(SOLVED) Simple circuit does not work in LTspice

Thread Starter

Danko

Joined Nov 22, 2017
1,834
Сircuit below oscillates in reality, but not oscillates in LTspice simulation.
How to simulate such kind of circuits in LTspice?

1703705390854.png

Oscillogram of real circuit:
1703706173861.png

LTspice simulation:
1703706352852.png
 

Attachments

panic mode

Joined Oct 10, 2011
2,733
what you have on board and what you have in simulation are two different things.
on board you have emitter of PNP transistor connected to +
but in simulation, emitter and collector are swapped on both transistors.
1703723964015.png

btw. not everything works in simulation.
here is a simple astable multivibrator that definitely works in real life but fails in LTSpice
which is why LTSpice includes its own 'educational' version...
base resistors are purposely mismatched to help start oscillations.
increasing their value some 10x does make the this circuit work as well.
1703724206997.png
 

Attachments

Last edited:

crutschow

Joined Mar 14, 2008
34,392
here is a simple astable multivibrator that definitely works in real life but fails in LTSpice
That's a common misconception.
Don't panic. ;)
It's not a simulation failure, it's a failure to communicate.
Many who use LTspice don't realize that it normally calculates the circuit DC operating point before the transient analysis.
In this case the circuit can settle into a quasi-stable DC state since there is no natural noise or asymmetries in the circuit.
If you omit the initial DC calculation by using either the startup or UIC option in the transient analysis, the oscillation occurs nicely, even if nothing is mismatched (below):

1703729807733.png
 
Last edited:

Thread Starter

Danko

Joined Nov 22, 2017
1,834
Don't see how.
The simulated transistors both have reverse polarity voltage applied to them.
Post your real circuit schematic.
For wide range of possible R1 value changes, without latching,
β of Q1, Q2 should be as close to 1 as possible.
Therefore Q1 and Q2 used in reverse mode (β=2-3).
See how oscillator works with transistors in normal mode -
one infinite pulse (latching):
1703740080794.png

on board you have emitter of PNP transistor connected to +
See:
1703735775156.png
 

Thread Starter

Danko

Joined Nov 22, 2017
1,834
Reverse mode is with the base and emitter interchanged.
"Reverse-active (or inverse-active or inverted)
By reversing the biasing conditions of the forward-active region,
a bipolar transistor goes into reverse-active mode.
In this mode, the emitter and collector regions switch roles."

https://en.wikipedia.org/wiki/Bipolar_junction_transistor#:~:text=Reverse-active (or inverse-,and collector regions switch roles.

You had the NPN and PNP reversed.
Every transistor is reversed (collector and emitter interchanged).
1703748922649.png
ADDED:
Nobody answer my question - how to simulate this particular circuit?
Seems reversed mode not detailed in LTspice BJT models...
 
Last edited:

panic mode

Joined Oct 10, 2011
2,733
same here...
starting with output low, C1 is charging and Q1 base potential is increasing.
at some point Q1 and Q2 turn on so the output (red) is high.
but capacitor voltage cannot change instantaneously so this rise means voltage increase on Q1 base as well.
this only helps drive both transistors into same direction which is steady state.
don't see how they will ever turn off... maybe something not obvious is taking place (tunneling?)
 
Last edited:

tiwag

Joined Jun 17, 2010
3
Сircuit below oscillates in reality, but not oscillates in LTspice simulation.
How to simulate such kind of circuits in LTspice?

View attachment 311056

Oscillogram of real circuit:


LTspice simulation:
Some basic understanding in electronics would help.
You need a bigger RLoad resistor in order to get enough basis voltage for Q2 to get into conduction.
Thats it, then it works fine

ScreenShot 20231229093044 831 LTspice XVII - osc1.asc.png

HTH Tiwag
 

Attachments

tiwag

Joined Jun 17, 2010
3
Some basic understanding in electronics would help.
You need a bigger RLoad resistor in order to get enough basis voltage for Q2 to get into conduction.
Thats it, then it works fine



HTH Tiwag
Here is an optimized version which works more robust
osc1_opt.png
 

Thread Starter

Danko

Joined Nov 22, 2017
1,834
Reverse β is set: BR=20 for Q1 and Q2.
Result: good coincidence.

LTspice diagram:
1703863771545.png

Oscillogram from real circuit:
------1703863856007.png
 

k1ng 1337

Joined Sep 11, 2020
960
We must subscribe to the requirements of Nature or it will not do what we want it to do!

Julius Sumner Miller


So I ask, did the circuit NOT work or did you just fail to understand it? :p
 
Top