Oscillators

UNIT-II OSCILLATORS

ANALOG ELECTRONICS 1

ELECTRONICS & COMMUNICATION ENGINEERING DEPARTMENT

Question Bank

1. Draw & explain construction of UJT. (3)

2. Draw & explain equivalent circuit of UJT. (2)

3. Explain working of UJT with its characteristics. (4)

4. Explain UJT relaxation oscillator. (4)

Uni-junction transistor:

Construction:

Figure: (a) Construction of UJT (b) symbolic representation

A UJT is made up of an n-type silicon base to which p-type emitter is embedded.

The n-type base is lightly doped whereas p - type is heavily doped.

The two ohmic contact provided at each end are called base-one B1 and base-two B2. So a

UJT has three terminals namely the emitter E, base-one B1 and base-two B2.

Equivalent circuit:

Figure: Equivalent circuit of UJT

UNIT-II OSCILLATORS



Between bases B1 and B2, the uni-junction behaves like an ordinary resistance. RB1 and

RB2 at the internal resistances respectively from bases B1 and B2 to eta point A.

When a voltage VBB is applied across the two base terminals B1 and B2, the potential of

point A with respect to B1 is given by

BBBBB2B1

B1AB1 ηVV

RR

RV

Where is called the intrinsic stand-off ratio. Typical values of are 0.51 to 0.82.

Interbase resistance RBB = RB1 + RB2 is of the order of 5-10 k.

Working and characteristics:

Figure: UJT equivalent circuit with VBB and VEE and typical static V-I characteristics

If emitter voltage Ve < VAB1, the E- B1 junction is reverse biased and the reverse emitter

current Ie is negative as shown by curve PS in figure. In this condition UJT is in ‘OFF’

state. The resistance between E – B1 junction is therefore very high.

At point S, Ve = VEE and Ie = 0, so drop across RE is zero.

When Ve = ηVBB + VD (at point B) the E – B1 junction gets forward biased to allow

forward current flowing through the diode. Here VD is the forward voltage drop across E-

B1, junction (usually 0.5 V).

Point B is called the peak point. Voltage Vp, and current Ip are called peak-point voltage

and peak-point current respectively.

After this peak point, the emitter injects holes from the heavily doped emitter E into the

lower base region B1. The lower base region B1 is filled up with additional current

carriers (holes). As a result, resistance RB1 of E - B1 junction decreases. The fall in RB1

causes potential of eta point A to drop.

UNIT-II OSCILLATORS



This drop in VAB1 causes Ve to fall.

As VEE is constant fall in Ve gives rises to more emitter current Ie ( = ( VEE-Ve) / RE).

This increased in Ie injects more holes into region B1, thereby further reducing the

resistance RB1 and so on.

The emitter current, limited by external resistance RE, is then given by

EB1

DEEe

RR

VVI

When RB1 has dropped to a very small value, indicated by point C, the UJT has reached

‘ON’ state. At point C, entire base region B1 is saturated and resistance RB1 cannot

decrease any more. This point C is called the valley point; Vv and Iv are the

corresponding emitter potential and current.

Between points B and C, emitter voltage Ve falls as Ie increases; UJT, therefore, exhibits

negative resistance between these two points.

At the valley point, the current is given by Vv/RB1. Valley-point current, also called

holding current, keeps UJT ON. When emitter current Ie falls below Iv UJT turns OFF.

UJT as a relaxation oscillator:

Figure: UJT relaxation oscillator Connection diagram

The UJT is a highly efficient switch; its switching time is in the range of nanoseconds.

Since UJT exhibits negative resistance characteristics, it can be used as a relaxation

oscillator.

The external resistances R1, R2 are external resistors.

R is the charging resistance.

When source voltage VBB is applied, capacitor C begins to charge through R

exponentially towards VBB. During this charging, emitter circuit of UJT is an open circuit.

UNIT-II OSCILLATORS



The capacitor voltage Vc, equal to emitter voltage Ve, is given by

RC

t

BBec eVVV 1

The time constant of the charge circuit is τ1 = RC.

When this emitter voltage Ve (or Vc) reaches the peak-point voltage Vp ( = η VBB + VD),

the junction between E – B1, breaks down. As a result, UJT turns ON and capacitor C

rapidly discharges through low resistance R1, with a time constant τ2 = RC. So, we can

get a pulse at the output as shown in the waveform.

Here τ2 is much smaller than τ1. When the emitter voltage Ve becomes less than the

valley-point voltage Vv, emitter current Ie falls below Iv and UJT turns OFF.

The time T required for capacitor C to charge from initial voltage Vv to peak-point

voltage Vp, through large resistance R, can be obtained as:

η1

1ln RC

f

1T

In case T is taken as the time period of output pulse duration (neglecting small discharge

time).

Figure: Voltage waveforms

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