Relaxation oscillators

Multivibrators

A multivibrator is an electronic circuit that switches rapidly by means of positive feedback between two or more states. There are three types of multivibrator circuit depending on the circuit operation:

Astable; in which the circuit is not stable in either state —it continually switches from one state to the other. It does not require an input such as a clock pulse.

Monostable, in which one of the states is stable, but the other state is unstable (transient). A trigger causes the circuit to enter the unstable state. After entering the unstable state, the circuit will return to the stable state after a set time. Such a circuit is useful for creating a timing period of fixed duration in response to some external event. This circuit is also known as a one shot.

Bistable, in which the circuit is stable in either state. The circuit can be flipped from one state to the other by an external event or trigger. The bistable multivibrator is simply a latch (flip-flop); it is added to this classification only for completeness.

A multivibrator consists of two main components — two passive networks and a bistable circuit, connected in a common feedback loop. The networks can be both resistive-capacitive (in the case of an astable circuit), a resistive-capacitive and a resistive (monostable), and both resistive (bistable).

Multivibrators find applications in a variety of systems where square waves or timed intervals are required.

Astable Multivibrator

An astable multivibrator consists of two amplifying stages connected in a positive feedback loop by two capacitive-resistive coupling networks. The amplifying elements may be junction or field-effect transistors, vacuum tubes, operational amplifiers, or other types of amplifier.

Regenerative switching circuits such as Astable Multivibrators are the most commonly used type of relaxation oscillator because not only are they are simple, reliable and easy to construct, they also produce a constant square wave output waveform.

The Astable Multivibrator has NO stable output states as it changes from one state to the other all the time.

The astable circuit consists of two switching transistors, a cross-coupled feedback network, and two time delay capacitors which allows oscillation between the two states with no external trigger signal to produce the change in state.

In Electronic Circuits, astable multivibrators are also known as Free-running Multivibrator as they do not require any additional inputs or external assistance to oscillate.

Astable’s produce a continuous square wave from its output or outputs, (two outputs no inputs) which can then be used to flash lights or produce a sound in a loudspeaker.

Time period of Astable multivibrator can be controlled by changing the values of feedback components such as coupling capacitors and resistors.

Circuit operation:

To begin, when power is applied, theoretically both T1 and T2 should turn on, since their base pins are connected through resistors (R2 and R3) to Vcc. However, due to small differences in the electric properties, one of the them will turn on slightly earlier than the other.

Assume anyone of the transistors Q1 or Q2 turns ON due to parameter variation or due to some switching transients, let it be Q1.

Then the collector voltage of Q1=Vce(sat)=0v(ground) approximately, it is cross coupled to base terminal of Q2 through C1, then Q2 remain in OFF state.

During Q1 ON, the current path through R1 charges the capacitor C1, the capacitor C1 voltage is coupled to base of transistor Q2.

While charging of C1, when the capacitor voltage exceeds 0.7V, Q2 become turns ON.

As soon as Q2 ON, its collector voltage falls to ground approximately, it is coupled to base terminal of Q1 then Q1 become OFF.

At the same time capacitor C2 starts charging through R2, when the C2 voltage exceeds 0.7 V, Q1 turns ON due to cross coupling.

This process continues.

We can also take output from collector terminals of the transistors as shown in figure.

Output waveformThis is the output taken from the collector terminal of Q1

Time and frequency and duty cycle of astable multivibrator

If the reference transistor is Q1, Time period of positive cycle of transistor based astable multivibrator is given by,Ton=0.69 R1xC1 sec Toff=0.69R2xC2 sec

We can also take output from Q2, if so Ton=0.69 R2xC2 sec Toff=0.69R1xC1 sec

T (Time period)=Ton + Toff = 0.69 (R1C1+R2C2)

Frequency (F) Duty cycle (D)

UNIJUNCTION TRANSISTOR (UJT)

Structure of a UJT Circuit symbol for a UJT

- A junction transistor (UJT) is a three- lead electronic semiconductor device with only one junction that acts exclusively as an electrically controlled switch.

-The UJT is not used as a linear amplifier. It is used in free-running oscillators, synchronized or triggered oscillators, and pulse generation circuits at low to moderate frequencies (hundreds of kilohertz). It is widely used in the triggering circuits for silicon controlled rectifiers.

-Its applications includes oscillators, pulse generators, saw-tooth generators, triggering circuits, phase control, timing circuits, and voltage- or current-regulated supplies.

The worth noting points about UJT:

1)The device has only one junction, so it is called the unijunction device.

2)The device, because of one P-N junction, is quite similar to a diode but it differs from an ordinary diode as it has three terminals.

3)The structure of a UJT is quite similar to that of an N-channel JFET. The main difference is that the gate surface of the JFET is much larger than emitter junction of UJT.

4)In a UJT the emitter is heavily doped while the N-region is lightly doped, so the resistance between the base terminals is relatively high, typically 4 to 10 kilo Ohm when the emitter is open.

