ELECTRONIC CIRCUIT COMPRISING A DIFFERENTIAL PAIR PROVIDED WITH DEGENERATION MEANS FOR DEGENERATING A TRANSCONDUCTANCE OF THE DIFFERENTIAL PAIR

Abstract: An electronic circuit comprising a differential pair (T1, T2) provided with degeneration means (DGMNS) for degenerating a transconductance of the differential pair (T1, T2), and an auxiliary circuit (B) for supplying a control voltage (U) to a control terminal (CNTRL) of the degeneration means (DGMNS). The auxiliary circuit (B) also accomplishes a DC-biasing of the differential pair (T1, T2). Said control voltage (U) and the DC-biasing are such that the transconductance of the differential pair (T1, T2) is virtually independent of the value of the control voltage (U) and virtually independent of the DC-biasing of the differential pair (T1, T2). The auxiliary circuit (B) comprises a further differential pair (T3, T4) provided with further degeneration means (FDGMNS) for degenerating a transconductance of the further differential pair (T3, T4). The auxiliary circuit (B) is arranged for supplying a further control voltage (UF) to a control terminal (FCNTRL) of the further degeneration means (FDGMNS). The control voltage (U) is dependent on the further control voltage (UF). The further control voltage (UF) and the control voltage (U) can, for instance, be the same voltage. The auxiliary circuit (B) comprises means (B2) for generating a desired current ratio (n) between a first branch (Br1) and a second branch (Br2) of the further differential pair (T3, T4). The auxiliary circuit (B) comprises means (B1) for generating a first DC-voltage (VP) to a first input terminal of the further differential pair (T3, T4), and a second DC-voltage (VM) to a second input terminal of the further differential pair (T3, T4).

Description: Electronic circuit comprising a differential pair provided with degeneration means for degenerating a transconductance of the differential pair The invention relates to an electronic circuit comprising a differential pair which is provided with degeneration means for degenerating a transconductance of the differential pair, and an auxiliary circuit for providing a control voltage to a control electrode of the degeneration means. Such an electronic circuit is known from US patent US 5, 642, 078. The known circuit is shown in Fig. 1 and comprises inter alia a differential pair consisting of a field effect transistor 22 and a field effect transistor 23. The differential pair further comprises a field effect transistor 16 which is connected by a source to the source of the field effect transistor 23 and which is connected by a drain to the source of the field effect transistor 22. The gate of the field effect transistor 16 is connected to a bias control circuit. The required direct currents for the differential pair are supplied by a current source 20 and a current source 21. The field effect transistor 16 is set for its linear operation range and serves to degenerate the transconductance of the differential pair. It is a disadvantage of the known electronic circuit that the transconductance of the differential pair is not accurately defined. It is an object of the invention to provide an electronic circuit comprising a differential pair whose transconductance is accurately defined. According to the invention, the electronic circuit mentioned in the opening paragraph is for this purpose characterized in that the auxiliary circuit is designed for providing a DC biasing of the differential pair, and in that the auxiliary circuit is designed such that the transconductance of the differential pair is substantially independent of the value of the control voltage and of the DC biasing of the differential pair. Since not only the control voltage at the control electrode of the degeneration means is regulated, as in the known circuit, but also the DC currents of the differential pair are regulated, it can be achieved that the transconductance of the differential pair becomes equal to a desired reference value. The reference value may be obtained, for example, through the use of a reference resistance. The reference resistance may be obtained in various alternative ways, for example through the use of a ratio of resistance values, a capacitor, a ratio of capacitance values, a voltage source, a ratio of voltage values, a current source, a ratio of current values, etc. An embodiment of an electronic circuit according to the invention is characterized in that the auxiliary circuit comprises a further differential pair comprising further degeneration means for degenerating a transconductance of said further differential pair, and in that the auxiliary circuit is furthermore designed for providing a further control voltage to a control electrode of the further degeneration means, and in that the control voltage is dependent on said further control voltage. An electronic circuit according to the invention is implemented thereby in a simple manner. If so desired, the degeneration means and the further degeneration means may be constructed in a similar manner. It is also possible to choose the control voltage