1. THEME 9: Multi-vibrators. Multi-vibrators in integral realization. Multi-vibrators (also called oscillators or pulse train generators) have two unstable (temporary) states which are changed continuously in time. The result is the pulse train generated at the output of the circuit. There are great variety of multi-vibrator circuits: composed with logic gates and RC- chains, with Schmidt flip-flop, with crystal resonators, multi-vibrators in integral realization, with specialized integrated circuits, with operation amplifiers and analogue comparators. Milti-vibrator with Schmidt flip-flop. This multi-vibrator is realized with one logic gate having embedded Schmidt flip-flop and a RC-chain. At the first moment, after switching on the power supply, the voltage on the capacitor is 0V and the output of the logic gate is in “1”. The capacitor starts the process of charging trough resistor R trying to reach the voltage of logic “1”, but at the moment it reaches the threshold level (Е1) the Schmidt flip-flop switches its output to “0”. Then the capacitor starts to discharge trying to reach 0V, but at the moment it reaches the threshold level (Е2) the Schmidt flip-flop switches its output to “1” and again starts charging. DIGITAL ELECTRONICS

2. The pulse duration (tu), pause duration (t П ), period (Т) and frequency (f) of generated pulse train are calculated using the following relations: where: Rout = 130Ω, E1=1.6V, E2 = 0.8V and Uo1= 3.6V for TTL circuits. Pulse train generators with stabilization of frequency Whenever the pulse train with stabilization of frequency is necessary the pulse generators is implemented with crystal resonator and logic gates. The crystal resonator is electro-mechanical system, composed by crystal plate, cut off with proper orientation according to the axes in the crystal structure, and attached electrodes. The resonator vibrates with fixed frequency determined by mechanical characteristics of the crystal plate. The frequency of vibration has high stability (in order of 10 -6 ), which determines very high stability of frequency of generated pulse train from this type of multi-vibrators. DIGITAL ELECTRONICS

3. Multi-vibrators in integral realization Many companies produce multi-vibrators in integral realization, which cover a wide range of frequencies often used in digital electronics and microprocessor technique. DIGITAL ELECTRONICS An example of Voltage Controlled Oscilator (VCO) - IC 74624.

4. All VCOs have one external capacitor and at least one input for voltage control of frequency (Uf). Some of VCOs have second input for voltage control (Ur) and one more additional enable input (Е). From shown on the figure internal structure of IC 74624 it is visible that except pulse generator it consists from additional logic responsible for synchronization of start and stop of generations. The result is that all pulses in the train are with equal duration. The frequency of pulse train is determined by the values of connected capacitor (С) and control voltages Uf and Ur. The presented characteristics show that for a given value of C, the voltage Uf controls the frequency linearly, while voltage Ur control the slope of characteristic. Multi-vibrators realized by two mono-vibrators in integral realization The mono-vibrators IC 74123 and 74221, discussed in previous lecture, are arranged in couples in one integrated circuit. This is very convenient for realization of multi-vibrators. The two mono-vibrators could be connected in such a way that every one of them activates the other when terminates the generated pulse. An example realization is presented at the following figure. The pulse duration for this circuit is determined by the RC-chain of mono- vibrator ЧМ2 (R2-C2), while the pause duration is determined by the RC-chain of mono- vibrator ЧМ1 (R1-C1). DIGITAL ELECTRONICS

5. The pulse duration, pause duration, period, frequency and duty cycle of generated pulse train are calculated using the following relations: Where к is coefficient of proportion in the range of 0,2÷0,7, depending on the type of TTL circuits. DIGITAL ELECTRONICS

6.  Multi-vibrator’ realization with specialized integrated circuits One of the specialized integrated circuits used for realization of mono- and multi- vibrators is integral timer 555. It is integrated circuit with mixed analogue and digital structure, composed by resistors divider, two analogue comparators, R-S type flip-flop, output inverter buffer stage and key transistor. The main characteristics and specific features of integral timer 555 are as follows: 1. Wide range for operation of power supply voltages - 3 - 15 V. 2. High stability of tu and f of generated pulse train. 3. Low output resistance for both logic levels – approximately 10Ω, which assures rapid transitions at the output (good shape of output pulses). 3 (Output) 84 (Reset) R S R Q K1 K2 6 5 2 1 7 5к 4. High output power – output current of 200mA, which allows direct control of relays, LED indicators, etc. 5. Low power consumption (without load) – 0.7mA/ V of power supply. DIGITAL ELECTRONICS

7.  Multi-vibrator with IТ555. The circuit of multi-vibrator with IT555 with timing diagrams of operation are shown on the figure below. Timing diagrams illustrate the process of generation of pulse train. When the power supply is switched on, because the voltage on capacitor is equal to 0V, the R-S flip-flop is in reset (‘0’) state on its complementary output  Q. Then the internal key transistor is blocked and the output voltage is high. At this moment the capacitor starts the process of charging from the positive pole of power supply source (+Ucс), through resistors R A and R B, capacitor С itself to negative pole of power supply source. Uc 2/3 Ucс 1/3 Ucс Uout t t1 t2 t3 t4 t 555 Output RARA RBRB C (8) +Ucc (4) (7) (6) (2) (3) (1) DIGITAL ELECTRONICS