5) The N-type silicon bar has a high resistance the resistance between emitter and base-1 is larger than that between emitter and base-2. It is because generally the emitter is closer to base-2 than base-1.

6) UJT is operated with emitter junction forward- biased while the JFET is normally operated with the gate junction reverse-biased.

7) UJT does not have ability to amplify but it has the ability to control a large ac power with a small signal. It exhibits a negative resistance characteristic and so it can be employed as an oscillator.

Equivalent circuit

• The equivalent circuit comprised of two resistors, one fixed (RB2) and one variable (RB1) and a single

diode (D).

• RB1 varies with IE.

• Variation of RB1 : 5 kΩ to 50 Ω for the corresponding

variation of 0 µA to 50 µA in IE.


Equivalent circuit of UJT:

Equivalent circuit

• RBB is the interbase resistance when IE = 0 i.e.

• Typical range of RBB : 4 kΩ - 10 kΩ




η is called standoff ratio (0.5-0.8 typical range) .It represents the ratio of r’B1 to r’BB with no current.

For VE > VRB1 by VD (0.35 → 0.70 V), the diode

will fire and IE will begin to flow through RB1.


The emitter firing potential VP is given by:


Characteristics of representative UJT:


The emitter characteristics:

For fixed values of η and VD, VP varies

with VBB.


Low voltage signal applied to emitter

A UJT (unijunction transistor) is a voltage-controlled switch that does not amplify the current in the load circuit.

emitter

Base 1

Base 2

e

b2

b1

UJT

pn n

0

Low current flow from base 1 through base 2emitter

pn ncurrentmeter

0

+ +

base 1base 2


currentmeter

OFF

OFF

Circuit operation of UJT :


emitter

pn ncurrentmeter

0

+ +

emitter

Base 1

Base 2

e

b2

b1UJT

pn n

00

Low current flow from base 1 through base 2

base 1base 2

applied to emitterHigh voltage signal

currentmeter

OFF

ON

High current flow from base 1 through emitter

base 1

A UJT (unijunction transistor) is a voltage-controlled switch that does not amplify the current in the load circuit. emitter


+

+

+

B2Current flows into this circuit branch until voltage at node becomes high enough to stop the flow.

p

n

n

B1+

+

capacitor

Always a small flow of current in this branch of circuit.

But not enough to turn the light on.

Unijunction transistor in a light flasher circuit


Current flows into this circuit branch until voltage at node becomes high enough to stop the flow.

+

+

B2

p

n

n

B1+

capacitor

(This charging current continues its flow into the capacitor until the capacitor becomes fully charged and node reaches the threshold voltage value.)


+

+

When node voltage reaches threshold value, the capacitor discharges current through the emitter to B1 circuit

vthreshold

Light is ON while capacitor discharge current is flowing.



vthreshold

When node voltage reaches threshold value, the capacitor discharges current through the emitter-B1 circuit

+

+

B2


capacitor


+

+

Voltage across capacitor drops as capacitor current discharges

Light is ON while capacitor discharge current is flowing.

Light is OFF when capacitor voltage is

+

p

n

n

below UJT’s threshold voltage value.

B1


+

+

B2


B1

capacitor


+

+

Voltage across capacitor drops as capacitor current discharges

p

n

n

When voltage across capacitor drops low enough current starts following in this branch again and cycle repeats itself.

+


UJT RELAXATION OSCILLATORS

Basic UJT relaxation oscilator


Assume that the initial

capacitor voltage, VC is

zero. When the supply

voltage VBB is first

applied, the UJT is in the

OFF state. IE is zero and

C charges exponentially

through R1 towards VBB.

Operation of UJT oscillator:


When the supply

voltage VC (= VE)

reaches the firing

potential, VP, the UJT

fires and C discharges

exponentially through

R2.

- when the emitter diode starts conducting, charge carriers are injected into the RB region of the bar.

- Since the resistance of a semiconductor material depends upon doping, the resistance of region RB decreases rapidly due to additional charge carriers (holes).

- With this decrease in resistance, the voltage drop across RB also decrease, cause the emitter diode to be more heavily forward biased.

- This, in turn, results in larger forward current, and consequently more charge carriers are injected causing still further reduction in the resistance of the RB region.

- Thus the emitter current goes on increasing until it is limited by the emitter power supply or some other mechanism like capacitor charging/discharging at emitter circuit.

- Since VE decreases with the increase in emitter current, the UJT is said to have negative resistance characteristic. It is seen that the base-2 (B2) is used only for applying external voltage VBB across it.

- When VE reaches the minimum potential (valley potential VV) the UJT turns OFF, IE goes to zero and the capacitor is recharged.

- This process repeats itself to produce the waveforms for vC and vR2 as shown in the next slide;



It can be shown that;

−−=

PBB

VBB

VV

VVCRt ln11

and;

( )

+=

V

PB V

VCRRt ln212


The periodic time;

21 ttT +=

In many cases, t1 >> t2, therefore;

−−=≅

PBB

VBB

VV

VVCRtT ln11

Engineering

Relaxation oscillators