SPICE question regarding astable multivibrator with transistor

[Michael Vick]

I have designed a astable multivibrator on breadboard with
transistors( npn , CEmitter config) and the gain is 10. powersupply =
dc 10v

when i designed the same circuit in spice , i am not seeing the
different states at the collector during the transient analysis..ie, i
am not seeing any oscillation between the 2 states at the coll .

could someone tell me what the problem could be???

thanks....

 

[Mike Engelhardt]

Michael,

I have designed a astable multivibrator on breadboard with
transistors( npn , CEmitter config) and the gain is 10.
powersupply = dc 10v

when i designed the same circuit in spice , i am not seeing the
different states at the collector during the transient analysis
..ie, i am not seeing any oscillation between the 2 states at
the coll .

could someone tell me what the problem could be???

Sometimes a circuit needs a kick-start to oscillate in SPICE.
The physical circuit has stochastic noise to do that but
the SPICE simulation might not. You can add the keyword
"startup" to the .tran command so that the power supplies
are ramped on over the 1st 20us of the simulation. Turning
the circuit on in the simulation is usually enough to get
something that oscillates to oscillate. There is an example
astable oscillator in the distribution as
../examples/Educational/astable.asc

--Mike

 

[Jim Thompson]

I have designed a astable multivibrator on breadboard with
transistors( npn , CEmitter config) and the gain is 10. powersupply =
dc 10v

when i designed the same circuit in spice , i am not seeing the
different states at the collector during the transient analysis..ie, i
am not seeing any oscillation between the 2 states at the coll .

could someone tell me what the problem could be???

thanks....

Yes ;-)


Transient analysis in most Spice variants *does not include noise*.
Thus many oscillator types will not self-start. Add a .IC statement
holding one node initially at some bias point away from the
equilibrium you are presently seeing.

...Jim Thompson

 

[Kevin Aylward]

Yes ;-)
Maybe.



Transient analysis in most Spice variants *does not include noise*.
Thus many oscillator types will not self-start. Add a .IC statement
holding one node initially at some bias point away from the
equilibrium you are presently seeing.

I would be surprised if this was the problem in any astable circuit run
in spice. An astable circuit don't have a stable DC state, so it cant
really stay put. The usual problem for these types of circuits is that
they don't converge, unless you physical hold it in one state at start
up. For "linear" oscillators, this is not the case. They do have a
stable DC state, hence spice might just stay there.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Kevin Aylward]

Michael,


Sometimes a circuit needs a kick-start to oscillate in SPICE.
The physical circuit has stochastic noise to do that but
the SPICE simulation might not.

Astables don't need a kick start. They are inherently not DC stable.
That is, they don't start due to noise.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Jim Thompson]

On Tue, 28 Oct 2003 11:26:15 -0000, "Kevin Aylward"

[snip]

Astables don't need a kick start. They are inherently not DC stable.
That is, they don't start due to noise.

Kevin Aylward

Where did you dream up that BS, Kevin?

...Jim Thompson

 

[Kevin Aylward]

On Tue, 28 Oct 2003 11:26:15 -0000, "Kevin Aylward"

[snip]

Astables don't need a kick start. They are inherently not DC stable.
That is, they don't start due to noise.

Kevin Aylward

Where did you dream up that BS, Kevin?

Years of experience, sonny

Because that's the way they are usually designed. Consider a 555, cant
get much more typical than that. The feedback path is totally dc, and as
such they don't have a DC stable state so they don't require any noise
to get them started. They would work with zero noise. Or what about the
Schmitt trigger and integrator. Again its inherently unstable. when one
discusses astables, on rarely is discussing ac coupled ones.

As I noted, the usually issue in spice with astables, is the thing won't
converge at t=0, that is it dosnt have a stable solution!

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Jim Thompson]

Jim Thompson wrote: [snip]

Where did you dream up that BS, Kevin?

Years of experience, sonny

Because that's the way they are usually designed. Consider a 555, cant
get much more typical than that. The feedback path is totally dc, and as
such they don't have a DC stable state so they don't require any noise
to get them started. They would work with zero noise. Or what about the
Schmitt trigger and integrator. Again its inherently unstable. when one
discusses astables, on rarely is discussing ac coupled ones.

As I noted, the usually issue in spice with astables, is the thing won't
converge at t=0, that is it dosnt have a stable solution!

Kevin Aylward

Not ALL astables have digital blocks, SONNY!!

...Jim Thompson

 

[Mike Engelhardt]

Kevin,

Astables don't need a kick start. They are inherently
not DC stable. That is, they don't start due to noise.

Nope. Depends on the astable. That's true for a 555
or ring oscillators, but not for a some discrete two
transistor astables. Attached is a counter example.

In any case, if your oscillator/astable doesn't
oscillate in SPICE, you should perturb the circuit in
simulation like the first two correct followups to the
original poster.

