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Three voltage mode universal filter biquads using digitally programmable second generation current conveyor (CCII) are presented in this paper. Salient features of proposed filter configurations include realization of all the filter functions except allpass, independently programmable filter parameters, no component matching constraint and low sensitivity figure. Component count of proposed filter configurations is proved to be minimal for offering aforementioned set of features. Workability of proposed circuit is verified by including the SPICE simulations.

Programmable characteristic of analog block is essential in many applications. It is required to compensate for the undesired parameter variation caused by temperature and process. Although analog programming is used in number of the applications but the limitation on the allowable range of analog tuning voltage makes it inconvenient for low-voltage applications. Hence, in these applications, the digital control is more attractive [

Digital programming techniques in contrast to analog programming techniques not only provide better accuracy in avoiding parameter race especially in low voltage environment [

The CCII has been most promising candidate for analog signal processing/generation since its inception in 1970 [

Digitally programmable universal filter (DPUF) structures are versatile and attractive from IC realization viewpoint. A systematic comparison presented in Section 5 shows that almost all the reported DPUFs have one or more than one of the following drawbacks: (1) realization of only three or lesser number of filter functions; (2) use of large number of active and/or passive elements; (3) programming of only one filter parameter or constrained programming.

Thus, the prime objective of this paper is to present DPUF configurations, which can provide all these essential features by using minimum possible numbers of active/passive components. Three voltage mode (VM) multiple input single output DPUFs are proposed in this paper. All these DPUFs realize all the five filter functions except allpass (AP) by proper selection of input variables. The proposed DPUFs offer attractive features such as (1) independently programmable filter parameters namely pole frequency (ω_{0}) and quality factor (Q); (2) cascadability; (3) no component matching constraint; and (4) low sensitivity figure. For obtaining all these features, proposed DPUFs use three digitally programmable CCIIs (DPCCIIs), one voltage follower (VF) and five passive components including two capacitors only. This component count is proved to be minimal for any current mode (CM)/VM DPUFs, which provides all these features. Some additional important facts of proposed DPUF configurations need to be highlighted. Actually, the first two proposed configurations do not need VF but it is required to incorporate cascadability feature. All the three DPUFs use one floating capacitor, which is less attractive for integration [

This paper is organized as follows. Starting from the Introduction, Section 2 briefly illustrates the use of DPCCII. Section 3 presents the realization of proposed DPUF. Section 4 discusses sensitivity performance and parasitic effects. Section 5 presents the systematic comparison of proposed DPUFs. Section 6 deals with simulation result and finally, paper is concluded in Section 7.

Literature reports number of DPCC structures [

where K denotes the decimal equivalent of applied n-bit codeword (=a_{n}_{−1}, a_{n}_{−2}, ・・・, a_{0}). It is given as

where j denotes the j^{th} bit of applied codeword. Parameter α (and β) denotes the non-ideal voltage (and current) transfer gains of DPCCII. Both the gain parameters α and β are ideally unity. Plus (+) and minus sign (−) in matrix (1) denotes the positive and negative DPCCII blocks respectively. Power integer m = −1 denotes current attenuation (K^{−1} = K^{−} = 1/K) in range “1 to 1/(2^{n}^{−1})” while m = +1 denotes current amplification (K^{+1} = K^{+} = K) in range “0 to (2^{n}^{−1})”.

DPCCII in symbolic form may be represented as four port block as shown in ^{th} DPCCII block with current gain K^{±} and applied codeword K.

This section introduces three VM second order DPUFs as depicted in

1. Realization of lowpass (LP), highpass (HP), bandpass (BP), band reject (BR) filter functions;

2. Independently programmable ω_{0} and Q;

3. No component matching constraint;

4. Cascadability with the aid of one VF in first two configurations;

5. Only single circuit configuration is used for programming all the filter functions.

The key difference of these configurations lays in the modularity of filter structure i.e. the employment of the identical active element, which simplifies circuit design of DPUF. All the configurations are different in the sense that they use different variants of DPCCII. The First DPUF (^{−}, DPCCIIK^{+} and negative DPCCIIK^{+}, the Second DPUF uses (^{−} and one negative DPCCIIK^{−}. All the features of these two configurations are same. One important feature in which these two configuration lacks is the cascadability. This feature can be introduced by employing one VF. The Third configuration (^{−} type only. The VF consists of same circuitry as used between port X and Y of DPCCII (2). The Third DPUF does not need any additional follower for cascadability because it is already equipped with this feature.