8. The capacitor is trying to charge to the value of power supply voltage (+Ucс), but at the moment of time (t1), when the voltage reaches 2/3Ucс, the comparator К1 switches from ‘0’ to ‘1’, which set the R-S flip-flop to state ‘1’ on the complementary output. Then the key transistor switches to saturation state and the output level becomes low. From this moment starts the process of capacitor discharging through resistor R B and saturated key transistor to negative pole of power supply. The process of capacitor discharging is going on till the moment of time (t2), when the voltage on capacitor reaches 1/3Ucс. At this moment the comparator К2 switches from ‘0’ to ‘1’ and reset again the R-S flip-flop to state ‘0’ on its complementary output. From this moment again starts the process of charging the capacitor. The processes of charging and discharging the capacitor are changed and this leads to generation of pulse train at the output. The pulse (tu) and pause duration (t П ) could be calculated using the basic equation for determination of time intervals during the transition process: DIGITAL ELECTRONICS

9. The period, frequency and duty cycle of the pulse train are calculated from the following relations: It is visible that for this circuit Kз > 0,5. When is necessary the duty cycle of pulse train to be Кз<0,5 or Кз=0,5, some modifications of the multi-vibrator circuit should be done. RARA RBRB C (8) +Ucc (1) 555 Output (4) (7) (6) (2) (3) П D Usually it is realized by connecting one or two diodes in the circuit in order to by-bass some of the resistors during the processes of capacitor charge or discharge. Example question: For pulse train generator with IT 555 shown on the figure, derive the equations for tu, t П, T, f and Кз for middle, extremely up and extremely low position of potentiometer slide. DIGITAL ELECTRONICS

10. Multi-vibrators with operation amplifier or analogue comparator Except multi-vibrators generating pulse trains with standard logic levels in some applications bipolar pulse train is necessary. Such type multi-vibrators could be realized using operation amplifier /or analogue comparator/ as an active device in the circuit. The principle is the same – the alternating with each other processes of capacitor charging or discharging. An example realization of multi-vibrator with operation amplifier (op. amp) and the timing diagrams showing the bipolar pulse train generation are presented on the figure below. After switching on the bipolar power supply voltage at the moment of time (t0), the capacitor voltage is equal to 0V and the voltage at the output of op. amp is in positive saturation equal to Uout(+). For conventional type op. amps the difference between Uout(+) and (+Ucc) is approximately 1V. Then the voltage on the non-inverting input of op. amp (in point A) will be positive and determined by the resistor divider R2-R1 as: DIGITAL ELECTRONICS From this moment of time starts the process of capacitor charging through the chain +Ucc - the op. amp. output - the resistor R3 - capacitor С – to the 0V pole. The process of charging continuous to the moment of time (t1), when the capacitor voltage reaches the value equal to U A (+). Then the output of op. amp. switches to negative saturation Uout(-), which is approximately equal to the negative power supply voltage (-Ucc) - the difference is app. 1V. Then the voltage on the non-inverting input of op. amp (in point A) will be negative and determined by the resistor divider R2-R1 as:

11. (Output) R3 R1 R2 OA С +Ucc -Ucc А Uc U A(+) Uout t t0 t1 t2 t3 t U A(-) Uout(+) Uout(-) DIGITAL ELECTRONICS From the moment of time (t1) starts the process of re-charging of capacitor through the chain – from 0V pole - capacitor С - the resistor R3 - the op. amp. output – to (-Ucc). The process of re-charging continuous to the moment of time (t2), when the capacitor voltage reaches the voltage U A (-). Then the output of op. amp. switches again to the positive saturation voltage Uout(+). The alternating processes of capacitor charging and re-charging are going further in the successive time intervals. The output of op. amp. switches from negative to positive saturation and vise versa, thus producing the bipolar pulse train. In both alternating processes the time constant is the same (R3.C).

12. From the timing diagrams it is visible that only the first pulse is shorter because the process of capacitor charging starts from 0V. The duration of all pulses in the pulse train (except the first) are equal to the pauses duration. Hence, the duty cycle for this multi- vibrator circuit is equal to 0,5. For practical calculations in order to determine the values of resistors and capacitor it is convenient to chose one of the following cases: 1. IfR1=R2, then 2. If 2R1=R2, then DIGITAL ELECTRONICS The pulse duration (tu) and pause duration (tп) are calculated using following relations:

13. Example question: For the bipolar pulse train generator shown on the figure, derive the equations for tu, t П, T, f and Кз. Then chose and calculate the values for resistors and capacitor in order to generate the pulse train with frequency f = 3kHz and duty cycle Кз=1/3. You can choose the value for capacitor from the following possibilities: C = (1; 1,5; 2,2; 3,3; 4,7; 6,8; 8,2) x 10 -i, where i depends on the dimension chosen – pF, nF, µF, etc. (Output) R3’ R1 R2 OA С +Ucc -Ucc А R3” D1 D2 DIGITAL ELECTRONICS For some applications pulse train with duty cycle Кз  0,5 is needed. Then it is necessary to modify the circuit in a way to change the time constants of two processes. Usually it is realized by suitable connection of one or two diodes in negative feedback of op. amp. The duration of slops of generated pulse train depends on the time characteristics of op. amp used. When steep slopes and high frequency pulse train are necessary, a high- speed op. amp. must be chosen with high value of parameter slew rate (SR) – the speed for increasing the output voltage [V/  s].