design audio amplifier circuit - Page 1
Forum section: electronics / beginners

author: NASK
author_rank: Contributor
timestamp: August 04, 2017, 07:06:07 am
content: I wish to design audio amplifier circuit. Hear I use two stage of amplifiers. 1st stage amplifies 100mv to 1.2V peak @ 2KHz.Q1. When I change the frequency to 50Hz output becomes to 210mv peak signal. At that time when I changed c3 cap to 100uf again it amplify 1.25V peak signal. I need to amplify audio signal therefore amplitude is changing according to frequency. What can I do for it?Q2. 2nd stage is power amplify part. Output of at that stage 180mv peak shows @ 2 KHz signal. That mean 2nd stage amplitude is decreased how it happened and solution for it.circuit it attached

author: Zero999
author_rank: Super Contributor
timestamp: August 04, 2017, 08:30:45 am
content: 1) C3 was too small. Now you've fixed it by increasing the value to something sensible.2) The transistors probably aren't getting enough base drive. Did you measure it with the speaker connected to the output?Why not use the LM386?

author: mikerj
author_rank: Super Contributor
timestamp: August 04, 2017, 10:47:53 am
content: Quote from: NASK on August 04, 2017, 07:06:07 amThe input coupling capacitor and the the input impedance of the amplifier forms a high pass filter which determines the lower cut off-frequency of your amplifier. If you need it to operate at a lower frequency then use a larger value capacitor, but since you have already done this I'm not sure what you are asking?Quote from: NASK on August 04, 2017, 07:06:07 amThis is because you are biasing the output stage using ridiculously low value resistors. The two 470 Ohm resistors are effectively paralleled so your first stage is trying to drive an impedance of 470/2 = 235Ohms (ignoring the impedance of the output transistors). Your first stage has an output impedance of approximately 1k235/(1000+235) * 1.25 = 237mV, very close to what you are measuring.You don't need to AC couple the first stage to the output stage. If you DC couple it, and correctly bias the first stage then the output stage will also be correctly biased since it is a follower, and won't need separate biasing. Also why have you used three capacitors to couple the stages together?

author: Zero999
author_rank: Super Contributor
timestamp: August 04, 2017, 11:00:14 am
content: Quote from: mikerj on August 04, 2017, 10:47:53 amYes, that's true. I forgot about the input capacitor.QuoteThat's true, but he said:Quote from: NASK on August 04, 2017, 07:06:07 amThat must have been with the first stage not connected to the second.

author: mikerj
author_rank: Super Contributor
timestamp: August 04, 2017, 11:41:56 am
content: Quote from: Hero999 on August 04, 2017, 11:00:14 amHis attached pdf shows that the first stage is disconnected from the output stage whilst he is measuring the voltage.

author: Zero999
author_rank: Super Contributor
timestamp: August 04, 2017, 01:18:53 pm
content: Quote from: mikerj on August 04, 2017, 11:41:56 amYes, you're right. I missed that. You're right, that's what you should expect to happen.The output stage should be DC coupled with no unnecessary coupling capacitors.There should be some emitter resistors to prevent thermal runaway.How much gain is required? This is a crappy circuit with not much power output and lots of distortion, at high output levels. The capacitors could be larger for better low frequency response, but there's no point going lower than couple of hundred Hz, because small speakers have no bass.