to be equal to the further control voltage. An embodiment of an electronic circuit according to the invention is characterized in that the auxiliary circuit comprises current bias means for providing a desired current ratio between a first current branch and a second current branch of the further differential pair. This renders it simpler to obtain a correct DC biasing of the differential pair. An embodiment of an electronic circuit according to the invention is characterized in that the current bias means are provided with a first input which is coupled to the first current branch, a second input which is coupled to the second current branch, and an output which is coupled to the control electrode of the further degeneration means. The current bias means control the control electrode of the further degeneration means such that the ratio of the respective currents of the first and the second current branch is equal to the desired current ratio. An embodiment of an electronic circuit according to the invention is characterized in that the auxiliary circuit furthermore comprises voltage bias means for providing a first bias voltage to a first input of the further differential pair and a second bias voltage to a second input of the further differential pair, and in that the further differential pair further comprises first current supply means connected in series with the first current branch and second current supply means connected in series with the second current branch, and in that the currents supplied by said first current supply means and second current supply means are dependent on the difference between the first bias voltage and the second bias voltage. It is achieved thereby that not only the desired current ratio of the first and the second current branch is equal to the desired current ratio, but that also the absolute values of the currents through the first and the second current branch are defined. An embodiment of an electronic circuit according to the invention is characterized in that said currents are approximately linearly dependent on the difference between the first bias voltage and the second bias voltage. This renders it possible to generate said currents in a simple manner. An embodiment of an electronic circuit according to the invention is characterized in that the degeneration means comprise a field effect transistor which is set for its linear operation range, and in that the further degeneration means comprise a further field effect transistor which is set for its linear operation range. A further linearizing of the differential pair is realized thereby, so that the distortion of the differential pair is reduced. An embodiment of an electronic circuit according to the invention is characterized in that the value of the first bias voltage corresponds to the highest value of the first bias voltage at which the electronic circuit can still function correctly, and in that the value of the second bias voltage corresponds to the lowest value of the second bias voltage at which the electronic circuit can still function correctly. This has the advantage that the differential pair continues to function correctly also at the widest possible signal control range at the inputs of the differential pair. A further advantageous embodiment of an electronic circuit according to the invention is defined in claim 10. The invention will be explained in more detail below with reference to the accompanying drawing, in which : Fig. 1 shows a known electronic circuit with a differential pair and a field effect transistor for degenerating the transconductance of the differential pair, Fig. 2 is a circuit diagram of a differential pair which is known per se and which is provided with a field effect transistor for degenerating the transconductance of the differential pair, and which can serve as a basic circuit for realizing a differential pair according to the invention, Fig. 3 is a circuit diagram of an auxiliary circuit designed for providing a DC biasing of a differential pair, Fig. 4 is a circuit diagram of voltage bias means for providing a first and a second bias voltage to the inputs of the differential pair of the auxiliary circuit, Fig. 5 shows an alternative circuit diagram of voltage bias means which supplies not only the first and the second bias voltage but also a common-mode voltage and a cascode voltage for the auxiliary circuit, Fig. 6 shows a circuit diagram of an embodiment of the further differential pair, Fig. 7 shows a circuit diagram of a current-generating circuit for supplying the current Ib, Fig. 8 shows a circuit diagram of a so-called gm/C filter known from the state of the art, Fig. 9 shows an electronic circuit comprising several differential pairs which are all controlled by an auxiliary circuit according to the invention, and Fig. 10 shows a circuit diagram which is an alternative to the auxiliary circuit of Fig. 3. The same components or elements have been given the same reference symbols in these Figures. Fig. 2 shows an embodiment of a differential pair which is known per se. The differential pair comprises a first