A circuit structure of the feedforward type with programmable zeroes, particularly for synthesizing time-continual filters

Abstract: This invention relates to a circuit structure of the feedforward type with programmable zeroes, particularly for synthesizing time-continual filters. This structure comprises a pair of amplification cells (14,15) interconnected at at least one interconnection node (A) and connected between a first signal (Vin) input (IN) of a first cell (14) and an output terminal (U) of the second cell (15, each cell (14,15) comprising a pair of transistors (10,2;6,7) which have a conduction terminal in common and have the other conduction terminals coupled respectively to a first voltage reference (Vcc) through respective bias members (3,4;9,11). The structure further comprises a circuit leg (13) connecting a node (X) of the first cell (14) to the output terminal (U) and comprising a transistor (8) which has a control terminal connected to the node (X) of the first cell (14), a first conduction terminal connected to the output terminal (U), and a second conduction terminal coupled to a second voltage reference (GND) through a capacitor (Cc). Thus, a released "zero" can be introduced in the right semiplane of the pole-zero complex plane to improve the flattening of group gain.

Description: This invention relates to a circuit structure of the feedforward type with programmable zeroes for synthesizing time-continual filters, delay chains, and the like, which structure comprises a pair of amplification cells interconnected at at least one interconnection node and connected between a first signal input of a first of said cells and an output terminal of the second of said cells, each cell in the pair comprising a pair of transistors which have a conduction terminal in common and have the other conduction terminals coupled respectively to a first voltage reference through respective bias members. Specifically, the invention relates to a circuit structure as above adapted to implement programmable delay cells having a broad pass-band and flat group delay. The description which follows will cover this specific field of application for convenience of illustration only. Prior Art As is well known, the frequency response of an amplified electric signal bears a relation of amplitude, or modulo, to phase of the signal. More particularly, as the band expands the phase changes, and the so-called group delay diminishes accordingly. Group delay is the derivative of phase with respect to frequency. In theory, a programmable delay of infinite bandwidth would be ideal. In practice, this cannot be achieved using prior art circuits. A programmable delay to meet an applicational requirement, and a sufficiently broad band, can be obtained at most. Conventional circuits designed to fill the demand have used operational amplifiers, which require feedbak control loops and filters of a high order, exhibit high power consumption, and occupy large circuit areas. In conventional feedback circuit structures, the differentiated signals are input to high-frequency circuit nodes, introducing "zeroes" in the transfer function and giving rise to the above-noted drawbacks. In an effort to obviate such drawbacks, a circuit structure has been proposed in the prior art which is of the feedforward type with programmables "zeroes". This prior structure can synthesize time-continual filters by expanding the signal band without appreciably distorting the group delay. Such a structure is described in European Patent Application No. 97830696.7 by the Applicant, for example. While in many ways advantageous and essentially achieving its objectives, the structure disclosed in the above patent application has first and second feedforward circuit legs which introduce a total of three "poles", and only one "zero", in the transfer function. The greater the offset, the greater the delay obtained. That structure represents something of a frequency booster expanding the band. This is achieved at the expense of some circuit complexity and enhanced power consumption. The underlying technical problem of this invention is to provide a circuit structure of the feedforward type with programmable zeroes, particularly for synthesizing time-continual filters, delay chains and the like, which structure has uniquely simple constructional and functional features effective to produce augmented group delay for a very moderate use of power, thereby overcoming the limitations of prior solutions. Another object of the invention is to provide a circuit structure of the feedforward type with programmable zeroes, for synthesizing time-continual filters, delay chains and the like, which is highly reliable in operation and relatively inexpensive to manufacture. Summary of the Invention The principle on which this invention stands is one of having a feedforward type of circuit leg connect the first cell of the structure to the output terminal, the feedforward circuit leg being effective to introduce a "zero" in the right-hand