[Solved]Emitter follower weirdness - Page 1
Forum section: electronics / beginners

author: Blaffetuur
author_rank: Regular Contributor
timestamp: April 09, 2016, 12:24:17 pm
content: Hi, i'm having trouble with following circuit:When I connect my function generator to this circuit, the sine wave drops 1.6V and I can't figure out whyI'm using a function generator that i made around the XR2206 chip, mabe that's the problem.In the attachments I have the simulated result, the generator output in the scope, and the generator connected to the circuit.Thanks !

author: danadak
author_rank: Super Contributor
timestamp: April 09, 2016, 12:49:23 pm
content: Because you are DC coupled to emitter follower means the most negative peak of theinput sine must not drop lower than the Vbeon of Q1, to stay in linear region.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: April 09, 2016, 12:57:32 pm
content: Example, attached.

author: 3db
author_rank: Frequent Contributor
timestamp: April 09, 2016, 12:59:24 pm
content: Quote from: danadak on April 09, 2016, 12:49:23 pmIn addition that circuit will be non-linear at the crossover region.Do you want this circuit to be AC coupled or DC ?I suggest AC and you bias the transistor so it operates in it's linear region.Regards3DB

author: danadak
author_rank: Super Contributor
timestamp: April 09, 2016, 01:00:44 pm
content: Example with not enough offset.A quick and easy fix is use a RRIO opamp as a follower.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: April 09, 2016, 01:20:31 pm
content: Example with RRIO OpAmp.Regards, Dana.

author: Blaffetuur
author_rank: Regular Contributor
timestamp: April 09, 2016, 01:50:58 pm
content: Quote from: danadak on April 09, 2016, 01:20:31 pmQuote from: 3db on April 09, 2016, 12:59:24 pmI'm following the LAoE book, and was making this circuit as an example.

author: Blaffetuur
author_rank: Regular Contributor
timestamp: April 09, 2016, 02:06:22 pm
content: Quote from: danadak on April 09, 2016, 12:49:23 pmSo why does my simulation not show this offet ?

author: BobsURuncle
author_rank: Regular Contributor
timestamp: April 09, 2016, 07:02:39 pm
content: Quote from: Blaffetuur on April 09, 2016, 02:06:22 pmYour simulated generator is AC and it looks like your real generator has a negative DC bias. I assume that is what the -1.59V Avg means in the lower right corner.

author: Blaffetuur
author_rank: Regular Contributor
timestamp: April 09, 2016, 07:12:22 pm
content: Quote from: BobsURuncle on April 09, 2016, 07:02:39 pmIf i connect my function generator straigt to my scope, I get a sine centered around 0V. When I connect it to my circuit I see it gradually moving down (2nd and 3rd attachments to my post)

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: April 09, 2016, 07:21:42 pm
content: Can you post a picture of the circuit you built up, including how the function generator and oscilloscope are connected?

author: BobsURuncle
author_rank: Regular Contributor
timestamp: April 09, 2016, 07:22:16 pm
content: If you have a capacitor coupling the generator into the circuit it is going to charge up with a DC bias since you are only conducting in one direction. This is because the circuit is biased in such a way as to act as a half wave rectifier. Into the scope alone it is conducting in both directions, i.e. average of zero DC. Or maybe you have a floating ground somewhere.

author: Audioguru
author_rank: Super Contributor
timestamp: April 09, 2016, 07:28:37 pm
content: That is what I was going to say.The base current of the transistor is charging the output capacitor of the signal generator. When the capacitor becomes fully charged then the transistor will have an extremely small output.

author: Zero999
author_rank: Super Contributor
timestamp: April 09, 2016, 08:32:39 pm
content: Bias the input at half the supply voltage and couple the input and output signals, via capacitors.

author: BobsURuncle
author_rank: Regular Contributor
timestamp: April 09, 2016, 08:40:22 pm
content: I don't think he is trying to build a proper linear amp, he is just doing an exercise from LTAOE which is supposed to produce what he sees in the simulation. His schematic is correct, the sim results are correct, but the circuit he built is not responding the same and he wants to understand why.Blaffetuur, are you sure the scope, generator and circuit are sharing the same ground? If your FG is battery or transformer supplied its probably floating, unless you tied it to something. I think the problem is capacitive charging somewhere. The circuit you built is not the same as the schematic - you are assuming something that is not so.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: April 09, 2016, 11:36:25 pm
content: Yes, most probably something to do with the function generator the OP is using. Here's what I get using my cheapo DDS MingHo MHS5200A FG and a HP721A power supply, exactly what is expected and the sim shows. There must be something fishy about the OP's FG ground system.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 10, 2016, 04:55:54 am
content: Note that the simulation source is ideal, i.e., zero ohms source resistance. Your real life generator is 50 ohms.Tim

