ujtrelaxationoscillators 150520184229 lva1 app6891

7/23/2019 Ujtrelaxationoscillators 150520184229 Lva1 App6891

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NIJUNCTION TRANSISTOR (UJT)



Physical structure




Equivalent circuit

• The equivalent circuit comprised of two

resistors, one fixed ( R B2) and one variable

( R B1) and a single diode ( D).

• R B1 varies with I E .




Equivalent circuit




Equivalent circuit

• R BB is the interbase resistance when I E = 0

i.e.

( )021 =

+= E I B B BB

R R R

• Typical range of R BB ! "Ω # $0 "Ω

• The position of the aluminum rod

determine the relative values of R B1 and

R B2.




021

1

1

=

=+

= E

B

I

BB BB

B B

B R V V

R R

RV η




021

1

=+

= E

I B B

B

R R

Rη




%or V E > V RB1 by V D (0.&' → 0.0 ), the

diode will fire and I E will begin to flow

through R B1.




The emitter potential V P is given by

D BB P V V V +=η




*haracteristics of representative +T




Programmable unijunction Transistor (PUT)

Although it has the same name as a UJT the programmable

unijunction transistor’s structure is not the same. It is

actually more similar to an SCR.



Programmable unijunction Transistor

The PUT can be “programmed” to turn on at a certain voltage

by an eternal voltage divider. This yields a curve similar to a

UJT.




!ternal "UT resistors R# and R$ replace uni%unction

transistor internal resistors R&# and R&$' respectively. These

resistors allo( the calculation o) the intrinsic stando)) ratio *.




VR is voltage divider (R1 and R2 can be specified)

Vc capacitor voltage

Wen Vc ! VR te PUT "ill conduct



UJT RELAXATION OSCILLATORS

&asic UJT relaation oscillator




Assume that the initial

capacitor voltage' V C

is +ero. ,hen thesupply voltage V BB is

)irst applied' the UJT

is in the - state. I E

is +ero and C charges

eponentially through

R to(ards V .

The operation




,hen the supply

voltage V C /= V E 0

reaches the )iring

potential' V P ' the UJT

)ires and C discharges

eponentially through

R2 until V E reaches

the valley potential V V .




,hen V E reaches the valley potential V V the

UJT turns -' I E goes to +ero and the

capacitor is recharged.This process repeats itsel) to produce the

(ave)orms )or vC and v R2 as sho(n belo(1




The waveform, vC




The waveform, v R2




Cn!itin "r s#itchin$%ON

To s(itch2on a UJT'

the emitter current I E

must be able to reach

the pea3 current I P i.e.

11 R I V P I I R

P E

>=




In other (ords' R1 must

be small enough such

that I E is not limited to a

value less than I P (hen

V C = V P .





Thus' to )ire the UJT1

P P BB V R I V +> 1

1 R I V V

P P BB >−

P

P BB

I

V V R

−<1





Cn!itin "r s#itchin$%O&&

To s(itch2o)) a UJT'

the emitter current I E

must drop belo( I V

(hen V C = V V .

4ence1

V V BB V R I V +<

1




Thus' to the UJT1

1 R I V V V V BB <−

V

V BB

I V V R −>1

Cn!itin "r s#itchin$%O&&




Thus' to ensure the s(itching -5 and -' the)ollo(ing condition must be met1

V

V BB

P

P BB

I V V R

I V V −>>− 1




-t can be shown that

−

−=

P BB

V BB

V V

V V C Rt ln

11

and

( )

+=

V

P

B

V

V C R Rt ln

212




The periodic time

21 t t T +=

-n many cases, t 1 >> t 2, therefore;

−−=≅

P BB

V BB

V V

V V C Rt T ln

11




/hen V BB and V P are much greater than V V ,then

−≅

P BB

BB

V V

V

C RT ln1

nd if η V BB

>> V pn

i.e. V P

V BB

, then

−

= BB BB

BB

V V

V C RT

η

ln1




or

−

=η 1

1ln1C RT

The frequency

==

η1

1ln

11

1C R

T f

ujtrelaxationoscillators 150520184229 lva1 app6891

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