Routine analysis of all the configuration yields following output function:

It can be seen from Equation (3) and

Setting codeword K_{2} = K_{3}, Equation (3) reduces to

From Equation (4), the ω_{0} and Q-factor can be expressed as

Filter Function | Input Combination |
---|---|

LP | V_{1} = V_{2} = 0, V_{3} = V_{i}* |

HP | V_{2} = V_{3} = 0, V_{1} = V_{i} |

BP | V_{1} = V_{3} = 0, V_{2} = V_{i} |

BR | V_{2} = 0, V_{1} = V_{3} = V_{i} |

AP^{#} | V_{1} = V_{3} = V_{i} = −V_{2} |

*V_{i} shows the actual applicable input. ^{#}AP needs one IVB.

It can be seen from Equation (5) that ω_{0} and Q of all the responses are independently programmable through codeword K_{2}(=K_{3}) and K_{1} respectively. One additional advantage offered by DPUF is the programming of ω_{0} in either direction by simply reversing the gain parameter of respective block i.e. changing the gain K^{+} (or K^{−}) of DPCCII-2, 3 by K^{−} (or K^{+}) or vice versa (Equation (5a)). This is particularly useful in achieving low frequency operation (for gain K^{−}) without requiring large passive component values.

Rewriting Equation (5) for C_{1} = 2C_{2} = C and equal resistor (R) results in following set of equation

where, ω_{c} and Q_{c} are defined as component dependent factors of ω_{0} and Q respectively. These are given as

Capacitance value C_{1} = 2C_{2} may be set at design level for quality factor of value _{c}) as given by Equation (6b) for K_{1} = 1. It is required for maximal flat LP and HP response. High-Q values required for BP and BR responses are obtained by proper choice of codeword K_{1}, which can be increased further by adding additional transistor arrays in corresponding DPCCII i.e. in K_{1} block. It is to be noted that n-transistor array can realize Q-value upto (2^{n}^{−1})Q_{c}.

This section discusses the effect of DPCCII non-idealities over the performance of proposed DPUF. These non-idealities results from (1) small error in unity transfer gains as described by Equation (2) and (2) DPCCII parasitics.

Taking the non-ideal transfer gain into account, the output function of proposed universal filters and corresponding parameter values are given in

Active and passive sensitivities of ω_{0} and Q for all the three configurations may be summarized as

where “x” denotes various active and passive elements i.e. α_{i}, β_{i}, K_{i}, resistances, and capacitances. It is evident from Equation (10) that the sensitivity figures are within reasonable limit.

Second set of non-idealities include DPCCII parasitics. DPCCII have high valued parasitic resistance R_{Y} (or R_{Z}) in parallel with low valued parasitic capacitance C_{Y} (or C_{Z}) at port Y (or Z), and a low valued series resistance R_{X} at port X. To simplify the discussion parasitic resistances at port Y and Z are not considered as these are much greater than the external resistance of circuit.

For fair comparison, VF used in DPUF of _{p}_{, }given as

Figure | Output function considering non-unity current/ voltage transfer and respective filter parameters | Output function considering parasitic elements of DPCC |
---|---|---|

2a | ||

2b | ||

2c |

Similarly, the operating frequency constraint of Second and Third DPUF configurations are given by Equations (9b) and (9c) respectively. These are given as

It can be seen from Equations (9) that the last two DPUF configurations suffer from slightly more number of parasitic effects. However, it does not pose any problem because of small values of C_{Y}, C_{Z} (order of few femto- farads) and R_{X} (of order of few ohms). Thus, the proposed DPUF configurations can be easily operated over the range of few megahertz.