semiplane of the pole-zero complex plane. Based on this principle, the technical problem is solved by a circuit structure as previously indicated being characterized in that it comprises a circuit leg to connect a node of the first cell to said output terminal, said circuit leg comprising a transistor which has a control terminal connected to said node of the first cell, a first conduction terminal connected to said output terminal, and a second conduction terminal coupled to a second voltage reference through a capacitor. The features and advantages of a circuit structure according to the invention will be apparent from the following description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 shows schematically a circuit structure embodying this invention; Figures 2A and 2B are plots of frequency response for the structure shown in Figure 1; and Figure 3 shows schematically a two-dimensional pole-zero map having a real axis on the abscissa and an imaginary axis on the ordinate. Detailed Description Referring to the drawings, in particular to the example of Figure 1, a feedforward type of a circuit structure having programmable zeroes, for synthesizing time-continual filters, delay chains and the like, according to this invention, is shown generally at 1 in schematic form. For example, the inventive structure can be used in a transverse filter to implement a Hilbert transform in the processing of data in PRML perpendicular recording read channels. Of course, other applications would be possible as well. The structure 1 comprises a first circuit portion 14 and a second circuit portion 15 connected to each other. These portions are essentially amplification cells interconnected into a single-ended configuration. The first portion, or cell, 14 comprises a first bipolar transistor 10 having a control or base terminal to receive an input signal Vin. This terminal represents the input terminal In of the circuit structure 1. The first cell 14 further comprises a second bipolar transistor 2 having an emitter terminal connected to the emitter terminal of the first transistor. In essence, the transistors 10 and 2 have respectively at least one conduction terminal in common. The first transistor 10 has the other conduction terminal, namely the collector terminal, coupled to a first supply voltage reference Vcc through a diode 3 which is forward biased to the power supply. The second transistor 2 has its collector terminal coupled to said first supply voltage Vcc through a bias current generator which will deliver a current I. The emitter terminals of the first 10 and second 2 transistors are coupled to a second voltage reference GND through a current generator 5 which will deliver a current 21. The base and collector terminals of the second transistor 2 are connected to each other. Connected in, between the base terminal of the second transistor 2 and the second voltage GND, is a capacitor C. The base terminal of the second transistor 2 also forms an interconnection node A between the first and the second cell of the circuit structure 1. The second cell 15 is essentially identical with the first cell 14, and comprises a first bipolar transistor 6 whose base terminal is connected to the base terminal of the second transistor 2 in the first cell and presents a voltage V1. The second cell further comprises a second bipolar transistor 7 having its emitter terminal connected to the emitter terminal of the first transistor 6. The first transistor 6 has a collector terminal coupled to the first supply voltage reference Vcc through a diode 9 which is forward biased to the power supply. The second transistor 7 has its collector terminal coupled to said first supply voltage Vcc through a bias current generator 11 which will deliver a current I. The emitter terminals of the first 6 and second 7 transistors in the second cell are coupled to the second voltage reference GND through a current generator 12 which will deliver a current 2I. The base and collector terminals of the second transistor 7 are connected to each other. Connected in, between the base terminal of the second transistor 7 and the second voltage GND, is an additional capacitor C. The base terminal of the second transistor 7 in the second cell is also an output terminal U of the circuit structure 1, from which output terminal the output signal Vout is obtained. Advantageously in this invention, the circuit structure 1 further comprises a circuit leg 13 connecting a node X of the first cell to the output terminal U. More particularly, the base terminal of a bipolar transistor 8 is connected to the collector X of the first transistor 10 of the first cell. This bipolar transistor 8 has its collector connected directly to the output terminal U of the circuit structure 1. Furthermore, the emitter terminal of the transistor 8 is connected to the second voltage reference GND through a capacitor Cc. The value of the transconductance of the transistor 8 is given by the product k * g m , where k is a design parameter and g m is