transistor T,, a second transistor T2, degeneration means DGMNS constructed with a field effect transistor TDG, a current source Il, and a current source 12. The field effect transistor Too is connected with its source and drain between the sources of the transistors T, and T2. The gate of the field effect transistor Too is connected to a control electrode CNTRL for receiving a control voltage U between the control electrodes CNTRL and a supply terminal Vss. The current source Il is connected between a further supply terminal VDD and the source of the transistor Ti. The current source 12 is connected between the further supply terminal VDD and the source of the transistor T2. The gate of the transistor T, is connected to a terminal 1 which is a first input of the differential pair. The gate of the transistor T2 is connected to a terminal 2 which is a second input of the differential pair. The drain of the first transistor T is connected to a terminal 3 which is a first output of the differential pair. The drain of the transistor T2 is connected to a terminal 4 which is a second output of the differential pair. The values of the currents supplied by the first and the second current source Il and 12 are referenced Ib. The field effect transistor Too is set for its linear operation range, so that it behaves approximately as a linear resistance whose value can be varied by means of the control voltage U. The control voltage U and the currents Ib are supplied from the auxiliary circuit B according to the invention shown in Fig. 3. Fig. 3 shows the circuit diagram of the auxiliary circuit B. The auxiliary circuit B comprises a further differential pair which is composed in a similar manner as the differential pair of Fig. 2. Transistors T3 and T4 correspond to the transistors T, and T2, respectively. A further field effect transistor TDGF corresponds to the field effect transistor Too. Current sources TLa and TL correspond to the current sources Il and 12, respectively. The auxiliary circuit B further comprises voltage bias means B,, current bias means B2, and a current-generating circuit B3. A common-mode voltage Vcm is supplied to the voltage bias means Bl. The voltage bias means B, supply a first bias voltage Vp to the gate of the transistor T3 and a second bias voltage VM to the gate of the transistor T4. The difference between the first bias voltage Vp and the second bias voltage VM is indicated with V. The first bias voltage Vp is equal to the common-mode voltage Vcm plus half the differential voltage V. The second bias voltage VM is equal to the common-mode voltage Vcm minus half the differential voltage V. The current-generating circuit B3 is connected so as to receive the differential voltage V and supplies the currents Ib to the further differential pair of the auxiliary circuit B and to the differential pair of Fig. 2. The current bias means B2 are provided with a first input inl which is connected to the drain of the transistor T3, a second input in2 which is connected to the drain of the transistor T4, and an output out which is connected to the gate of the further field effect transistor TDGF and to the further control electrode FCNTRL for supplying a further control voltage UF. The circuit operates as follows. The value of the currents Ib supplied by the current-generating circuit B3 can be calculated from equation 1: Ib = m.V/R (1) in which : R is the value of a reference resistance, and m is a scaling value. The current bias means B2 supply the currents through the transistor T3 and the transistor T4 such that they have a ratio of 1 : n through modification of the further control voltage UF at the gate of the further field effect transistor TDGF Since the current ratio of the transistors T3 and T4 is controlled by the current bias means B2, and since the currents supplied by the current sources TL, and TL2 are also controlled by the current-generating circuit B3, the transconductance gm of a differential pair controlled by the auxiliary circuit B is defined by equation 2 : 1, t-l "' The scaling factor m is preferably independent of the temperature and independent of process parameters. This may be realized in a simple manner through standard IC design techniques. The reference value R may be accurately defined in a simple manner, for example through the use of a discrete resistor. It follows from equation 2 that the transconductance gm of a differential pair is accurately defined then. Fig. 4 is a circuit diagram of a first embodiment of the voltage bias means B,. The voltage bias means B, comprise transistors T5 to Tg, a diode D, a current source 13, and a voltage source Veau. The sources of the transistors T5 and T7 are connected to the further supply terminal VDD. The drains of the transistors T5 and T7 are interconnected, as are the sources of the transistors T6 and T8. The drain of the transistor T6 is connected to the gate of the transistor T5, to a terminal Vp, and to a first electrode of the diode D. The drain of the transistor T8 is connected to the gate of the transistor T7, to a terminal VM, and to a second electrode of the diode