Distortion in darlington amplifier

[Stan2020]

I have assembled a circuit published before:

With an input of 500Hz sin wave and 20 mv amplitude the output on 8 ohms speaker looks like this:

I am not good at electronics; should it be a single transistor I'd suggest the wrong operating point choice;
but what is this in case of compound transistor?
I noticed that distortions are getting smaller if I decrease emitter resistor up to 60 ohms.
Is this circuit diagram wrong?

 

[Nigel Goodwin]

It's an absolutely horrible 'circuit', and is never going to sound any good - there's no negative half cycles as you've got no negative drive capacity.

And yes, the circuit diagram is wrong.

 

[audioguru]

The horrible circuit is designed by a student who knows nothing about electronics and writes with very poor English.

 

[crutschow]

Build a proper audio amp with a push-pull output if you want to power a speaker.

 

[audioguru]

You can build a low power poor-sounding 3 transistors push-pull amplifier or you can buy a mono LM386 (0.14W into 8 ohms at low distortion when powered from 5V) or buy a stereo PAM8403 (2.5W per channel into 4 ohms when powered from 5V). The IC's cost $1.00US or less.

 

[Stan2020]

Thank you everyone for responses!
I did have a feeling there is something wrong with this schematics, although I found if emitter resistor is decreased to 50 ohm then distortions disappear and amplifier produces a decent sound from 15 mV input.
I don't think 'poor English' of the original author has anything to do with his knowledge of electronics.
My English is quite far from being 'perfect' - does this disqualify me?
Of course I tried making an amplifier with LM386 - to be honest, I am not that impressed. The standard circuit (from here for example) shows a strong tendency for self-generation even without capacitor between 1 and 8.
I ended up yesterday assembling this simple circuit and it sounds quite good, no distortions:

The whole idea is to play with nostalgic memories of when I was a kid and trying to make AM radios, mostly failing
I already made simple one-stage RF amplifier and pre-amp as well, not perfect ones but local AM broadcasting sounds loud enough (although with a lot of hissing). And yes, I tried MK484 as well

 

[Nigel Goodwin]

Thank you everyone for responses!
I did have a feeling there is something wrong with this schematics, although I found if emitter resistor is decreased to 50 ohm then distortions disappear and amplifier produces a decent sound from 15 mV input.
I don't think 'poor English' of the original author has anything to do with his knowledge of electronics.
My English is quite far from being 'perfect' - does this disqualifies me?
Of course I tried making an amplifier with LM386 - to be honest, I am not that impressed. The standard circuit (from here for example) shows a strong tendency for self-generation even without capacitor between 1 and 8.
I ended up yesterday assembling this simple circuit and it sounds quite good, no distortions:
View attachment 125995
The whole idea is to play with nostalgic memories of when I was a kid and trying to make AM radios, mostly failing
I already made simple one-stage RF amplifier and pre-amp as well, not perfect ones but local AM broadcasting sounds loud enough (although with a lot of hissing). And yes, I tried MK484 as well

There's still a LOT wrong with that circuit as well, but at least it's got a vague similarity to a working circuit - your first one didn't.

 

[Stan2020]

There's still a LOT wrong with that circuit as well

May I ask for detailed explanation of your bold statement, otherwise it sounds quite, excuse me, arrogant?

 

[Nigel Goodwin]

For a couple of quick things, no negative feedback from the output, so high distortion and not very stable, plus no bootstrapping of R8, so greatly reduced power output and even more distortion. It's a really, really, poor circuit - on a scale of 1 to 10?, perhaps a 1 or 2, where your first post would be struggling to get up to a zero. Using four transistors (and less passive components) you can make a 'proper' push pull amplifier, as used in commercial radios etc. for decades.

 

[Stan2020]

For a couple of quick things, no negative feedback from the output, so high distortion and not very stable, plus no bootstrapping of R8, so greatly reduced power output and even more distortion. It's a really, really, poor circuit - on a scale of 1 to 10?, perhaps a 1 or 2, where your first post would be struggling to get up to a zero. Using four transistors (and less passive components) you can make a 'proper' push pull amplifier, as used in commercial radios etc. for decades.

Forgive me my little knowledge of electronics...what is 'bootstrapping R8'? Were you trying to say in this highly professional way 'I'd connect a capacitor in parallel with R8'? Please use simple words, if possible, I'd appreciate that. Why do we need this capacitor? Something related to thermal stability?
Can you give a link to ' four transistors (and less passive components) ... used in commercial radios ' schematics _without transformers_ that would have sensitivity of 10mV, output to 8 ohm speaker and satisfy you in regards to the scale from 1 to 10?
Please point out an example of the 'negative feedback from the output' in 4 transistors push-pull amplifier (no transformers)?
How I can see 'high distortions' you've mentioned? I applied up to 10 kHz 20 mV sine input, did not notice anything distorted on 8 ohms speaker output.
How I can see the mentioned 'unstability'? It works every time I turn it on, no self-generation, no spontaneous output change.

 

[unclejed613]

you could linearize that a bit more by adding an emitter resistor to Q1, and running a feedback signal through a voltage divider and coupling cap from the connection of R9+R10. this will reduce distortion even more because global feedback will take into account all the nonlinearities between Q1 and the output. R8 as Nigel said doesn't allow the positive half of the waveform to reach near the positive rail. the "bootstrap" circuit he's talking about splits R8 into 2 resistors, and adds a capacitor to the output and the middle of the two resistors and boosts the + side voltage swing. another way to get the same result is the use of a constant current source in place of R8.

