Feedforward structure with programmable zeros for synthesizing continuous-time filters, delay lines and the like

Abstract: A feedforward structure with programmable zeros for synthesizing continuous-time filters, delay lines and the like, whose particularity is that it comprises a first cell and a second cell which are cascade-connected, each one of the first and second cells comprising a first pair (1, 2) of bipolar transistors in which the emitter terminals are connected to a current source (5), the first pair of transistors being connected to a second pair of transistors (6, 7), a current source (11) being connected to the emitter terminals of the second pair of transistors, a first high-impedance element (C) being connected between the first and second pairs of transistors, a second high-impedance element (C) being connected in output to the second pair of transistors, a fifth transistor (8) being connected between the collector terminal of a first transistor (1) of the first pair of transistors and a collector terminal of a second transistor (2) of the second pair of transistors, the base terminal of the fifth transistor (8) receiving a signal which is taken from the collector terminal of the first transistor of the first pair of transistors and is taken with a positive sign in the first cell and with a negative sign in the second cell, in order to determine a transfer function with a pair of singularities at the numerator, the second transistors (2, 7) of the first and second pairs being controlled respectively by current sources (4, 9) which have mutually different values.

Description: The present invention relates to a feedforward structure with programmable zeros for synthesizing continuous-time filters, delay lines and the like. More particularly, the invention relates to a feedforward structure with programmable zeros for providing programmable delay cells. It is known that the frequency response of an amplified signal entails a correlation between the frequency response of the amplitude (modulus) of the signal and the frequency response of the phase of said signal. In particular, as the band increases, the phase changes and therefore the group delay is reduced. Group delay is defined as the derivative of the phase with respect to the frequency. Theoretically, the desirable result is a programmable delay with an infinite band. In applications in which it is necessary to have a delay which can be programmed according to requirements while maintaining a wide signal band, this last requirement cannot be achieved by means of conventional circuits. In particular, conventional circuits designed to provide the above conditions resort to operational-amplifier structures which require the use of control loops, with high orders of filtering, high consumption and large areas for their implementation. In conventional structures of the fedback type, derivative signals are introduced in high-frequency points of the circuit, introducing zeros in the transfer function, with the above-described drawbacks. The aim of the present invention is to provide a feedforward structure with programmable zeros for synthesizing continuous-time filters, delay lines and the like, which allows to widen the band of the signal without distorting group delay appreciably. Within the scope of this aim, an object of the present invention is to provide a feedforward structure with programmable zeros for synthesizing continuous-time filters, delay lines and the like which allows to obtain a programmable delay while maintaining a wide signal band. Another object of the present invention is to provide a feedforward structure with programmable zeros for synthesizing continuous-time filters, delay lines and the like which allows frequency amplification of the modulus of a signal without altering its phase response. Another object of the present invention is to provide a feedforward structure with programmable zeros for synthesizing continuous-time filters, delay lines and the like in which said structure is obtained in an open-loop mode. Another object of the present invention is to provide a feedforward structure with programmable zeros for synthesizing continuous-time filters, delay lines and the like having low current consumption and reduced area occupation. Another object of the present invention is to provide a feedforward structure with programmable zeros for synthesizing continuous-time filters, delay lines and the like which is highly reliable and relatively easy to manufacture at low costs. This aim, these objects and others which will become apparent hereinafter are achieved by a feedforward structure with programmable zeros for synthesizing continuous-time filters, delay lines and the like, characterized in that it comprises a first cell and a second cell which are cascade-connected, each one of said first and second cells comprising a first pair of bipolar transistors in which the emitter terminals are connected to a current source, said first pair of transistors being connected to a second pair of transistors, a current source being connected to the emitter terminals of said second pair of transistors, a first high-impedance element being connected between said first and second pairs of transistors, a second high-impedance element being connected in output to said second pair of transistors, a fifth transistor being connected between the collector terminal of a first transistor of said first pair of