D. The current source I is connected between the terminal VM and the supply terminal Vss. The voltage source Vcas is connected between the gate of the transistor T6 and the further supply terminal VDD The transistors T6 and T8 are connected as a differential pair. Since the gates of the transistors T6 and T8 are interconnected, and the sources of the transistors T6 and T8 are interconnected, the transistors T6 and Tg pass equal currents. The current supplied by the current source IB has a value 21b, so the currents running through the transistors T and T8 are equal to lIb. (Since the gate-source voltages of the transistors T5 and T7 are different, the currents through the transistors T5 and T7 are different, but the sum of the currents through the transistors T5 and T7 is equal to 21b.) So the current flowing through the diode D is equal to lIb. The voltage drop across the diode D is equal to the differential voltage V. Since the gates of the transistors T5 and T7 are coupled to the terminals Vp and VM, respectively, the voltages Vp and VM are defined. The diode D may be replaced by an alternative element across which a voltage drop can be generated such as, for example, a resistor. Fig. 5 shows a circuit diagram of a second embodiment of the voltage bias means B I. The circuit comprises transistors T5, T6, T9, and T, o. The circuit further comprises the diode D, resistors R, and R2, and current sources 14 and I5. The sources of the transistors T5 and Tq are connected to the further supply terminal VDD. The sources of the transistors T6 and TIO are connected to the drain of the transistor T5 and the drain of the transistor T9, respectively. The gates of the transistors T6 and Tio and the drain of the transistor Tio are interconnected. The gate of the transistor T5 is connected to the drain of the transistor T6, to the terminal Vp, to the first electrode of the diode D, and to a first electrode of the resistor R I The second electrode of the resistor R, is connected to a first electrode of the resistor R2 and to the gate of the transistor T9. The second electrode of the diode D is connected to a second electrode of the resistor R2 and to the terminal VM. The current source 14 is connected between the terminal VM and the supply terminal Vss. The current source 15 is connected between the drain of the transistor Tao and the supply terminal Vss. The voltage source Vcas required in Fig. 4 is not necessary in the circuit of Fig. 5 because the voltage Vcas is generated by the circuit itself. A further advantage of the circuit of Fig. 5 is that it supplies the required common-mode voltage Vcm (see Fig. 3). Again, the diode D may be replaced by an alternative element in this circuit. It is also possible, however, to leave out the diode D and not replace it with another element. The function of the diode D is taken over in that case by the series arrangement of the resistors R, and R2. It should be noted that it may be necessary in the case of a differential pair as shown, for example, in Fig. 2, that there is a common mode voltage control at the inputs thereof. The manner in which this is realized is generally known. A circuit may be used for this, for example, such as the arrangement of transistors T5, T6, T7, Tg and the voltage source Vcas as shown in Fig. 4. To define the transconductance gm of a differential pair as accurately as possible (so that no significant deviation arises from the calculated transconductance in accordance with equation 2), it is advisable to choose the DC voltages at terminals 1 and 2 (see Fig. 2) to be equal as much as possible to the DC voltages of the first bias voltage Vp and the second bias voltage VM, respectively (see Fig. 3). Fig. 6 shows the circuit diagram of an embodiment of the further differential pair. The difference with the further differential pair as shown in Fig. 3 is that the current bias means B2 are constructed with a current mirror CM. The input in 1 of the current mirror CM is connected to the drain of the transistor T3. The output out of the current mirror CM is connected to the drain of the transistor T4 and to the gate of the further field effect transistor TDGF. The further differential pair may alternatively be constructed with other types of differential pairs, for example cascoded differential pairs. The current mirror CM may simply be composed from two transistors in accordance with the state of the art. More complicated types of current mirrors may also be used, for example cascoded current mirrors. Fig. 7 is a circuit diagram of an embodiment of a current-generating circuit B4 for supplying the currents Ib. The circuit comprises transistors T ;) to T) ?, a resistor R3, and a diode D2. The sources of the transistors T13 to Tn are connected to the supply tenminal Vss The gates of the transistors TI3 to Tj ? are interconnected. The drain of the transistor T14 is connected to the gate of the transistor T ; 4 and to the drain of the transistor Tri2. The resistor R3 is connected between the source of the transistor T12 and the further supply terminal Voo. The drain of the transistor Tl3, the drain of the