Filter circuit comprising an amplifier and a capacitor

Abstract: Filter circuit comprising an amplifier and a capacitor (C). The base-emitter path of a first bipolar transistor (1) is incorporated in a first current path between the output electrode of an amplifier element and the capacitor and the base-emitter path of a second bipolar transistor (1) is incorporated in a second current path, the base of this second transistor being connected to the capacitor. The capacitor receives charge and discharge currents both of which are base currents and are thus very small so that the capacitor can be given a small capacitance. The capacitor can thus be incorporated in an integrated circuit. The circuit can be used as a coincidence detector and as a peak detector.

Description: The invention relates to a filter circuit as defined in the precharacterizing part of claim 1. In a generally known circuit the amplifier element, for example a transistor, forms part of a differential amplifier. In operation the voltage across the capacitor is continuously compared with the input voltage. The capacitor is charged or discharged by an output current of the differential amplifier, which current is dependent on the difference between the input voltages. In a balanced state of the circuit the voltage across the capacitor is substantially equal to the input voltage and the output current is substantially zero, with approximately as much charge being applied to the capacitor as being withdrawn from it. If this state is disturbed for some reason, so that the said voltages become unequal, it is restored after some time. This time depends on the capacitance of the capacitor and on the amplification of the differential amplifier. It is apparent therefrom that the circuit has a time constant and behaves as a low-pass filter. If a comparatively large time constant is desired for the circuit, the capacitor should have a considerable capacitance. However, the capacitance may be kept small so that the capacitor can be incorporated in an integrated circuit comprising the other components of the circuit if the charge and discharge currents of the capacitor are small. For this purpose circuits have been proposed with current mirror circuits having a more or less complicated structure in which a current of moderate strength is converted into a very small current. These circuits require a considerable surface area in the integrated circuit. Another generally known circuit is a peak detector in which a capacitor is charged by the peak values of the input voltage of the circuit whereafter it is discharged. Here, too, a time constant is important. A filter circuit known from the patent US 3863080, comprises two input transistors, constituting a differential amplifier. The difference voltage detected between the bases of the input transistors results in a charge or discharge current to a voltage storage circuit. Said currents are, or are reproductions of, the relatively large collector currents of said transistors. So the capacitor used in the voltage storage circuit has a relatively large value. It is, inter alia, an object of the invention to provide an economical filter circuit which is kept small enough to be integrated. To this end, a first aspect of the invention provides a filter circuit as defined in claim 1. Since transistors usually have a large amplification factor β, namely of the order of 100 to several hundreds, the charge and discharge currents of the capacitor are very small, as desired. The circuit is very simple and has no critical components. Advantageous embodiments are defined in the dependent claims. The invention is based on the recognition that in several applications (for instance: a coincidence detector, a peak detector) said current conversion need not be very accurate because the value of the time constant is not critical, so that a much simpler circuit which does not require much surface area in the integrated circuit may be sufficient. The invention will now be described in greater detail by way of example with reference to the accompanying drawings in which Fig. 1 is a basic circuit diagram of a filter circuit according to the invention, Fig. 2 is a block diagram of a coincidence detector including a slightly modified filter circuit, Fig. 3 is a more detailed diagram of some parts of the diagram of Fig. 2, Fig. 4 shows a peak detector including a modification of the filter circuit of Fig. 1 and Fig. 5 shows a peak detector including another modification of the filter circuit. In Fig. 1 the references T1 and T2 denote two amplifier elements, for example, bipolar transistors of the npn type. The base of each of these transistors constitutes a first input electrode, the emitter constitutes a second input electrode and the collector constitutes an output electrode. The emitters of transistors T1 and T2 are interconnected and connected to the collector of a further npn transistor 5. The collector of transistor T1 is connected to the emitter of an npn transistor 1 whose collector is connected to the positive terminal of a power supply source V B and whose base is connected to the base of a pnp transistor 3. The emitter of transistor 3 is connected to the said terminal of source V B and the collector is connected to the emitter of a further pnp transistor 4. The collector of transistor T2 is connected to the emitter of an npn transistor 2 whose collector is connected to the said positive terminal and whose base is connected to a capacitor C, to the base of transistor 4 and to the gate of a field-effect transistor 6 of the NMOS type. The drain of transistor 6 is connected to the positive terminal and the source is connected to the base of transistor T2 and to a resistor 7. The other ends of capacitor C and resistor 7 as well as the negative terminal of source V B and the collector of transistor 4 are connected to ground. The emitter of transistor 5 is connected to a resistor 8 and the base is connected to the junction point of two resistors 9 and 10. The other end of resistor 9 is connected to a positive