Electronics > Beginners
NPN C-E Buffer Output in LT Spice
eev_carl:
Hi,
(Big edit from the original post; had the wrong circuit breadboarded)
Is there a way in LT spice for me to AC couple the output as with my scope? When I breadboard this, I get the same 3.4V offset but AC coupling brings this back to 0V.
Thanks,
Carl
StillTrying:
The cap time constants might be much too long, change the 1000u to about 1n.
Ian.M:
C2*R4 gives a time constant of one second. Your sim runs for 0.1 seconds. Don't use the .tran startup option unless your sim duration is greater than five times the coupling network time constant!
.tran startup prevents LTspice precalulating the steady state DC bias across the coupling capacitor, because it starts V1 at 0V.
Edit: struck out bad maths, see below. |O
StillTrying:
--- Quote from: Ian.M on November 17, 2018, 02:28:28 pm ---C2*R4 gives a time constant of one second.
--- End quote ---
I guesstimate it at 20 minutes! or 1ms with a 1n. :)
Ian.M:
Why would you guestimate it? The effective output impedance of the emitter follower is very small compared to R4, so its contribution can be ignored, and the component values of R4 and C2 couldn't be easier to multiply by mental arithmetic.
N.B. Five time constants is simply the time it takes for a RC exponential to settle to within 1% of its final value - good enough for practical work. Add another five time constants settling time for every additional two digits of accuracy when taking a precision measurement.
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