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Topic: What is this amplifier doing, exactly? (Read 8422 times) previous topic - next topic

afremont


I think I've entered the twilight zone.

Okay, one more time.... I did not design the circuit to be turned on and left that way.

So, since every single person who looks at this is assuming that they will be forced against there better judgement to use it incorrectly, I will update the design and test it to make sure it is idiot proof.



I intended no offense, just offering up my thoughts and trying to be sure that I understand the circuit.  :)  When it comes to these microcontrollers, anything can happen.  The output pin being left high wouldn't be an unusual situation during development, so how else should the circuit be analyzed?  I'm thinking that just adding a blocking cap on the base along with a pulldown and it should be well protected from DC.  At 50% duty cycle, everything should be fine, but you might want to use a smaller resistor on the base since hfe at large currents is as low as 20, unless I got it all wrong.
Experience, it's what you get when you were expecting something else.

BillO

I do understand what you are saying, at least partially.  I'm not sure where you got the hfe of 20.  My spec sheet (Farichild 09/99) states a worst case hfe of 40 @ 500mA and shows a typical of around 80 @ 500ma.  The transistor I am using actually measures at 121, but that is besides the point.  Looking at DC characteristics in a circuit like this is pointless.  The average current through the transistor is much less than 500ma.  In fact, at 400hz the circuit only draws about 200ma.

I've come up with a 'better' circuit that will be (almost) idiot proof.  I am just going to build it and test it to make sure it meets the design specs.  However, I am cannot seem to come up with fool-proof design based on the simple switch we've been discussing here, so the new circuit is an AC-coupled class 'A'.  I'll post the schematic when I'm done the tests.
Facts just don't care if you ignore them.

afremont

Oops, you're right the minimum hfe is around 40.  But average current isn't the point, this is about idiot-proof non-self-destructive design. ;)  If the average is 200mA then the max is 400mA which will melt things if it gets stuck on. 

I don't see any need for a class A amp, common emitter class B should be plenty good enough and use less power.  Sound quality certainly isn't the issue.  An emitter follower would be good enough I think.
Experience, it's what you get when you were expecting something else.

CrossRoads

Sounds like the discussion has come back around to this.
AC coupled, transistor turns off when Input gets held High or Low, no DC going thru the speaker either.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

CrossRoads

I have 5 of these running, I have two Tones to make a nice output warble like a loud cell phone ring. Have been running since Jan 2011, so 2+ years. Ignore the header/switch at the top left, just use a series resistor from V+ to the transistor.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

CrossRoads

This article goes into depth of the single gate amplifier. Scroll down to page 8.
http://www.ece.ucsb.edu/Faculty/rodwell/Classes/ece2c/labs/Lab_2_2C_2007.pdf
We don't need it biased to operate in the linear region since the tone output is either on or off - hence the amplifiers output can be on or off as well.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

afremont

Yeah, like Crossroads showed.  Class B and no wasted energy with DC idle currents.
Experience, it's what you get when you were expecting something else.

BillO


Oops, you're right the minimum hfe is around 40.  But average current isn't the point, this is about idiot-proof non-self-destructive design. ;)  If the average is 200mA then the max is 400mA which will melt things if it gets stuck on.


Well, I'm not sure that would be self-destructive.  In any case, as I said before, just rate the collector resistor at 3w.  Problem solved.  But read on... 

I don't see any need for a class A amp, common emitter class B should be plenty good enough and use less power.  Sound quality certainly isn't the issue.  An emitter follower would be good enough I think.


Class 'A' because I'm sure someone will likely find fault with any other solution.  Since they have not seen it before, they will assume it just doesn't work.  Not that they won't find fault with a class 'A'.  Also, class 'A' will only require a couple of extra parts.  Since the very simple was apparently too simple, why not go the extra step and make it something everyone can look up and wrap their heads around. BTW, I can't seem to get my head around an emitter follower solution.  Not that it won't work.

So, below is the class 'A' I came up with.  Draws 210mA @ 5V and delivers 125mw into 8ohms from 20Hz to 6KHz.  Has typical single stage class 'A' distortion on sine a triangle waveforms and is biased to start clipping more or less symmetrically at Vin = about 4.5V.  Square waves produce an expected over-shoot spike which is controlled by C3.  If you don't care about the over-shoot, you can leave C3 out.  The over-shoot on square waves causes and appreciable increase in gain at frequencies above 6KHz.  Increasing C3 can help with this.

It works as described and does not get hot, explode, glow, kick the dog or beat the wife despite what you may believe.  You can leave the drive pin high or low as you wish.  If you want to prove it is does anything other than what I've stated, build one and show me actual results.  So, all you 'Bash Street Kids', have at it. 
Facts just don't care if you ignore them.

BillO


This article goes into depth of the single gate amplifier. Scroll down to page 8.
http://www.ece.ucsb.edu/Faculty/rodwell/Classes/ece2c/labs/Lab_2_2C_2007.pdf
We don't need it biased to operate in the linear region since the tone output is either on or off - hence the amplifiers output can be on or off as well.


I agree.
Facts just don't care if you ignore them.

oric_dan

Quote
... I will update the design and test it to make sure it is idiot proof.

From a practical viewpoint, this should be Rule #1, since everybody who's ever done
anything at all knows how large the idiot population really is.

retrolefty


Quote
... I will update the design and test it to make sure it is idiot proof.

From a practical viewpoint, this should be Rule #1, since everybody who's ever done
anything at all knows how large the idiot population really is.


"Make it idiot proof and someone will make a better idiot."


CrossRoads

I have a card at home that someone here in the forum sent me 2 years ago that uses a 754410  H-bridge chip as the amplifier.

http://www.ti.com/product/sn754410
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

oric_dan



Quote
... I will update the design and test it to make sure it is idiot proof.

From a practical viewpoint, this should be Rule #1, since everybody who's ever done
anything at all knows how large the idiot population really is.


"Make it idiot proof and someone will make a better idiot."

Yeah well, such is life in the trenches. OTOH, adding one small feature can have an
enormous impact - case in point, the input cap on the ckt being discussed here ad
infinitum. Leave off that cap and the consequences can be easily disasterous.

For years, all of my pcbs have used simple series-R protection on the I/O pins, and
no one ever blew up one of my boards [at least that owned up to it]. OTOH, back
in the days of yore, another guy was selling similar boards with no protection and
all smt parts, and every single day on the associated forum, some poor guy came
on crying he had blown his xxxx board. Some designs are "more" idiot-proof than
others, I guess.

retrolefty


Quote

For years, all of my pcbs have used simple series-R protection on the I/O pins, and
no one ever blew up one of my boards [at least that owned up to it]. OTOH, back
in the days of yore, another guy was selling similar boards with no protection and
all smt parts, and every single day on the associated forum, some poor guy came
on crying he had blown his xxxx board. Some designs are "more" idiot-proof than
others, I guess.


No finer example of design for protection then the Ruggeduino. There is a lot one can learn
from reading it's schematic.

http://ruggedcircuits.com/html/ruggeduino.html

http://ruggedcircuits.com/AM010/am010.pdf

oric_dan

There are a lot of good features on the ruggeduino board. One can argue about
this, but from my years of experience, the single most important one is the
series-Rs in the I/O lines. Those alone save the microcontroller 20 different ways.

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