DC voltage. The other connections of resistors 8 and 10 are connected to ground. The base of transistor T1 is connected to an input terminal I for the circuit. In operation this terminal is connected to a DC voltage V. Transistor 5 and resistor 8 operate as a current source S1 and transistor 6 operates as a buffer circuit for feeding back the voltage across capacitor C to the base of transistor T2 and for making the output voltage of the circuit available, which voltage is substantially equal to the voltage across the capacitor, at an output terminal 0 which is connected to the junction point of the source of transistor 6 and the base of transistor T2 and resistor 7. Transistors T1 and T2 form part of a differential amplifier. The voltage across capacitor C is thereby continuously compared with voltage V. A current i₁ flows through the collector of transistor T1 and the emitter of transistor 1 and a current i₂ flows through the collector of transistor T2 and the emitter of transistor 2, the sum of i₁ and i₂ being always equal to the current of source S1. A current i₁/β n flows to the base of transistor 1. β n is the current gain factor of the npn transistor 1. Similarly, a current i₂/β n , which is a discharge current for capacitor C, flows to the base of transistor 2. The base current of transistor 1 is also the base current of transistor 3 so that the collector current of transistor 3, which is also the emitter current of transistor 4, has the value β p *i₂/β n . β p is the current gain factor of the pnp transistor 3. A current, which is a charge current for the capacitor and whose value is β p times as small as the emitter current of transistor 4, flows from the base of transistor 4 to capacitor C. In this case it is assumed that transistors 1 and 2, which are incorporated in one and the same integrated circuit, have the same β n and that transistors 3 and 4 have the same β p for the same reason. For all transistors of Fig. 1 it is also assumed that the difference between the emitter and collector currents is negligibly small and for transistor 6 the gate current is assumed to be zero. It is apparent from the foregoing that the charge current has the value i₁/β n , i.e this current is equal to the base current of transistor 1. β p does not occur in this expression. If the voltages at the bases of transistors T1 and T2 are unequal, capacitor C is charged or discharged by a current (i₂-i₁ )/β n . Since β n has a large value, this current is very small. The voltage across the capacitor increases or decreases, dependent on the sign of the difference i₂-i₁ so that one of the current i₁ and i₂ increases while the other decreases. As a result the difference i₂-i₁ becomes smaller and after some time it becomes substantially zero. In the balanced state which is then reached the voltage across capacitor C is substantially equal to V. It is apparent from the foregoing that the circuit of Fig. 1 behaves as a circuit having a given time constant and thus as a low-pass filter with a differential amplifier which has a gain factor of approximately 1 due to the feedback. Transistors 1 and 2 can be considered as elements for supplying a current, namely to the base, which current is β n times as small as the current which is applied to the emitter. Similarly, transistor 4 converts an input current into an output current which is β p times as small. Transistors 1 and 3 may be considered as a current mirror circuit for mirroring the collector current of transistor T1, and transistors 3 and 4 may be considered as a current mirror circuit for mirroring the base current of transistor 1. Due to the spreads among components and due to temperature effects, ageing and the like the β's are not well-defined so that the time constant is not well-defined either. The circuit of Fig. 1 is thus only usable in applications in which an exact time constant is not required. Fig. 2 is a block diagram of a circuit in which a filter circuit F is used which is derived from the circuit of Fig. 1. It is a so-called coincidence detector in a picture display device, for example, a television receiver. Such coincidence detectors are described in European Patent Specification 91,719 (PHN 10.326) and in Netherlands Patent Application 8600945 (PHN 11.725) in the name of the Applicant. The contents of both documents are herein incorporated by reference. The detectors described in these documents require capacitors having a large capacitance. In Fig. 2 the reference AND denotes a stage having an AND function, i.e. a coincidence stage receiving two pulsatory signals, namely a line-synchronizing signal LS which is derived from an incoming video signal by means of a synchronizing signal separating stage and a gate signal GS which originates from a line oscillator. The picture display device comprises a number of parts which are not shown, inter alia , a line-synchronizing circuit for synchronizing the signal of the line oscillator with signal LS in known manner. The pulses of gate signal GS have the same repetition frequency as the pulses of signal LS, for example, 15.625 kHz in accordance with the European television standard and a duration which is slightly longer than that of these pulses, for example, 5 µs as compared with 4.7 µs. An output of gate AND operates a switch SW whose master contact is connected to the input of the circuit of Fig. 1. A first selection contact of switch SW is connected to a DC voltage V₁ and a second selection contact is connected to a DC voltage V₂ which is lower than V₁. The signal GS is also applied to the filter circuit F in such a manner that the differential amplifier therein is only operative during the occurrence of the gate pulses. The output of circuit F is connected to a non-inverting input of an amplifier CP, an inverting input of which is connected to a DC voltage V₃. Amplifier CP operates in known manner as a comparison stage for comparing the output voltage of circuit F with voltage V₃ and for supplying a signal which depends on the established difference. If the pulses applied to gate AND