Designing an amplifier - Page 1
Forum section: electronics / beginners

author: adem
author_rank: Newbie
timestamp: April 29, 2020, 10:08:48 pm
content: Hi @everyone in the world.Our teacher gave us students a new job. He gave us a circuit with the parts we should take and we should look up the technical docs and pick up the voltages and currents, then calculate the rest.In the file Hinweisendstufe.pdf, you see the circuit.In the other file, I wrote some currents down. The currents from the top half are picked from another project, where they made sense.In this circuit it seems to be bullshit and this is where my questions starts.I could add the calculations too, but I think, everything is bullshit.How do I design such a circuit?How do I look up for the right diagram in the technical docs?Can you recommend a book about this topic?I built up the circuit in LTSpice and it's in the attachments, too.Thanks

author: Wimberleytech
author_rank: Super Contributor
timestamp: April 30, 2020, 06:35:18 pm
content: I dont understand what your assignment was. Must use the architecture shown?Your LTSpice file is missing some symbols and models for those transistors.Regardless...I spent a couple of minutes on your problem.You got it biased all wrong.I made a couple of assumptions on bias current, changed the gain, and replaced the parts with what I have in my library.Maybe this will help you unwrap yourself from around the axle.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: April 30, 2020, 07:49:23 pm
content: Hi,I think that the circuit is closer to this:50mA quiescent current in the output stageVoltage gain 5.1Input 0.775 V (RMS) = sqrt(2) * 0.775 peakOutput = 4V (RMS) into 32 Ohms for 0.5Wgain required 4/0.775 = 5.16I have switched the parts to one that are in the standard LTspice distribution.I have attached the LTspice modelRegards,Jay_Diddy_Bheadphone amplifier JDB.asc(3.47 kB - downloaded 59 times.)

author: adem
author_rank: Newbie
timestamp: April 30, 2020, 11:56:39 pm
content: Quote from: Wimberleytech on April 30, 2020, 06:35:18 pmThanks for the effort, but I cannot choose other Transistors and I didn't thought about the fact, that you don't have the models (BC107, BD135, BD136, BC177).

author: adem
author_rank: Newbie
timestamp: May 01, 2020, 12:09:58 am
content: Thank you for your effort.The Datasheets for these Transistors are way better than the old ones.Can you please explain, how to choose the value for RBE?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 01, 2020, 12:13:09 am
content: Hi,Here is the BC/BD version.I have added the BD135 / 136 and BC107 BC177 models to the schematic.and the LTspice is belowJay_Diddy_Bheadphone amplifier JDB BD BC.asc(4.55 kB - downloaded 92 times.)

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 01, 2020, 12:21:44 am
content: Hi,RBE is chosen like thisThe quiescent current in the output transistor is chosen to be 50mAIb (Q2) = 50mA / HFE (Q2)= 50mA / 200 = 250uAChoose I rbe = Ib/20.7 / 125uA = 5.6kuse 5.6K or 4.7KJay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 01, 2020, 12:42:53 am
content: Hi,To answer your next question.I have replace the generic op-amp model with the LF356 model obtained from the TI website.Regards,Jay_Diddy_Bheadphone amplifier JDB BD BC LF356_NS.asc(7.71 kB - downloaded 52 times.)