transistors and a collector terminal of a second transistor of said second pair of transistors, the base terminal of said fifth transistor receiving a signal which is taken from the collector terminal of said first transistor of the first pair of transistors, said signal being taken with a positive sign in said first cell and with a negative sign in said second cell, in order to determine a transfer function with a pair of singularities at the numerator, the second transistors of said first and second pairs being controlled respectively by current sources which have mutually different values. Further characteristics and advantages of the invention will become apparent from the description of preferred but not exclusive embodiments of the feedforward structure according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein: Figure 1 is a circuit diagram of a first embodiment of the feedforward structure according to the present invention; and Figure 2 is a circuit diagram of a second embodiment of the feedforward structure according to the present invention. With reference to the above figures, the feedforward structure according to the present invention is illustrated in Figure 1 as regards the provision of a singularity (zero) in the left half-plane and in Figure 2 as regards the provision of a singularity (zero) in the right half-plane. The circuit of Figure 1 comprises a first circuit section and a second circuit section which are mutually connected. The first circuit section comprises a first bipolar transistor 1, whose base terminal receives an input signal Vin. A diode (3) and a first current source 4 are respectively connected to the collector terminals of the pair of bipolar transistors 1 and 2; the first current source 4 supplies a current I 0 . The emitter terminals of the transistors 1 and 2 are then connected to a second current source 5. A first capacitor C is connected to the base terminal of the bipolar transistor 2. The second circuit section comprises a second pair of bipolar transistors, which are referenced to as third transistor 6 and fourth transistor 7. In the third bipolar transistor 6, the base terminal is connected to the base terminal of the transistor 2 and the collector terminal is connected to the supply voltage. The emitter terminals of the transistors 6 and 7 are biased by a third current source 11 whose value is equal to the second current source. The bipolar transistor 7 is diode-connected, and a second capacitor C, for example equal in value to the first capacitor, is connected to the base terminal of said transistor. A fourth current source 9, whose value is (1+K) times the value of the first current source 4, is connected to the common node between the base terminal and the collector terminal of the transistor 7. Finally, a fifth bipolar transistor 8 is connected, by means of its base terminal, to the collector terminal of the transistor 1 and, by means of its collector terminal, to the common node between the base terminal and the collector terminal of the bipolar transistor 7, thus providing an additional connection between the first and second circuit sections. A fifth current source 10 is connected to the emitter terminal of the bipolar transistor 8 and its value is K times the value of the first current source 4. In Figure 2, identical reference numerals designate identical elements. The difference between the circuit of Figure 1 and the circuit of Figure 2 resides in the fact that the fifth bipolar transistor 10 receives in input a signal which is taken from the collector terminal of the bipolar transistor 1, with its sign changed. In this manner, the transfer function of the circuit of Figure 1 is given by: H1(s) = Vout/Vin = (1 + sK/W 0 ) / (1 + s/W 0 ) 2 while the transfer function of the circuit of Figure 2 is given by: H2(s) = Vout/Vin = (1 - sK/W 0 ) / (1 + s/W 0 ) 2 The global transfer function of the two above-described cells, which are cascade-connected and shown respectively in Figures 1 and 2, is thus given by the product of the two transfer functions H1(s) and H2(s). It is thus possible to obtain: H(s) = [1 - (Ks/W 0 ) 2 ] / (1 + S/W 0 ) 4 This transfer function produces a pair of real zeros which are arranged respectively in the left half-plane and in the right half-plane if the value of K is chosen equal for the circuit of Figure 1 and for the circuit of Figure 2. Otherwise, if one chooses K = K1 and K = K2, with K1 different from K2, then the resulting transfer function has a term in "s" at the numerator and is of the 1+βs-alpha s 2 type. In practice, it has been observed that the structure according to the invention fully achieves the intended aim and objects, since it allows, by means of a simple cascade connection of two cells, to obtain a transfer function with two real zeros. In this case, the group delay remains flat for the band of interest of the signal. If the transfer functions of the two cells provided according to the present invention are instead obtained so that their respective numerators have mutually different terms in K, i.e., so that K=K1 for a first transfer function and K=K2 for a second transfer function, it is possible to obtain a global transfer function (determined by the product of the transfer functions of the two cascade-connected cells) wherein a polynomial of the 1+βs-alpha s 2 is present at the numerator. In this second case, by varying the terms K1 and K2 it is possible to have a preset variation of the group delay without varying the band of the signal. The feedforward structure thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept. Thus, for example, the capacitors C of the two cells can be chosen so as to have mutually different values. Finally, all the details may be replaced with other technically equivalent elements. