Abstract
A new current-mode sinusoidal oscillator employing a single voltage differencing transconductance amplifier (VDTA), two grounded capacitors, and one grounded resistor has been proposed. The proposed sinusoidal oscillator offers the following advantageous features: (i) use of grounded capacitors which are attractive from the view point of IC implementation as well as eliminating/accommodating parasitic capacitances, (ii) independent control of frequency of oscillation and condition of oscillation, (iii) ready availability of explicit current-mode output, (iv) low active and passive sensitivities, and (v) a very good frequency stability. The workability of the proposed configuration has been established by PSPICE simulations.
1. Introduction
In the synthesis of signal processing/signal generation circuits, current-mode (CM) operation has received much attention over the conventional voltage-mode (VM) operation due to its wider bandwidth and high linearity [1]. There is a growing interest in the realization of sinusoidal oscillators with explicit CM output because these oscillators may be employed as test signal generators for the testing of CM filters, CM precision rectifier, and so forth, which would otherwise require additional voltage to current converter when tested by using conventional VM oscillators [2]. Sinusoidal oscillators find numerous applications in communication, control systems, signal processing, instrumentation, and measurement systems; see [3–5] and the references cited therein. Explicit current-mode sinusoidal oscillators (ECMSOs) based upon different active single building blocks are available in the literature; see [6–13] and the references cited therein.
In [14], many active building blocks have been introduced, VDTA is one of them. Although a CMOS realization of VDTA and its RF filter application has been introduced in [15], to the best knowledge and belief of the authors no other application has been reported in the open literature so far. Therefore, the purpose of this paper is to propose a new ECMSO using a single VDTA along with three passive components (two grounded capacitors and one grounded resistor), which offers (i) use of grounded capacitors which are attractive from the viewpoint of IC implementation as well as eliminating/accommodating parasitic capacitances, (ii) independent control of frequency of oscillation and condition of oscillation (iii) ready availability of explicit current-mode output, (iv) low active and passive sensitivities, and (v) a very good frequency stability. Apart from realizing the intended type of ECMSO, the proposed topology has other modes of operation also. The workability of the new circuit has been verified by PSPICE simulations.
2. The New Oscillator Configuration
The symbolic notation of the VDTA is shown in Figure 1, where and are input terminals and , , and are the output terminals. All terminals of VDTA exhibit high impedance values [15]. The VDTA can be described by the following set of equations: A routine circuit analysis yields the following characteristic equation: Thus, the condition of oscillation (CO) and frequency of oscillation (FO) are given by Therefore, it is seen that CO can independently be controlled by resistor while FO is electronically controllable by transconductance .
With the feedback link broken at node “” and considering the terminal of VDTA as the input, the two open loop transfer functions realized by the proposed circuit are given by Thus, in this mode, the same configuration can be used to realize low-pass and band-pass filters simultaneously. From (4), the natural frequency () and bandwidth (BW) are given by Thus, it is seen that and BW are independently tunable.
In the third mode of operation, the current transfer functions obtained from Figure 2 are given by For sinusoidal steady state, (6) becomes Thus, the phase difference between ( and ) is and between ( and ), and ( and ) is .
Hence, the currents ( and ), ( and ) and ( and ) are in the quadrature form. To extract the currents , and explicitly another device with their inputs virtually grounded will be required due to which although the capacitors , and resistor will not be physically connected to ground but they will still be virtually grounded [16].
3. Nonideal Analysis
It is useful to count the effect of various VDTA nonideal parameters on the CO and FO of the proposed oscillator. Taking into account the finite -terminal parasitic impedance consisting of a resistance in parallel with capacitance , the finite -terminal parasitic impedance consisting of a resistance in parallel with capacitance , the finite -terminal parasitic impedance consisting of a resistance in parallel with capacitance and the parasitic impedance at the -terminal consisting of a resistance in parallel with capacitance , then the non-ideal expression of CO and FO can be given by where denotes , then, the active and passive sensitivities of can be found as From the above mentioned sensitivity values, it is easy to figure out that all the active and passive sensitivities are no more than half in magnitude.
4. Frequency Stability
Using the definition of the frequency stability factor as given in [5], (where is the normalized frequency and represents the phase of the open-loop transfer function of the oscillator circuit), with , , and ; the of the proposed oscillator is found to be . Therefore, very good frequency stability is obtainable by selecting larger value of .
5. Simulation Results
To confirm theoretical analysis, the proposed ECMSO was simulated using CMOS VDTA from [15]. Power supply voltages were taken as V and μA biasing currents are used. The transistor aspect ratios are taken from [15]. The passive elements of the configuration were selected as nF and k. The transconductances of VDTA were controlled by bias currents. PSPICE generated output waveforms indicating transient and steady-state responses are shown in Figures 3(a) and 3(b), respectively. These results, thus, confirm the validity of the proposed configuration. The total harmonic distortion (THD) of the proposed oscillator is found to be 2.66% (Figure 4). A comparison with other previously known ECMSOs using single active building block has been given in Table 1. Figure 5 shows the frequency responses of the BP and LP filters obtained from the proposed configuration. From Figures 6, 7, and 8, it is clear that the two currents are in quadrature.
(a)
(b)
As a sample example, Figure 8 shows that the two currents are in quadrature and the measured value of phase shift between two waveforms is .
6. Concluding Remarks
A new application of VDTA has been introduced in the realization of a novel ECMSO. In oscillator mode, the circuit offers (i) independent control of frequency of oscillation and condition of oscillation, (ii) use of grounded capacitors which are attractive from the viewpoint of IC implementation as well as eliminating/accommodating parasitic capacitances, (iii) ready availability of explicit current-mode output, (iv) low active and passive sensitivities, and (v) a very good frequency stability. In filter mode, the structure realizes LP and BP filter responses simultaneously with independent control of and BW. In third mode of operation, the circuit offers quadrature current-mode outputs. The validity of the proposed configuration in various modes of operation has been established by the PSPICE simulations.
Acknowledgment
The authors gratefully acknowledge Professor Dr. Raj Senani, the Head of the Division of Electronics and Communication Engineering, and the Director of NSIT, New Delhi, for useful discussions/suggestions.