^{1}

^{*}

^{2}

^{*}

^{2}

^{*}

^{2}

^{*}

In this paper, current differencing buffered amplifier (CDBA) based bistable multivibrators are introduced. Each presented circuit is constructed using single CDBA as the basic active building block and three resistors. Two applications namely an astable and a monostable multivibrator are also realized to demonstrate the usefulness of the proposed bistable multivibrators. The presented circuits are simulated using PSPICE from Cadence Orcad16.2 to verify their functionality. Simulation results agree well with the theoretical analysis.

Inherent wide bandwidth which is virtually independent of closed loop gain, greater linearity, and large dynamic range are the key performance features of current mode technique [

Bistable multivibrator, commonly known as Schmitt trigger, finds extensive applications in the fields of communication systems, instrumentation measurement systems, and power conversion control circuits [

It may be observed from the table that

- the op-amp based structures [

- the structure proposed in [

- the configurations of [

- the structures presented in [

- the OTRA based structures [

- the CDBA based structure provides further flexibility as it can be driven by both voltage and current inputs and can provide both voltage and current outputs

Above discussion suggests that CDBA based design is one of the most suitable choice. To the best of authors’ knowledge no CDBA based schmitt trigger circuit is available in literature. Thus this paper aims at introducing new CW and CCW Schmitt Triggers, using single CDBA and three resistors which will provide further flexibility to circuit designers. The PSPICE simulation results are also shown, which are in correspondence to the theoretical analysis. To show the usefulness of the presented circuits, the applications of the Schmitt triggers as square wave/triangular wave generator and monostable multivibrator are introduced.

The remaining paper is organized as follows. In Section 2 the function of a CDBA is introduced followed by the description of proposed circuits. The PSPICE simulations and experimental results to investigate the circuit performances are presented in Section 3 which are in confirmation with the theoretical propositions. In Section 4, application examples of the proposed circuits are given. The concluding remarks are presented in Section 5.

The circuit symbol of CDBA is shown in

Ref. | No. of active blocks used | No. of passive components | Hysteresis type | Output type | Output Impedance | Temperature sensitivity |
---|---|---|---|---|---|---|

[ | 1 Op-amp | 3R | CW, CCW | Voltage | Low | No |

[ | 2 OTA | 2R | CW | Voltage | High | Yes |

[ | 1 CC II+ | 3R | CW | Voltage | High | No |

[ | 1CC II | 3R | CW | Voltage | high | No |

[ | 1 OTRA | 2R | CW, CCW | Voltage | Low | No |

Proposed | 1 CDBA | 3R | CW, CCW | Voltage | Low | No |

The proposed CW Schmitt Trigger conﬁguration is shown in _{1} at n terminal of the CDBA and output v_{o} is taken across w terminal. A high value resistance R_{Z} is connected at the z terminal of the CDBA which forces the circuit into saturation. Resistor R_{2} forms a positive feedback loop to the ‘p’ input of the CDBA. Thus, the CDBA output saturates either at +V_{sat}, the positive saturation level or at the negative saturation level ?V_{sat}. This circuit realizes the CW hysteresis characteristic as shown in

For the CW hysteresis operation, the output V_{o} is initially assumed to be at the positive saturation level_{p} and I_{n} of CDBA are given by

As V_{i} increases from zero, V_{o} remains at _{i} reaches the upper threshold voltage V_{TH} thereby changing the output level from _{i} is greater than the lower threshold voltage V_{TL}. Assuming that V_{o} is at _{i} is smaller than V_{TH} initially, I_{p} can be determined from Equation (3) as

As v_{i} increases, current I_{n} gets closer to I_{p} and when I_{n} exceeds I_{p}, the output V_{o} switches to its negative saturation level _{TH} can be computed when I_{p} is equal to I_{n} and can be expressed as

The current of p terminal can now be computed as

However, I_{n} remains same as Equations (3). By equating I_{p} and I_{n} the lower threshold voltage V_{TL} can be determined as

The output level will switch back to _{p} gets more positive than I_{n}.

The CCW Schmitt Trigger is shown in _{p} and I_{n} can be computed as

Assuming that V_{o} is at _{i} is initially larger than V_{TL}, then V_{TL} can be computed as

When V_{i} is smaller than V_{TL}, output V_{o} switches to _{i}, I_{p} also increases and forces the output to change its state when I_{p} exceeds I_{n}. The upper threshold voltage V_{TH} can thus be derived as

Hysteresis Curve for CCW Schmitt Trigger is shown in

For the circuits shown in _{TH} = −V_{TL} and hence the switching voltage (V_{ST}) defined as (V_{TH} + V_{TL})/2 is zero. Some applications require that V_{TH} and V_{TL} both should either be positive or negative resulting in finite value of V_{ST}. This can be accomplished by adding a reference voltage to the circuit of

1. CW Schmitt Trigger with positive V_{ST}, shown in

2. CW Schmitt Trigger with negative V_{ST}, depicted in

3. CCW Schmitt Trigger with positive V_{ST}, given in

4. CCW Schmitt Trigger with negative V_{ST}, shown in

For the circuit of _{p} and I_{n} are given as

Using routine analysis the V_{TH} and V_{TL} can be computed as

where

Similarly the V_{TH} and V_{TL} for negative switching, as given in

For CCW Schmitt Trigger with positive switching voltage, the threshold voltages V_{TH} and V_{TL} are given by Equations (14) and (15) respectively whereas for CCW configuration with negative switching voltage are given in Equations (17) and (18) respectively.