transistor TI, and the gates of the transistors Tn and T) 2 are interconnected. The diode D2 is connected between the source of the transistor T, 1 and the further supply terminal VDD. The drains of the transistors T, 5 to T ; 7 supply the desired currents Ib. It should be noted that the current-generating circuit B4 may be provided with a start circuit in accordance with the state of the art for preventing starting problems. The current-generating circuit B4 of Fig. 7 will now be explained in more detail in combination with Fig. 10. In Fig. 10, the current-generating circuit B3 of Fig. 3 is replaced by a current-generating circuit B4. The differential voltage V between the terminals Vp and VM is not supplied to the current-generating circuit B4 in the auxiliary circuit of Fig. 10. A further difference with Fig. 3 is that a current Ib is supplied to the voltage bias means 8). So the current 21b supplied by the current source 13 in Fig. 4 is supplied from the currentgenerating circuit B4 or, if the voltage bias means B, are constructed in accordance with Fig. 5, the currents Ib supplied by the current sources 14 and 15 are supplied from the currentgenerating circuit B4 here. Now if the diode D2 of Fig. 7 is constructed so as to be identical to the diode D of Fig. 4 or Fig. 5, the voltage V across the diode D2 will be substantially equal to the differential voltage V of Fig. 3 or Fig. 10. The voltage V across the diode D2 of Fig. 7 accordingly is not directly a result of the voltage V of Fig. 3 or Fig. 10, but is generated by the current-generating circuit B4. This renders it possible for an electrode of the diode D2 to be at a fixed potential. In the present case, the anode of the diode D2 is connected to the further supply terminal VDD. This renders it easier to realize the current-generating circuit B4 as compared with the current-generating circuit B3. Fig. 7 shows that three currents Ib are supplied from the drains of the transistors T ; 5 to Tn. This is merely an example, the desired number of currents Ib may be adapted in a simple manner. Suppose, for example, that the auxiliary circuit B of Fig. 10 is used, and that three differential pairs as shown in Fig. 2 are controlled by means of the further control signal UF, and that the circuit of Fig. 5 is used for the voltage bias means BX. Each differential pair requires two currents Ib. The further differential pair also requires two currents Ib. The voltage bias means B, also require two currents Ib. The number of currents Ib to be supplied by the current-generating circuit B4 in this example is equal to 10. It should be noted that the currents Ib (in this example) cannot be derived directly from the current-generating circuit B4 for the differential pairs and the further differential pair. This is because these currents are to be reversed in direction first, for example by means of a current mirror. Fig. 8 is a circuit diagram of a gm/C filter which is known per se. In this example, the gm/C filter comprises three differential pairs DFI, DF2, and DF3, which may be constructed, for example, as shown in the circuit diagram of Fig. 2. The outputs of the differential pairs DF to DF3 are coupled to respective capacitors Ci to C3. The gm/C filter further comprises a building block TS. The building block TS supplies control signals U, to U3 to the gates of the degeneration field effect transistors of the differential pairs DF, to DF3. Such gm/C filters are used, for example, in so-called sigma-delta AD converters. A disadvantage of known gm/C filters is that the transconductance of the differential pairs DF, to DF3 is not accurately defined. This may lead to a wide spread in the zero points with the use in a sigma-delta AD converter. This disadvantage is eliminated in that the differential pairs DF, to DF3 are controlled by the auxiliary circuit B according to the invention as shown in Fig. 9. The gates of all degeneration field effect transistors are controlled here with the same control voltage U. Moreover, each differential pair DF, to DF3 receives the same current Ib. Starting from the assumption that the differential pairs DF, to DF3 are of the same construction, the transconductance values of said differential pairs will be the same. If it should be desirable to obtain twice the transconductance for the differential pair DF2 only, for example, a simple solution is to connect two differential pairs in parallel. The circuit of Fig. 9 may be used, for example, for realizing an improved gm/C filter. The differential pairs DF) to DF3 are coupled to one another in the same manner as in Fig. 8, and capacitors C, to C3 are also added in the same manner. The electronic circuit may be implemented both with discrete components and in the form of an integrated circuit. Except for the degeneration field effect transistors, the other field effect transistors may be replaced with bipolar transistors. It is also possible to replace all p-conductivity type transistors with n-conductivity type transistors provided all nconductivity type transistors are replaced with p-conductivity type transistors at the same time.