--Mike

Version 4
SHEET 1 888 1240
WIRE -560 976 -560 848
WIRE -560 848 -480 848
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WIRE -224 896 -224 848
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WIRE -96 848 -96 832
WIRE -96 848 -96 976
WIRE -480 976 -480 1024
WIRE -480 1024 -496 1024
WIRE -224 976 -224 1024
WIRE -224 1024 -160 1024
WIRE -432 1024 -480 1024
WIRE -240 1024 -224 1024
WIRE -368 1024 -304 848
WIRE -304 848 -224 848
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WIRE -368 848 -480 848
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WIRE -720 864 -720 720
WIRE -720 720 -560 720
WIRE -560 720 -560 752
WIRE -560 720 -96 720
WIRE -96 720 -96 752
FLAG -96 1104 GND
FLAG -560 1104 GND
FLAG -720 1104 GND
SYMBOL RES -576 736 R0
SYMATTR InstName R1
SYMATTR Value 2K
SYMBOL res -112 736 R0
SYMATTR InstName R2
SYMATTR Value 2K
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SYMATTR InstName R3
SYMATTR Value 100K
SYMBOL res -240 880 R0
SYMATTR InstName R4
SYMATTR Value 100K
SYMBOL cap -368 1008 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value .01µ
SYMBOL cap -240 1008 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C2
SYMATTR Value .01µ
SYMBOL VOLTAGE -720 848 R0
SYMATTR InstName V1
SYMATTR Value 5
SYMBOL NPN -160 976 R0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL npn -496 976 M0
SYMATTR InstName Q2
SYMATTR Value 2N3904
TEXT -768 1176 Left 0 !.tran 10m

 

[Kevin Aylward]

Kevin,


Nope. Depends on the astable.

Of course. But there *is* a general assumption of what is typical of
astables. I would say the 555 sets the standard nowadays.

That's true for a 555
Yes.

or ring oscillators,

Nope. Ring oscillators are not usually astables they are er..oscilators.
They are usually, imo, odd inverter chains which have a stable dc point
at mid supply.

Schmitt trigger/Integrators are another example of an astable.

but not for a some discrete two
transistor astables. Attached is a counter example.

Ho humm...teaching your granny to suck eggs again I see.

In any case, if your oscillator/astable doesn't
oscillate in SPICE, you should perturb the circuit in
simulation like the first two correct followups to the
original poster.

Nope. *Only* if the astable is not of the 555/Schmitt trigger and
integrator type, or other similar. If it is of that type, perturbations
won't make the slightest difference, so your basic advice was incorrect.
That is, it does not apply to astables in general.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Kevin Aylward]

Jim Thompson wrote: [snip]

Where did you dream up that BS, Kevin?

Years of experience, sonny

Because that's the way they are usually designed. Consider a 555,
cant get much more typical than that. The feedback path is totally
dc, and as such they don't have a DC stable state so they don't
require any noise to get them started. They would work with zero
noise. Or what about the Schmitt trigger and integrator. Again its
inherently unstable. when one discusses astables, on rarely is
discussing ac coupled ones.

As I noted, the usually issue in spice with astables, is the thing
won't converge at t=0, that is it dosnt have a stable solution!

Kevin Aylward

Not ALL astables have digital blocks, SONNY!!

I agree. But astable, imo, automatically brings to mind 555 or
Schmitt/integrator.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Jim Thompson]

Kevin,
[snip]
In any case, if your oscillator/astable doesn't
oscillate in SPICE, you should perturb the circuit in
simulation like the first two correct followups to the
original poster.

Nope. *Only* if the astable is not of the 555/Schmitt trigger and
integrator type, or other similar. If it is of that type, perturbations
won't make the slightest difference, so your basic advice was incorrect.
That is, it does not apply to astables in general.

Kevin Aylward

Newbies should be aware that this is NOT an argument amongst PEERS.
Kevin, as usual, has his head up his ass and can only see 555's ;-)

...Jim Thompson

 

[Michael Vick]

Thanks a lot to all these quick replies...The initial condition
problem does infact occur in spice....and once i corrected it , i saw
oscillations immediately...one step in the right direction.
However when i ran a Parametric sweep of the DV voltage from 0-10 V,
the frequency of the oscillations remain the same , with just change
in the amplitude... this is a interesting problem since the same
circuit in realtime has varying frequencies(obvious) at diff voltages.

being a new user to Spice , i have not come across this kind of error.
does anyone have further ideas on tht.. I tried to change the gain to
around 100 , to see if it had any effect on the frequency.but it didnt
change a bit...
I am thinkking it could probably be because of the transistor model(
q2n2222)...

thanks...
mike..

 

[Jim Thompson]

Thanks a lot to all these quick replies...The initial condition
problem does infact occur in spice....and once i corrected it , i saw
oscillations immediately...one step in the right direction.
However when i ran a Parametric sweep of the DV voltage from 0-10 V,
the frequency of the oscillations remain the same , with just change
in the amplitude... this is a interesting problem since the same
circuit in realtime has varying frequencies(obvious) at diff voltages.

being a new user to Spice , i have not come across this kind of error.
does anyone have further ideas on tht.. I tried to change the gain to
around 100 , to see if it had any effect on the frequency.but it didnt
change a bit...
I am thinkking it could probably be because of the transistor model(
q2n2222)...

thanks...
mike..

