power amp circuit  - Page 1
Forum section: electronics / beginners

author: NASK
author_rank: Contributor
timestamp: July 19, 2018, 12:00:32 pm
content: HiI need to design simple power amp circuit using Darlington class AB push-pull amplifier with having input resistance 10k to drive 8 Ω speakers. I followed book Electronic devices Thomas L. Floyd and it gives Rin = βac(r e + RL) // R1 // R2 (page 356) equation to design it. In my design output gain is reducing in results, how correct this issue.

author: Zero999
author_rank: Super Contributor
timestamp: July 19, 2018, 01:11:29 pm
content: There's some crossover distortion there. Add some emitter resistors to the output stage and increase the base voltages by reducing the resistor values, adding an extra diode or a BJT with a VBEmultiplier.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: July 19, 2018, 01:55:49 pm
content: Hmm, don't you maybe need some voltage gain too? there's no voltage gain in this circuit, thus not much power gain, if the input signal doesn't already have sufficent amplitude

author: NASK
author_rank: Contributor
timestamp: July 20, 2018, 07:44:01 am
content: QuoteTherefor i change the value but required input resistance is 100k , current system has 247R how improve it?QuoteI do not need voltage gain it is doing by pre-amp. pre amp circuit have sufficient amplitude.

author: Zero999
author_rank: Super Contributor
timestamp: July 20, 2018, 09:57:55 am
content: The output stage should be in the feedback loop of the entire amplifier circuit, which will both boost the input impedance and reduce the distortion.Those compound output transistors might not be needed. A similar thing has been done before, see the following thread:https://www.eevblog.com/forum/projects/guitar-headphone-amplifier/

author: BrianHG
author_rank: Super Contributor
timestamp: July 20, 2018, 04:50:16 pm
content: 1. Place a 100uf cap across the 2 bases of the complimentary pair transistors to boost your rail-rail output voltage range performance.2. Without the 2 transistors mounted together on a small heatsink with a thermal readback of them to shrink the voltage across the transistor bases, this circuit will go through thermal runaway. Your other choice to mitigate this is to add 2 small series resistors on the emitter outputs.3. Optionally add a resistor between T1's and T2's base with a cap in parallel on top of that resistor to further control/improve distortion and thermal runaway.These tips need to be checked in the real world as they still work in the simulator, however, the simulator might be treating your components as perfect ideal components and my tricks help in extending the performance at the edges.

author: NASK
author_rank: Contributor
timestamp: July 24, 2018, 11:13:12 am
content: Thank you all for reply.Hero999 given solution have an op-amp. Inherently op-amp input impedance is high because of that it is suitable to get high resistance preamp output. And also it is in negative feedback loop in buffer amp stage because of that it boosts the impedance. My target understands the basic transistor design concepts; therefor I wish to use common collector amp (emitter follower circuit) method for increase input impedance. And also add a negative emitter feedback bias circuit to provide additional gain total output. I believe these methods are equal to single op-amp method. I have attached my circuit and results.BrianHG, you say adding 100uf cap across the 2 bases of the complimentary pair, any way previously I have used Darlington pair method not the Sziklai Pair (I think you are telling about it) or I am not understanding you told thing where to put 100uf cap, if it is could you tell it with diagram please. As you told way I add small resistors to emitter output to prevent thermal runaway.Anyone could you tell me how to calculate R11 feedback resistor value, I choose it just trial & error method I need to know how calculate it (I cal value is equaling to 5K)

author: NASK
author_rank: Contributor
timestamp: July 25, 2018, 03:48:01 am
content: First I thought emitter follower is required to impedance match, without it also working with negative feedback since it boost the signal. Any way power amp side values are choose in trail and error method any one could you help me to calculate it? (Vout=2v) second thing, why is it not symmetrical?

author: Audioguru
author_rank: Super Contributor
timestamp: July 25, 2018, 01:48:35 pm
content: Quote from: NASK on July 25, 2018, 03:48:01 amThe bottom half of the output waveform is squashed because the input transistor has no negative feedback so its output squashes the top half of its output signal.

author: NASK
author_rank: Contributor
timestamp: July 26, 2018, 03:02:34 am
content: Thank you for replyYou are telling input transistor are not in negative feedback(T3&T7), but I think both are in negative feedback. If I am wrong please rectify the issue with a diagram. Many thanks.I think this away also, pre amp stage having squash . For small signal it is not showing but high ampitide it is showing.

author: Audioguru
author_rank: Super Contributor
timestamp: July 26, 2018, 03:36:23 am
content: R11 provides negative feedback to the second transistor but not to the first transistor. My simulation shows the distorted result.You have a tiny microphone input level and a high speaker output level so the gain of your amplifier is very high requiring not much negative feedback. For less distortion then the preamp transistor should have two transistors in a circuit with its own high gain and a lot of negative feedback.

author: NASK
author_rank: Contributor
timestamp: July 26, 2018, 04:55:38 am
content: Many thanks again to above post. I have never experienced to calculate negative feedback transistor base circuit plz could I know where to find tutorial or ebook to desing this type of amps

author: Audioguru
author_rank: Super Contributor
timestamp: July 26, 2018, 10:40:04 am
content: Go to Google and search for Transistor Negative Feedback.

author: NASK
author_rank: Contributor
timestamp: July 31, 2018, 09:16:47 am
content: Now it is working with symmetrical!

author: Audioguru
author_rank: Super Contributor
timestamp: July 31, 2018, 12:32:09 pm
content: The sounds will be clearer without the distortion you had before.

author: Zero999
author_rank: Super Contributor
timestamp: July 31, 2018, 03:07:04 pm
content: Quote from: NASK on July 31, 2018, 09:16:47 amThat looks pretty badly distorted, better than before, but still not good and the waveform is far from symmetrical.EDIT:It's more obvious it isn't symmetrical, after the bottom half of the signal is flipped over, i.e. fullwave rectification. Look the amplitude of the negative half, doesn't even match the positive and it's skewed horribly.

author: JS
author_rank: Frequent Contributor
timestamp: July 31, 2018, 03:34:21 pm
content: You could use an opamp inside the loop to increase the open loop gain, getting more feedback to correct the distortion, here is a simple circuit that works nice, as you see it allows you to add a DC servo to eliminate DC and not needing an external cap if running from dual rails. You could use it or ignore it and use the thing like an opamp.http://www.jlmaudio.com/JLM%20Hybrid%20Opamp%20circuit.pdfJS

author: Zero999
author_rank: Super Contributor
timestamp: July 31, 2018, 03:43:51 pm
content: Quote from: JS on July 31, 2018, 03:34:21 pmI already suggested a similar thing a few posts ago, but the original poster wants a discrete design. I'd suggest a discrete op-amp, but it might be too complicated.

author: JS
author_rank: Frequent Contributor
timestamp: July 31, 2018, 05:11:22 pm
content: Quote from: Hero999 on July 31, 2018, 03:43:51 pmA discrete opamp might not be too complicated, 5 transistors could do. Here ia a design I've used in the past, for low bandwidth should be good enough, like audio band, something like 40dB of open loop but it can reduce crossover distortion quite a bit.JS