Claims: CLAIMS : 1. An electronic circuit comprising a differential pair (T"T2) which is provided with degeneration means (DGMNS) for degenerating a transconductance of the differential pair (T,, T2), and an auxiliary circuit (B) for providing a control voltage (U) to a control electrode (CNTRL) of the degeneration means (DGMNS), characterized in that the auxiliary circuit (B) is designed for providing a DC biasing of the differential pair (TI, T2), and in that the auxiliary circuit (B) is designed such that the transconductance of the differential pair (T,, T2) is substantially independent of the value of the control voltage (U) and of the DC biasing of the differential pair (T,, T2). 2. An electronic circuit as claimed in claim 1, characterized in that the auxiliary circuit (B) comprises a further differential pair (T3, T4) comprising further degeneration means (FDGMNS) for degenerating a transconductance of said further differential pair (T3, T4), and in that the auxiliary circuit (B) is furthermore designed for providing a further control voltage (UF) to a control electrode (FCNTRL) of the further degeneration means (FDGMNS), and in that the control voltage (U) is dependent on said further control voltage (UF). 3. An electronic circuit as claimed in claim 2, characterized in that the auxiliary circuit (B) comprises current bias means (B) for providing a desired current ratio (n) between a first current branch (Brl) and a second current branch (Br2) of the further differential pair (T3, T4). 4. An electronic circuit as claimed in claim 3, characterized in that the current bias means (B2) are provided with a first input (inl) which is coupled to the first current branch (Br,), a second input (in2) which is coupled to the second current branch (Br2), and an output (out) which is coupled to the control electrode (FCNTRL) of the further degeneration means FDGMNS). 5. An electronic circuit as claimed in claim 4, characterized in that the auxiliary circuit (B) furthermore comprises voltage bias means (B l) for providing a first bias voltage (Vp) to a first input of the further differential pair (T3, T4) and a second bias voltage (VM) to a second input of the further differential pair (T3, T4), and in that the further differential pair (T3, T4) further comprises first current supply means (TLI) connected in series with the first current branch (Br,) and second current supply means (TL2) connected in series with the second current branch (Br2), and in that the currents supplied by said first current supply means (TLs) and second current supply means (TL2) are dependent on the difference (V) between the first bias voltage (Vp) and the second bias voltage (VM). 6. An electronic circuit as claimed in claim 5, characterized in that said currents are approximately linearly dependent on the difference (V) between the first bias voltage (Vp) and the second bias voltage (VM). 7. An electronic circuit as claimed in claims 1, 2, 3, 4, 5, or 6, characterized in that the degeneration means (DGMNS) comprise a field effect transistor (ToG) which is set for its linear operation range, and in that the further degeneration means (FDGMNS) comprise a further field effect transistor (TDGF) which is set for its linear operation range. 8. An electronic circuit as claimed in claim 2, 3, 4, 5, 6, or 7, characterized in that the value of the control voltage (U) is approximately equal to the value of the further control voltage (UF). 9. An electronic circuit as claimed in claim 5, 6, 7, or 8 in as far as it is dependent on claim 5, characterized in that the value of the first bias voltage (Vp) corresponds to the highest value of the first bias voltage (Vp) at which the electronic circuit can still function correctly, and in that the value of the second bias voltage (VM) corresponds to the lowest value of the second bias voltage (VM) at which the electronic circuit can still function correctly. 10. An electronic circuit as claimed in claim 3, characterized in that the current bias means (B2) comprise a current mirror (CM) which is provided with an input (inl) coupled to the first current branch (Br,) and an output (out) coupled to the second current branch (Br2), said output (out) being coupled to the control electrode (FCNTRL) of the further degeneration means (FDGMNS).

Inventors: BLANKEN PIETER G, BLANKEN, PIETER, G.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2001
Main classification: H03F3
IPC codes: H03F3, H03G3