author: Blaffetuur
author_rank: Regular Contributor
timestamp: April 10, 2016, 08:50:05 am
content: I build up the circuit again today and hooked up my adjustable powersupply to the input of the circuit. After palying with the voltage knob I got the response out of the circuit that I was expecting so the problem had to be my function generatorYou can see in the Schematic.jpg that there is a 100nF cap in series with the output, so I decided to take the signal before the cap. And sure enough now I get the scopeview that I wantedThe function generator schematic is something I found online, so when I build it I didn't know the function of that capacitor, so do I just leave that cap out ?

author: danadak
author_rank: Super Contributor
timestamp: April 10, 2016, 11:17:48 am
content: If you are trying to build a usable func gen then it should be capable of -1) Adjustable amplitude2) Adjustable offset with respect to ground3) 50 ohm output Z4) Sine, triangle, square, ramp outputsSo that means split supplies, and a buffer operating off split supplies. The Zoutyou want to achieve is that of a voltage generator, as close to 0 as you can, andthe ability to source current from either rail, which the buffer will take care of.Thats just the basics.As an aside on eBay and the like are DDS based generators, that have fairly precise fand V and are sweepable, PC controllable over USB. Make for very handy bench aids.Here is example but w/o sweep, PC controlhttp://www.ebay.com/itm/DDS-Function-Signal-Generator-Sine-Square-Triangle-Sawtooth-Wave-Low-Frequency-/221924868290?hash=item33abc0c0c2:g:7jcAAOSwstxU2GqzRegards, Dana.

author: MrSlack
author_rank: Frequent Contributor
timestamp: April 10, 2016, 12:59:52 pm
content: +1.Watch out for your average XR2206 schematic online. Know the IC's limits: It has a roughly 600 ohm very non-linear non-ideal impedance by default with a DC offset unless it has dual supplies and an output follower.

author: BobsURuncle
author_rank: Regular Contributor
timestamp: April 10, 2016, 04:06:14 pm
content: Quote from: Blaffetuur on April 10, 2016, 08:50:05 amYou are welcome.

author: Blaffetuur
author_rank: Regular Contributor
timestamp: April 10, 2016, 07:19:30 pm
content: Quote from: danadak on April 10, 2016, 11:17:48 amI'm not 100% sure, but maby I will just buy a propper function generator (lets sleep over it a couple of nights...). But which one to get?Is the Siglent SDG805 any good? (http://www.batronix.com/shop/waveform-generator/Siglent-SDG805.html)or should I spend a bit more and get a Rigol ? (http://www.batronix.com/shop/waveform-generator/Rigol-DG1022.html)

author: Blaffetuur
author_rank: Regular Contributor
timestamp: April 10, 2016, 07:24:13 pm
content: For making all the examples in LAoE I also need a sweep controll so another reason to go shopping

author: MrSlack
author_rank: Frequent Contributor
timestamp: April 10, 2016, 07:28:35 pm
content: If you've got an XR2206 you can control the frequency with a voltage (see the data sheet). You can use a ramp generator to provide the frequency control voltage and the trigger for the scope using the ramp generator described by w2aew here:https://youtu.be/bVahBmsvirEAnd there's your sweeping function generator!I rather like cheaping out like this - it's more fun

author: Blaffetuur
author_rank: Regular Contributor
timestamp: April 10, 2016, 07:52:23 pm
content: Quote from: MrSlack on April 10, 2016, 07:28:35 pmI had build his schematic up already and have parts shipped to put it in a nice box

author: Blaffetuur
author_rank: Regular Contributor
timestamp: April 10, 2016, 07:57:29 pm
content: Quote from: MrSlack on April 10, 2016, 07:28:35 pmBut I like looking at new stuff

author: danadak
author_rank: Super Contributor
timestamp: April 10, 2016, 08:33:21 pm
content: Choice of generator really driven by user tasks you want to accomplish.Look at youtube, usually someone has done an evaluation. Do a side byside spec comparison and choose from that, Excel a great way of accom-plishing that.Regards, Dana.