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims: A feedforward structure with programmable zeros for synthesizing continuous-time filters, delay lines and the like, characterized in that it comprises a first cell (Fig 1) and a second cell (Fig 2) which are cascade-connected, each one of said first and second cells comprising a first pair (1, 2) of bipolar transistors in which the emitter terminals are connected to a current source (5), said first pair of transistors being connected to a second pair (6, 7) of transistors, a current source (11) being connected to the emitter terminals of said second pair of transistors, a first high-impedance element (C) being connected between said first and second pairs of transistors, a second high-impedance element (C) being connected in output to said second pair of transistors, a fifth transistor (8) being connected between the collector terminal of a first transistor (1) of said first pair of transistors and a collector terminal of a second transistor (7) of said second pair of transistors, the base terminal of said fifth transistor receiving a signal which is taken from the collector terminal of said first transistor of the first pair of transistors, said signal being taken with a positive sign in said first cell and with a negative sign in said second cell, in order to determine a transfer function with a pair of singularities at the numerator, the second transistors (2, resp. 7) of said first and second pairs being controlled respectively by current sources (4, resp. 9) which have mutually different values. A feedforward structure according to claim 1, characterized in that the current sources of said second transistors of said first and second pairs of transistors are connected to the collector terminals of said second transistors. A feedforward structure according to claim 1, characterized in that said second transistors of said first and second pairs of transistors are diode-connected. A feedforward structure according to claim 2, characterized in that said fifth transistor has a current source connected to its emitter terminal. A feedforward structure according to claim 4, characterized in that said current source connected to the emitter terminal of said fifth transistor has a value equal to K times the value of the current source connected to the collector terminal of said second transistor of the first pair of transistors. A feedforward structure according to claim 1, characterized in that the current sources that control the second transistors of said first pair of transistors and of said second pair of transistors are connected to the collector terminals of said second terminals, the value of the current source of said second pair of transistors being equal to (1+K) times the value of the current source of said first pair of transistors. A cell with feedforward structure having programmable zeros, characterized in that it comprises a first pair (1, 2) of bipolar transistors and a second pair (6, 7) of bipolar transistors which are mutually connected with a first high-impedance element (C) interposed, a second high-impedance element (C) being provided at the output of said second pair of transistors, a further transistor (4 Fig 1, 8 Fig 2) being connected between a first transistor of said first pair and a second transistor of said second pair. A cell with feedforward structure according to claim 7, characterized in that in said further transistor the base terminal is connected to the collector terminal of said first transistor of the first pair of transistors and the collector terminal is connected to the collector terminal of the second transistor of said second pair of transistors. A cell with feedforward structure according to claim 8, characterized in that a signal taken from the collector terminal of said first transistor of the first pair of transistors is input to the base terminal of said further transistor. A cell with feedforward structure according to claim 8, characterized in that a signal taken from the collector terminal of said first transistor of the first pair of transistors is input, with its sign changed, to the base terminal of said further transistor. A cell with feedforward structure according to claim 7, characterized in that current sources are respectively connected to the collector terminals of said second transistors of the first and second pairs of transistors, the value of the current source connected to the second transistor of said second pair being equal to (1+K) times the value of the current source connected to the second transistor of said first pair. A cell with feedforward structure according to claim 1, characterized in that a current source having a value which is K times the value of the current source connected to the collector terminal of the second terminal of said first pair of transistors is connected to the emitter terminal of said further transistor.

Inventors: PISATI VALERIO, PORTALURI SALVATORE, PISATI, VALERIO, PORTALURI, SALVATORE
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1997
Main classification: H03F1
IPC codes: G05B19, H03H11