Post your schematic on alt.binaries.schematics.electronic

...Jim Thompson

 

[Kevin Aylward]

Kevin,
[snip]
In any case, if your oscillator/astable doesn't
oscillate in SPICE, you should perturb the circuit in
simulation like the first two correct followups to the
original poster.

Nope. *Only* if the astable is not of the 555/Schmitt trigger and
integrator type, or other similar. If it is of that type,
perturbations won't make the slightest difference, so your basic
advice was incorrect. That is, it does not apply to astables in
general.

Kevin Aylward

Newbies should be aware that this is NOT an argument amongst PEERS.

Do you mean, that if we assume that we have a 555 or a Schmitt gate with
RC feedback, or similar astable, that they require noise to get them
started?

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Jim Thompson]

Mike Engelhardt wrote:
Kevin,
[snip]
In any case, if your oscillator/astable doesn't
oscillate in SPICE, you should perturb the circuit in
simulation like the first two correct followups to the
original poster.

Nope. *Only* if the astable is not of the 555/Schmitt trigger and
integrator type, or other similar. If it is of that type,
perturbations won't make the slightest difference, so your basic
advice was incorrect. That is, it does not apply to astables in
general.

Kevin Aylward

Newbies should be aware that this is NOT an argument amongst PEERS.

Do you mean, that if we assume that we have a 555 or a Schmitt gate with
RC feedback, or similar astable, that they require noise to get them
started?

Kevin Aylward

No. Your implication was ALL astables.

...Jim Thompson

 

[Mike Engelhardt]

Kevin,

[snip]

In any case, if your oscillator/astable doesn't
oscillate in SPICE, you should perturb the circuit in
simulation like the first two correct followups to the
original poster.

Nope. *Only* if the astable is not of the 555/Schmitt trigger and
integrator type, or other similar. If it is of that type,
perturbations won't make the slightest difference, so your basic
advice was incorrect. That is, it does not apply to astables in
general.

Kevin Aylward

Newbies should be aware that this is NOT an argument amongst PEERS.
Kevin, as usual, has his head up his ass and can only see 555's ;-)

Do you mean, that if we assume that we have a 555 or a Schmitt gate with
RC feedback, or similar astable, that they require noise to get them
started?

I assumed he was just saying that you have your head up your ass.
No one said that a 555 needs noise to start the oscillation. You
assumed astables were 555's. It was an assumption you made to be
argumentative about the help and well-placed advice was humbly
offered by JT and myself to the original poster. Hopefully one day
you will hear a loud pop -- or maybe just a quiet squish -- and see
daylight.

--Mike

 

[Jim Thompson]

Kevin,
[snip]

Do you mean, that if we assume that we have a 555 or a Schmitt gate with
RC feedback, or similar astable, that they require noise to get them
started?

I assumed he was just saying that you have your head up your ass.
No one said that a 555 needs noise to start the oscillation. You
assumed astables were 555's. It was an assumption you made to be
argumentative about the help and well-placed advice was humbly
offered by JT and myself to the original poster. Hopefully one day
you will hear a loud pop -- or maybe just a quiet squish -- and see
daylight.

--Mike

Everyone help Kevin get his head out of his ass, send him some Milk of
Magnesia ;-)

...Jim Thompson

 

[Kevin Aylward]

Kevin,

[snip]
In any case, if your oscillator/astable doesn't
oscillate in SPICE, you should perturb the circuit in
simulation like the first two correct followups to the
original poster.

Nope. *Only* if the astable is not of the 555/Schmitt trigger and
integrator type, or other similar. If it is of that type,
perturbations won't make the slightest difference, so your basic
advice was incorrect. That is, it does not apply to astables in
general.

Kevin Aylward

Newbies should be aware that this is NOT an argument amongst PEERS.
Kevin, as usual, has his head up his ass and can only see 555's ;-)

Do you mean, that if we assume that we have a 555 or a Schmitt gate
with RC feedback, or similar astable, that they require noise to get
them started?

No one said that a 555 needs noise to start the oscillation.

There was certainly an implication made that astables require noise to
start them, when what was actually meant was only some astables might
need noise to start them.

You
assumed astables were 555's.

Not at all. I made an assumption of a class of astables.

It was an assumption you made to be

argumentative about the help and well-placed advice was humbly

Engelhardt with humble is a violation of the known laws of physics.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Mike Engelhardt]

Kevin,

There was certainly an implication made that astables require noise to
start them, when what was actually meant was only some astables might
need noise to start them.

Nonsense. It was a guess to the problem based on the original
poster's circuit description. I certainly never thought nor
intended that 555 type astables needed noise to start. No one
thinks that.

Not at all. I made an assumption of a class of astables.

A *wrong* assumption about the original poster's circuit. His
circuit did need a kick start. You might read his post and his
follow-up.

OK, possibly overly optimistic, but one can still hope.

--Mike