why is this amplifier ocsillating, not work? - Page 1
Forum section: electronics / beginners

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 06, 2016, 07:27:44 pm
content: hi everyone,do any one know why this transistor amp not working?note: all values are in the pic below>Q1 & Q2 are BC107 transistors.

author: AndyC_772
author_rank: Super Contributor
timestamp: March 06, 2016, 07:47:43 pm
content: You've not assigned a specific type to either transistor, nor values for the mic capacitor or R6. The simulation model is, therefore, incomplete.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: March 06, 2016, 07:49:03 pm
content: FWIW... here's an image of the schematic.

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 06, 2016, 08:06:08 pm
content: R6 is 220 ohmand the mic is a normal electrec mic and this is not mean for simulation but as a schimatic,I will post the test results i'm getting soon...But in general the first stage which is the preamp part is working quite well giving an output of 1v pk-pk,I think the problem is in the scond stage where the DC ofset is add.And for the oscillation it was oscillating at a frequincy of around 1 to 3.5khz with a laud humming sound.

author: Zero999
author_rank: Super Contributor
timestamp: March 06, 2016, 08:23:29 pm
content: Are you sure it isn't acoustic feedback?This circuit isn't very good. The sound quality will be poor and power consumption high.The LM386 would be better.

author: wiss
author_rank: Frequent Contributor
timestamp: March 06, 2016, 08:25:16 pm
content: Inductive loads on a colector might be a bad idea, move the load to the emitter side?

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 07, 2016, 08:14:52 pm
content: Quote from: wiss on March 06, 2016, 08:25:16 pmi did it and it works nicely! thanks, and i changed R6 to 110 ohms.Quote from: Hero999 on March 06, 2016, 08:23:29 pmmaybe it is acoustic feedback because of the buzzing sound that drops when i block the microphone.well, i would if i could but the instructor told us not to us op amps for now, so i can't use it.so how to deal with this acoustic feedback because the only way it seems to be working is just lowering the volume which is not helpful i also tried filtering it with no good results.

author: Audioguru
author_rank: Super Contributor
timestamp: March 07, 2016, 09:09:28 pm
content: Of course if the mic can hear the speaker then you will have acoustical feedback. Put the mic in one room, put the speaker in another room and close the door between them.Or record it and play it back later with the mic turned off.I have new hearing aids behind my ears where the mics are. The speakers are inside my ear canal and there was feedback only when the feedback cancelling mode was turned off and the gain was high.

author: Zero999
author_rank: Super Contributor
timestamp: March 07, 2016, 11:15:41 pm
content: Quote from: ali6x944 on March 07, 2016, 08:14:52 pmHow many transistors are you aloud to use? More than two transistors is required to build a decent quality audio amplifier which can drive a speaker.

author: danadak
author_rank: Super Contributor
timestamp: March 08, 2016, 12:02:48 am
content: An electret mic generally speaking wants to use a charge amp,which in this ap note is a TIA.http://www.ti.com/lit/ug/tidu765/tidu765.pdfhttps://cdn.sparkfun.com/datasheets/BreakoutBoards/SparkFun_Mic-Preamp-v12.pdfhttp://www.epanorama.net/circuits/micamp.htmlTake a look at this for some of the design challenges. Even thoughit uses an OpAmp it covers some of the general design issues.Regards, Dana.

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 08, 2016, 04:02:14 am
content: Quote from: Hero999 on March 07, 2016, 11:15:41 pmHe said 5 transistors max and i alraedy used 2 of the 5!But i think it was ok to use opamp comparators so i tried it an see my thread the peaple there helped alotBut non of them worked properly all of them made a nasty hushing sound it was like hearing an crapy AM radio in a room full of crapy low quality SMPS.

author: wiss
author_rank: Frequent Contributor
timestamp: March 08, 2016, 08:09:57 am
content: I assume that the electret mic is not a bare but a commercial component, then it will have a JFET amp inside and wants to feed a rather low impedance.You could keep the speaker on the collector side of the output transistor is you make that point voltage driven, i.e. heavy feedback on that transistor. You could probably improve quality by increasing feedback on the first stage as well, assuming the gain will be enough.

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 08, 2016, 09:59:13 am
content: so mosft instaed of bjt for the second stage?i will post the capture photos from the scope.

author: wiss
author_rank: Frequent Contributor
timestamp: March 08, 2016, 11:16:38 am
content: No MOSFET, they are usually very slow in these applications, BJT will be fine.

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 08, 2016, 08:15:34 pm
content: here is the capture:

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 08, 2016, 08:22:35 pm
content: it is working nicely, i got a good mark, the only thing that the instructor was commenting about is the hushing/noise sound and volume adjustment.thanks for everybody it was really nice of you guys to help me in this project thanks very much

author: Zero999
author_rank: Super Contributor
timestamp: March 08, 2016, 09:00:59 pm
content: Quote from: ali6x944 on March 08, 2016, 04:02:14 amWhy are you just using 2 transistors then? Does your project need to do something else too?You can build a pretty reasonable low power amplifier with 5 transistors.

author: Audioguru
author_rank: Super Contributor
timestamp: March 08, 2016, 11:45:54 pm
content: Quote from: wiss on March 08, 2016, 08:09:57 amThe Drain of the Jfet has an impedance of about 3.3k and when effectively in parallel with the 10k resistor that powers it produces 2.5k. Then its load should be 25k or more to avoid signal loss.This transimpedance amplifier idea from Texas Instruments is something good that I have never seen before. It is too bad that the student is not allowed to use it.