To validate the theoretical predictions, the proposed bistable multivibrator circuits have been simulated using PSPICE. The CDBA is realized using current feedback operational amplifier (CFOA) IC AD 844 as shown in

_{Z} = 500 kΩ, R_{1} = 5 kΩ, R_{2} = 10 kΩ. The

saturation levels are ±6.3 V. The input voltage is a 50 Hz sinusoid with signal swing from −8 V to +8 V. The simulated threshold levels are ±3 V which are in accordance with the theoretically computed value of ±3.15 V.

The transient responses for CW and CCW configurations are shown in

The frequency response of CW Schmitt Trigger is shown in

Transient response for CCW for positive switching voltage is shown in _{TH} = 6 V and V_{TL} = 2 V and corresponding transfer characteristics is shown in _{Z} = 500 kΩ, R_{1} = 3.17 kΩ, R_{S} = 4.6 kΩ, R_{2} = 10 kΩ and V_{dc} = 5 V. Transient response and hysteresis curve for CW configuration with negative switching voltage for V_{TH} = 6 V and V_{TL} = 2 V are shwon in

_{Z} = 100 kΩ, R_{1} = 4.7 kΩ, R_{2} = 4.7 kΩ with supply voltage of ±12 V. The saturation levels are ±10 V. The input voltage is a 1 KHz sinusoid with signal swing from −10 V to +10 V. The observed threshold levels are ±10 V which are same as the theoretically computed value. _{Z} = 100 kΩ, R_{1} = 4.7 kΩ, R_{2} = 4.7 kΩ and supply voltages are ±8 V. Observed saturation voltages are ±6 V giving threshold voltages as ±6 V and are equal to theoretical values.

In the following subsections two well known applications of Schmitt trigger namely triangular/square wave Generator and monostable multivibrators are developed to demonstarte the utility of proposed work in circuit applications.

The circuit of CDBA Schimitt trigger based triangular/square wave generator is shown in

cuit comprising of CDBA II, is a simple integrator. The Schmitt trigger continuously compares the current I_{p}_{1 }and I_{n}_{1 }and accordingly the output V_{o}_{1} swings repetitively between positive saturation level _{o}_{1} to be at_{o}_{2} that is linearly rising. As a result the current I_{n}_{1} will rise and when exceeds I_{p}_{1} the output V_{o}_{1} switches to_{o}_{2}. As soon as I_{n}_{1} falls below I_{p}_{1}, V_{o}_{1} switches back to _{o}_{2} become a positive going ramp again. For Schmitt trigger the V_{TH} and V_{TL} can be computed as _{2}/R_{1} and _{2}/R_{1} respectively. Using the routine analysis the time period of the waveform can be computed as

This gives frequency of oscillation as

The simulated square wave output V_{o}_{1} and triangular output V_{o}_{2} are shown in _{1} = 10 kΩ, R_{2} = 100 Ω, R_{3} = 20 kΩ, R_{4} = 5 kΩ, and C = 1 µF.

The realization of the monostable multivibrator is shown in _{c} is clamped by diode. To ensure the stable-state operation, R_{Z} must be high enough to make output voltage V_{o} switch to positive saturation level_{z} and I_{p} are given by_{ }

Now if a positive-edge triggering signal I_{trig} is applied at terminal n of CDBA, the circuits enter into the quasi- stable State. As I_{n} is more positive than I_{p} the output voltage V_{o} jumps to _{F}. In the quasi-stable state, the expressions of I_{p} and I_{z} are given by

And capacitor discharging equation can be expressed as

at t = T_{2} the capacitorvoltage reaches the threshold voltage V_{TL}, when output voltage switches back to_{TL} can be derived by equating Equations (23) and (24) and is given by

From Equations (25) and (26) the pulse width T (T_{2} − T_{1}) for which the circuit remains in quasi stable sate can be computed as

_{F} = 20 kΩ, R_{Z} = 500 kΩ, C = 10 nF having T = 59 µs as against calculated value of 64 µs.

In this paper single CDBA based bistable multivibrator configurations are proposed which include CW, CCW Schmitt Triggers with and without reference voltage. Two applications namely square wave/triangular wave generator and monostable multivibrator are realized to demonstrate the usefulness of the proposed bistable multivibrators. The simulation and experimental results are found to be in close agreement to theoretical predictions. The proposed configurations are one of the best choices for voltage mode applications. Also, due to inherent flexibility of signal usage in CDBA the proposed configurations can easily be extented to current/transimped- ance/transadmittance mode depending upon the applications.

Rishi Pal,Rajeshwari Pandey,Neeta Pandey,Ramesh Chandra Tiwari, (2015) Single CDBA Based Voltage Mode Bistable Multivibrator and Its Applications. Circuits and Systems,06,237-251. doi: 10.4236/cs.2015.611024