the transconductance of the first transistor 10 in the first cell 14 and the first transistor 6 in the second cell 15. All the transistors in the structure 1 are bipolar transistors of the npn type. A dual structure comprising pnp transistors is well within the capabilities of skilled persons in the art. The operation of the circuit structure 1 according to the invention will now be described. The transfer function of the inventive circuit structure is given as: H(s) = Vout Vin = [kg m 3 +s*g m 2 *Cc-2*C*Cc*kg m *s 2 ] [(kg m +sCc)*(g m +2*s*C) 2 ] and if Cc=kcC is selected, with ωo=g m /(2C), then (1) can be written as: H(s) = Vout Vin = [(1+s*kc/(2k*ωo))-(kc*s)/(2*ωo 2 )] [(1+s*kc/(2k*ωo))*(1+s/ωo) 2 ] This transfer function produces a pair of real "zeroes", respectively lying in the right semiplane and the left semiplane of the complex plane pole-zero diagram. As regards the "poles", the structure has two complex conjugate "poles" and one real "pole", all in the left semiplane. As can be seen in Figure 3, the possibility afforded by the circuit structure according to the invention of having a "zero" on the left which lies symmetrically with respect to the "zero" on the right allows an offset to be obtained for the equivalent group delay having substantially four "poles". By associating the capacitor Cc with the transistor 8, the position of the "zero" in the right semiplane can be released. It is as if the signal were further differentiated. In other words, by modifying the capacitance of Cc, and hence the ratio k/Cc, the position of that "zero" can be changed. Thus, a transfer function is obtained wherein the group delay is substantially flat for the signal band of interest, and is unaffected by the real "pole" located in the left semiplane. As a practical example, the flatness of the group delay is maximized when the following set of values are used: k = 1.2; kc = 2; ωo = 2*π*300MHz = 1.88Grad/sec with a group delay error of 1% at 132 MHz. The graphs of Figures 2A and 2B have been plotted using these values. The circuit structure of this invention is quite simple, and has proved capable of a very fast response. Modifications and changes may be made unto the structure described hereinabove, within the invention scope as defined in the appended claims. In particular, different capacitance values may be used for C in each cell, or the inventive structure could be embodied in CMOS technology by replacing each npn bipolar transistor with a corresponding N-channel MOS transistor.

Claims: A circuit structure of the feedforward type with programmable zeroes, particularly for synthesizing time-continual filters, delay chains, and the like, which structure comprises a pair of amplification cells (14,15) interconnected at at least one interconnection node (A) and connected between a first signal (Vin) input (IN) of the first (14) of said cells and an output terminal (U) of the second (15) of said cells, each cell in the pair (14,15) comprising a pair of transistors (10,2;6,7) which have a conduction terminal in common and have the other conduction terminals coupled respectively to a first voltage reference (Vcc) through respective bias members (3,4;9,11), characterized in that it comprises a circuit leg (13) connecting a node (X) of the first cell (14) to said output terminal (U), said circuit leg comprising a transistor (8) which has a control terminal connected to said node (X) of the first cell (14), a first conduction terminal connected to said output terminal (U), and a second conduction terminal coupled to a second voltage reference (GND) through a capacitor (Cc). A structure according to Claim 1, characterized in that said transistor (8) in the circuit leg (13) is an npn bipolar transistor. A structure according to Claim 1, characterized in that said node (X) is a conduction terminal of the first transistor (10) of the first cell (14) coupled to said first voltage reference. A structure according to Claim 1, characterized in that the capacitance value of said capacitor (Cc) is related to the capacitance of a capacitor (C) connected between the output terminal (U) and said second voltage reference (GND). A structure according to Claim 1, characterized in that the transconductance value (g m ) of said transistor (8) in the circuit leg (13) is related to the transconductance of the first transistor (10) in the first cell (14) and of the first transistor (6) in the second cell (15). A structure according to Claim 5, characterized in that said mutual relation is defined by a multiply parameter k. A structure according to Claim 1, characterized in that said transistor (8) in the circuit leg (13) is a bipolar transistor, and that the conduction terminal connected to the output terminal (U) is the collector.

Inventors: CASTELLO RINALDO, CAZZANIGA MARCO, PISATI VALERIO, PORTALURI SALVATORE, CASTELLO, RINALDO, CAZZANIGA, MARCO, PISATI, VALERIO, PORTALURI, SALVATORE
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 2001
Main classification: H03H11
IPC codes: H03H11, H03F1