NPN C-E Buffer Output in LT Spice - Page 1
Forum section: electronics / beginners

author: eev_carl
author_rank: Regular Contributor
timestamp: November 17, 2018, 02:09:31 pm
content: 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

author: StillTrying
author_rank: Super Contributor
timestamp: November 17, 2018, 02:25:45 pm
content: The cap time constants might be much too long, change the 1000uto about 1n.

author: Ian.M
author_rank: Super Contributor
timestamp: November 17, 2018, 02:28:28 pm
content: C2*R4 gives a time constant ofone 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.

author: StillTrying
author_rank: Super Contributor
timestamp: November 17, 2018, 02:47:49 pm
content: Quote from: Ian.M on November 17, 2018, 02:28:28 pmI guesstimate it at 20 minutes! or 1ms with a 1n.

author: Ian.M
author_rank: Super Contributor
timestamp: November 17, 2018, 03:02:07 pm
content: 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.

author: StillTrying
author_rank: Super Contributor
timestamp: November 17, 2018, 04:18:42 pm
content: "Why would you guestimate it?"OP said:"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."I'm guesstimating that with 0.5M to 10M of scope impedance on point Vout in the 1st post, thatthe DC blockingof the 1000uF will appear to be not working, because it's taking so very long to charge the 1000uF.Or something..

author: Audioguru
author_rank: Super Contributor
timestamp: November 17, 2018, 05:37:01 pm
content: 1000uF feeding 1Meg has the u and M cancel each other. Then the capacitor charges to 63% of a full charge in 1000 seconds which is 16.7 minutes. It takes 5 times as long (83.3 minutes or 1.39 hours to be almost fully charged but the capacitor leakage current will prevent that.

author: Ian.M
author_rank: Super Contributor
timestamp: November 17, 2018, 05:51:58 pm
content: Audioguru,Many thanks for spotting my three orders of magnitude brainfart. Maybe I'm getting too old for math!

author: Zero999
author_rank: Super Contributor
timestamp: November 17, 2018, 08:46:20 pm
content: It's not what you asked, but the bias resistors (R1 and R2) are far too low. The input impedance will be equivalent to R1, R2 and the transistor's base impedance in parallel. The transistor's base impedance is equal to the Hfe * R3, which is a few Megs, so can be ignored, leaving R1 and R2 in parallel, which would give R = (1k2*1k)/(1k2+1k) = 1k2/2k2 = 0.545k = 545RAssuming the Hfe is around 200, the base impedance will be 2M, so the impedance of the R1 and R2 in parallel can be1/10of that, which is around 200k, about 400 times the values of R1 and R2 in parallel, so they can be multiplied by 400, giving R1 = 470k and R2 = 390k as the nearest standard E24 values. The total input impedance will now be around 200k and the value of C1 can be reduced to reflect that, so use 100nF.