occur simultaneously, switch SW is in the state shown in Fig. 2 in which voltage V₁ is applied to the differential amplifier. Whenever a gate pulse occurs, the output voltage of circuit F increases and this voltage substantially does not change therebetween. Capacitor C thus has a hold function. If coincidence subsists, the output voltage of circuit F reaches the target value V₁ after some time and subsequently remains constant. Voltage V₃ has a value between V₁ and V₂. At the instant when the voltage at the output of circuit F exceeds the value V₃, amplifier CP supplies a positive signal to the line-synchronizing circuit (not shown) for switching certain parts of this circuit in known manner. If there is no coincidence between the input signals of gate AND, switch SW is in the other state in which voltage V₂ is applied to circuit F. The final value at the output of circuit F is substantially equal to V₂. In this case or in the case when an initially prevailing coincidence state is lost, the output signal of amplifier CP is negative. The operation of the circuit of Fig. 2 can be explained with reference to the following numerical example. A value of 25 pF is chosen for the capacitance of capacitor C so that the capacitor can be incorporated in the same integrated circuit as the other components of the circuit. If circuit F is to have a reaction time of 300 line periods and if the difference between voltages V₁ and V₂ is equal to 2 V, then it holds that: I * Δt = C * ΔV in which I is the charge of discharge current of capacitor C and Δt = 300 * 5 µs, C = 25 pF and ΔV = 2 V. Consequently, I is approximately equal to 33 nA. If β n = 200, this corresponds to a collector current of approximately 7 µA. The period for generating the switching signal at the output of amplifier CP after coincidence has been established for the first time can be adjusted by way of the choice of voltage V₃. This period is not critical for the application described. Fig. 3 shows a more detailed diagram of a part of an embodiment according to Fig. 2. In Fig. 3 the same elements as in Figs. 1 and 2 have the same reference symbols. As compared with Fig. 1, the filter circuit F is modified because resistor 9 is not connected to a power supply voltage but to the emitter of an npn transistor 11 and because resistor 8 is not connected to ground but to the emitter of a further npn transistor 12 and to a resistor 13 the other end of which is connected to ground. The collector of transistor 11 is connected to a positive DC voltage and the gate signal GS is applied to the base. The base of transistor 12 is connected to that of transistor 5 and the collector is connected to the emitters of two npn transistors 14 and T3. The base of transistor T3 is connected to a positive DC voltage and the line-synchronizing signal LS is applied to the base of transistor 14. The pulses of the two applied signals are directed positively. The collector of transistor 14 is connected to the positive terminal of source V B . A resistor R is connected to the said terminal at one end and to the collector of transistor T3 and to the base of transistor T1 at the other end. It is apparent from Fig. 3 that transistors 5 and 12 are only turned on during the occurrence of the gate pulses so that circuit F only then operates. If the pulses in signal LS occur simultaneously with the pulses in signal GS, transistors 12 and 14 are turned on during the occurrence of the pulses in signal LS while transistor T3 is turned off. A given voltage is present at the base of transistor T1. If the said pulses do not occur simultaneously, transistor T3 is turned on, while transistor 14 is turned off. A voltage which is lower than the first-mentioned voltage is now present at the base of transistor T1. These are the above-mentioned voltages V₁ and V₂, while elements 12 and T3 constitute the gate AND in Fig. 2 with the collector current of transistor T3 as an output signal of the gate. It will be evident that some details of the circuits described may be implemented in a different way. For example, the amplifier elements T1, T2 and T3 may be replaced by elements other than bipolar npn transistors, for example, pnp or field-effect transistors. The circuit of Fig. 1 may also be modified in such a way that it has a completely different function, with capacitor C still being charged and discharged by base currents. Fig. 4 shows a part of such a modification in which a modification with respect to Fig. 1 relates to the base of transistor T2 which is not connected to the output terminal of the circuit but to the collector and to a current source S2, while the input voltage at the base of transistor T1 is not a DC voltage but an AC voltage, for example, a modulated television signal having a negative modulation. The source of transistor 6 connected to the output terminal 0 is not connected to the base of transistor T2. In the case of a suitable dimensioning of the circuit it behaves as a peak detector for supplying a demodulated video signal because capacitor C is charged by the base current of transistor 4 during the occurrence of the synchronizing pulses in the video signal and is discharged therebetween by the base current of transistor 2. The last-mentioned current is determined by source S2 and is substantially constant. Fig. 5 shows another embodiment of a peak detector in which no feedback is used. As compared with Fig. 1, elements 6 and 7 have been omitted. The emitter of transistor 2 is connected to the output terminal 0 and to a current source S3. The base is connected to capacitor C. The base of transistor T2 is connected to a reference voltage V ref and the collectors of transistors T2 and 2 are connected to the positive terminal of source V B . In this case there is also a charge transport for capacitor C in one direction by means of the base current of transistor 4 and in the other direction by means of the base current of transistor 2, which current is substantially constant. In both cases the voltage across capacitor C follows the peak value of the video signal.