Literature reports large number of DPUFs based on different active elements. Thus, for fair comparison, active elements used in all the DPUFs (proposed and reported) are given in terms of equivalent numbers of digitally programmable current follower (DPCF) and VF. For example, a DPCC uses two followers-one DPCF and one VF. Similarly, digitally controlled current differencing buffered amplifier (CDBA) of [

Criteria: (1) independently programmable filter parameters ω_{0} & Q; (2) cascadability; (3) number of passive components; (4) number of filter functions; (5) number of active components; (6) equivalent number of followers; (7) CM/VM or mixed mode (MM).

Comparison summary given in _{0}) [

It can be seen from

Reference, Year/Criteria | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
---|---|---|---|---|---|---|---|

[ | Y | Y | 7 | 3 | 4CF + 4DPCF = 8 | 8 | CM |

[ | Y | N | 5 | 3 | 3DPCCII = 6 | 6 | CM |

[ | Y* | Y | 5 | 3 | 3DP − DOCCII = 6 | 6 | CM |

[ | Y | Y | 4 | 3 | 3MOCCII + 5DPCF = 11 | 11 | CM |

[ | Y* | Y | 4 | 5 | 3DP − CDBA = 9 | 9 | CM |

[ | Y* | Y | 8 | 3 | 2DPCF + 1CF + 3VB = 6 | 6 | VM |

[ | Y | Y | 16 | 3 | 5DP − FDCCII = 10 | 10 | VM |

[ | Y | Y | 8 | 3 | 5DPCCII = 10 | 10 | VM |

[ | Y | Y | 5 | 2 | 4DPCCII = 8 | 8 | VM |

[ | Y | Y | 6 | 2 | 3DPCCII = 6 | 6 | VM |

[ | Y* | Y | 8 | 3 | 4DPCF + 3VF = 7 | 7 | VM |

[ | Y | Y | 8 | 5 | 5DPCCII = 10 | 10 | MM |

Proposed (all three configurations) | Y | Y | 5 | 4 | 3DPCCII + 1VF^{#} = 7 | 7 | VM |

Y: yes; N: no; DP: digitally programmable; FDCCII: fully differential CCII; *ω_{0 }programming only; ^{#}VF is required in first two DPUFs to obtain cascadability feature.

component count also needs quite large number of additional active/passive components. Apart from this, DPUF of [

The comparison presented herein shows that for the offered set of features, which comprises almost all the important features, proposed DPUFs use least number of active and passive component.

Performance of proposed generalized DPUF is verified by Tanner EDA version 14 using supply voltage ±0.75 V, TSMC 0.25 µm parameters, aspect ratio and bias details as given in [_{1} = R_{2} = R_{3} = 5 KΩ and C_{1} = 50 pF, C_{2} = 25 pF. It results in component dependent filter parameters as _{2}(=K_{3}). _{0} programming (in range 900 KHz to 6.3 MHz) of various responses for appropriate Q-values. Q-values are chosen depending on the type of response. Similarly Q-pro- gramming using codeword K_{1} (Equation 6(b)) is depicted in

Filter Function | Codeword Programming | |
---|---|---|

Pole Frequency | Q-Factor | |

HP | K_{2} = K_{3} = (1, 4, 7), K_{1} = 1 | NA |

LP | K_{2} = K_{3} = (1, 4, 7), K_{1} = 1 | NA |

BP | K_{2} = K_{3} = (1, 4, 7), K_{1} = 7 | K_{1} = (1, 4, 7), K_{2} = K_{3} = 4 |

BR | K_{2} = K_{3} = (1, 4, 7), K_{1} = 7 | K_{1} = (1, 4, 7), K_{2} = K_{3} = 4 |

This paper presents three VM DPUF configurations using least possible number of active and passive components. All DPUFs provide common attractive features such as the realization of four filter functions from same configurations, independently programmable filter parameters (ω_{0}, Q) and no component matching constraint. Programmable characteristic of DPUF may be useful for various applications in mixed mode (analog/digital) VLSI system. All the circuits are realized and simulated by using 0.25 μm TSMC technology parameters.

DeveshSingh,NeeloferAfzal,DineshPrasad,RichaSrivastava,KuldeepPanwar, (2015) Digitally Programmable Voltage Mode Universal Filters—A Minimal Realization. Circuits and Systems,06,213-223. doi: 10.4236/cs.2015.610022