Sonsivri

Hi.
a simulation with ORCAD and PROTEUS for Astable multivibrator.
you acn change RB1,2 and/or C1,2 and see its effect on output frequency.

until c1=c2 , RB1=RB2  output is square.(T=2PW=2SW)

f=1/T=1/PW

PW=RB.C.ln2  (approximately)

           
LINK:   http://rapidshare.com/files/82187869/Astable-simulations.zip.html

Hi ali... I'm just starting with electronics.. And have some problems simulating a Astable multivibrator that actually works on my board.
Do you know if I must configure my Multisim application to simulate oscilators?

Hi.
In simulation cause of symmetry in circuit both of transistors will be in saturation state.and oscillation will not be created.
In simultion you can use a pulse generator for holding emitter of one of the transistors in VCC voltage for a short time (for example 10us).by this way you turn off a transistor and the other is on (and in saturation mode).
for pulse source sw should be very greater than pw.
for example : pw=10us  sw=10s.
In 10us value of pulse is VCC and for 10s value should be 0(GND).
Therefore after 10us the emmiter is grounded and circuit return to its actual state. and Pulse source will have no extra effect in circuit just will provide ground for emitter.
Attention: in Astable multivibrator we dont need any source( just DC power supply is needed) but in simulation for deleting effect of symmetry in circuit we have used pulse source.
In actual circuit if we obsrve saturation in both of transistors we can connect base of one the transistors to GND for moments, it caused to turning off this transistors.
If you use ORCAD you should tick "skip the initial transient bias point calculation"  option.

best regards.

In proteus you must define initial conditions (via an IC=xxx label on a wire) so as to establish an initial
state - without this the circuit is totally symmetrical
and will not oscillate.

please check the proteus sample Ff.DSN for more details.