 

[Stan2020]

you could linearize that a bit more by adding an emitter resistor to Q1, and running a feedback signal through a voltage divider and coupling cap from the connection of R9+R10. this will reduce distortion even more because global feedback will take into account all the nonlinearities between Q1 and the output. R8 as Nigel said doesn't allow the positive half of the waveform to reach near the positive rail. the "bootstrap" circuit he's talking about splits R8 into 2 resistors, and adds a capacitor to the output and the middle of the two resistors and boosts the + side voltage swing. another way to get the same result is the use of a constant current source in place of R8.

Thank you! I am still a bit baffled with these efforts to 'linearize' this circuit. I mean, how I can see that it needs linearization? Practically? I mean, it works, no distortion( I am using scope to watch output waveform)..then what?
About R8, are you saying because of it the upper half of the sine will be smaller then the bottom part so the output waveform will be asymmetrical vertically? I failed to notice that on a screen... After all, this is a simple low-end amp, not a hi-fi gizmo, one shouldn't demand too much from it.
'Nigel' just claimed he knows better and simpler circuits, but failed to give an example (I am not surprised).

 

[audioguru]

You did not see distortion because C3 was cutting all the high frequency distortion harmonics. C3 also cut high frequency audio making the amplifier muffled like an old AM radio.
I fixed it and got an output power 5 times more than before. The output power is still very low because the power supply voltage is very low.
Bootstrapping is when the output reaches down with C7 ands pulls up the bootstrap voltage on R8 and R14 allowing a higher output signal swing.

 

[Stan2020]

You did not see distortion because C3 was cutting all the high frequency distortion harmonics. C3 also cut high frequency audio making the amplifier muffled like an old AM radio.
I fixed it and got an output power 5 times more than before. The output power is still very low because the power supply voltage is very low.
Bootstrapping is when the output reaches down with C7 ands pulls up the bootstrap voltage on R8 and R14 allowing a higher output signal swing.

Thanks a lot! You know, resistors 0.22 Ohm are not kind of realistic. Does this mean emitters should be just connected?
That's why I don't like simulations.
And yes, this amp IS designed for old muffled AM radio.

 

[audioguru]

An LM386 amplifier IC and most other amplifier ICs will probably oscillate when built on a solderless breadboard because of all the stray capacitance between all the rows of contacts and the wires all over the place.
The 0.22 resistors help balance the output transistors.
Why use a muffled amplifier circuit to make a muffled old AM radio even more muffled?? Who listens to an old AM radio anymore?

 

[Stan2020]

An LM386 amplifier IC and most other amplifier ICs will probably oscillate when built on a solderless breadboard because of all the stray capacitance between all the rows of contacts and the wires all over the place.

Even with gain factor of 20?
MK484 does not. The amplifier I built above does not.
Even with minimal extras, strictly by the specs example app, this is what I see with LM386 with 400 Hz sine input (20 gain mode):

Is this oscillation?

 

[audioguru]

Less gain means more negative feedback. The datasheet of the LM386 says it is not stable if its gain is less than 9 times (when it is built properly on a pcb).

 

[Stan2020]

Less gain means more negative feedback. The datasheet of the LM386 says it is not stable if its gain is less than 9 times (when it is built properly on a pcb).

AFAIK the minimal, built-in gain of LM386 is 20.
Can you please point out where exactly in specs it says about instability at 9?
Although, from what I managed to learn, 'less gain' might mean less positive feedback. And AFAIK 'negative feedback'=less gain, more stability, 'positive feedback'=more gain, less stability. Did I learn wrong?

 

[unclejed613]

How I can see 'high distortions' you've mentioned? I applied up to 10 kHz 20 mV sine input, did not notice anything distorted on 8 ohms speaker output.

since the second harmonic of 10khz is 20khz, which is at the limit of most people's hearing range, you probably wouldn't hear it.... try a 1 or 2khz tone instead. with your amplifier set up the way it is in the schematic, there will be plenty of second harmonic (and likely some 4th and 6th as well) present in the output... the reason for this is primarily from R8... as you approach the positive rail, the current through R8 drops a lot, reducing the drive current to Q3, which has the effect of compressing the positive peak of the output waveform. this is why people have been suggesting the use of a bootstrap circuit or a current source in place of R8. there is a way you can see some of the distortion, if you have a half way decent dual trace oscilloscope. the method is to feed the amplifier input signal into channel "a", and the speaker output signal into channel "b", and adjust the gain controls so both waveforms are pretty close to the same size on the screen. then switch the vertical mode to "add" and you will get one waveform twice the original size, then (it's sometimes channel "a" and sometimes channel "b that has this control) use the "invert" switch on whichever channel that has it... this changes "add" mode to "subtract" mode, and the resulting waveform on the screen is the differences between the input and output waveform (i.e. distortion).

 

[Stan2020]

since the second harmonic of 10khz is 20khz, which is at the limit of most people's hearing range, you probably wouldn't hear it.... try a 1 or 2khz tone instead. with your amplifier set up the way it is in the schematic, there will be plenty of second harmonic (and likely some 4th and 6th as well) present in the output... the reason for this is primarily from R8... as you approach the positive rail, the current through R8 drops a lot, reducing the drive current to Q3, which has the effect of compressing the positive peak of the output waveform. this is why people have been suggesting the use of a bootstrap circuit or a current source in place of R8.

by saying ' did not notice anything distorted on 8 ohms speaker output' I meant I did not see any _visual_ distortions on the scope screen. It has nothing to do with "the limit of most people's hearing range". Pardon me my Klatch, I am still not that stupid to estimate distortions by the ear