4 are identical and no spurious pulse occurs.

At time t.sub.4, signal PB/REC switches to 1 to select the active mode of the circuit. Switch S1 returns to its initial state and signal MUTE is activated until time t.sub.5 to attenuate the parasitic effects caused by the switching.

According to an alternative embodiment of the invention, as represented in FIG. 3, the transconductance amplifier 16 receives a signal G allowing to select a low or a high transconductance. A low transconductance is selected in steady state to obtain a high time constant for charging capacitor CI, thus preventing the offset compensation loop from following the oscillations at the lowest frequency of the processed signal. In contrast, a high transconductance is selected when capacitor CI must be rapidly charged or discharged during mode transitions of the processing circuit.

FIG. 4 also represents the waveform of signal G, which is the waveform of signal MUTE. At time t.sub.2, when the circuit switches from the active mode to the inactive mode, signal G is activated to select the high transconductance until time t.sub.3. In this case, the voltage across capacitor CI rapidly reaches value Vref, before time t.sub.3 when signal MUTE is deactivated.

Similarly, at time t.sub.4, when the circuit switches from the inactive mode to the active mode, signal G is again activated until time t.sub.5 so that the voltage across capacitor CI rapidly reaches the correction voltage Vc.

As represented between times t.sub.O and t.sub.1 (G=1), the high transconductance is also selected when powering the circuit.

FIG. 5 illustrates an exemplary circuit allowing to obtain signals S1, S2 and G from signals PB/REC and MUTE that are conventionally available in a tape recorder.

Signal G directly corresponds to signal MUTE.

Signal S1 is provided by an OR gate 25 receiving signals PB/REC and MUTE.

Signal S2 is provided by a NAND gate 26 receiving the complement of signal PB/REC and signal MUTE.

Having thus described particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended to., be limiting. The invention is limited only as defined in the following claims and the equivalents thereto.