Claims: A filter circuit comprising: an amplifier (T1,T2) provided with an amplifier element (T1) having an input electrode (base T1) coupled to an input terminal (I) of the filter circuit for receiving an input voltage (V), current converting means (1,3,4,2) having an input (emitter 1) coupled to an output electrode (collector T1) of said amplifier element (T1) to receive a current (i1), and having an output for charging and discharging a capacitor (C), an output terminal (O) of the filter circuit coupled to the capacitor (C) for making an output voltage available, characterized in that the current converting means (1,3,4,2) for charging and discharging the capacitor (C) comprise: first current converting means (1,3,4) for generating a capacitor charge current in one direction, said current corresponding to a base current of a first bipolar transistor (1) having an emitter coupled to said input (emitter 1) of the current converting means, and a collector coupled to a power supply source (Vb), second current converting means (2) for generating a capacitor charge current in the opposite direction, said current corresponding to a base current of a second bipolar transistor (2) having an emitter coupled to a second input of the current converting means (1,3,4,2) to receive a second current (i2,S3), a base coupled to the capacitor (C), and a collector coupled to the power supply source (Vb). A filter circuit as claimed in Claim 1, characterized in that the first current converting means (1,3,4) comprise a current mirror circuit (3,4) having an input (base 3) coupled to a base of the first bipolar transistor (1) and having an output (base 4) coupled to the capacitor (C). A filter circuit as claimed in Claim 2, characterized in that the current mirror circuit (3,4) comprises, a third transistor (3) having an emitter coupled to a first terminal (Vb) of the power supply source, a base coupled to the base of the first bipolar transistor (1), a fourth transistor (4) having an emitter coupled to a collector of the third transistor (3), a base coupled to the capacitor (C), and a collector being coupled to a second terminal (ground) of the power supply source. A filter circuit as claimed in Claim 3, characterized in that the first (1) and the second (2) bipolar transistors are of a given conductivity type (npn), the third (3) and the fourth (4) bipolar transistor being of the opposite conductivity type (pnp). A filter circuit as claimed in Claim 1, characterized in that a buffer circuit (6) is arranged between the capacitor (C) and an input electrode (base T2) of a second amplifier element (T2) for feeding back the voltage across the capacitor (C) to said first input electrode (base T2) to be compared with the input voltage of the filter circuit (V), said first input electrode (base T2) being coupled to the output terminal (O) of the filter circuit. A filter circuit as claimed in Claim 1, wherein the input voltage (V) of the filter circuit (F) is switchable between two values (V1,V2) in dependence upon an output signal of a coincidence stage (AND) having a first (GS) and a second (LS) pulsary input signal, the filter circuit further comprising a first current source (S1) only being operative during the occurrence of said first pulsary input signal (GS), said first current source (S1) being coupled to a second input electrode (emitter T1) of the amplifier element (T1). A filter circuit as claimed in claim 6, characterized in that the coincidence stage (AND) comprises a further amplifier element (T3) having an input electrode (emitter T3) for receiving the second pulsary input signal (LS) and being only operative during the occurrence of the first pulsary input signal (GS), an output electrode (collector T3) of the third amplifier element (T3) being coupled to a resistor (R) and to the first-mentioned input electrode (base T1) of the amplifier element (T1). A filter circuit as claimed in claim 6, wherein the output terminal (O) of the filter circuit (F) is connected to an input terminal of a comparison stage (CP) for comparing the output voltage of the filter circuit (F) with a reference voltage (V3) within a range between the two values (V1,V2) of the input voltage (V) and for generating a switching signal at an output of the comparison stage (CP). A filter circuit as claimed in claim 1, wherein the filter circuit further comprises a second amplifier element (T2) having first (base T2) and second (emitter T2) input electrodes and an output electrode (collector T2), and a first current source (S1) coupled to second input electrodes (emitters T1,T2) of the first-mentioned (T1) and the second (T2) amplifier elements, and wherein said emitter of said second bipolar transistor (2) is coupled to an input electrode (base T2) and an output electrode (collector T2) of a second amplifier element (T2) and to a second current source (S2). A filter circuit as claimed in Claim 1, characterized in that a second input electrode (emitter T1) of the amplifier element (T1) is coupled to a first current source (S1), and the emitter of the second bipolar transistor (2) is connected to a second current source (S2) and to the output terminal (O) of the filter circuit.

Inventors: MOTTE BRUNO PIERRE JEAN-MARIE, MOTTE, BRUNO PIERRE JEAN-MARIE, Motté, Bruno Pierre Jean-Marie
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1990
Main classification: H03H11
IPC codes: H03H11