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Old 2nd July 2008, 11:26 PM   #1
dododio is offline dododio  France
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Default How to get the less distortion with BJT?

Hello,
Everything is in the subject: i try to build a low frequencies preamplifier with a differential structure (40dB, input voltage swing: from 0.01V to 0.1V and a good THD over the entire bandwith(audio)).
I simulate my circuits in ORCAD Pspice.
With a simple diff pair with current sink followed by a class A amplifier DC coulped with the previous stage (for the feedback) i can't lower the thd specialy when the input voltage rise and i would like to get a good thd over the dynamic of the signal.
When Vce swing Ic swing too so béta swing too=>distortion,
I heard somewhere that cascode could minimize the third order distortion is that right?, is there any other solution?
I also heard about cross coupled amplifier...
If somebody can bring some theory about linearisation of the bjt diff amp...
Sincerely,
dd
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Old 3rd July 2008, 12:12 AM   #2
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Default Re: How to get the less distortion with BJT?

Quote:
Originally posted by dododio
Hello,
When Vce swing Ic swing too so béta swing too=>distortion,

dd
Hwo about using a constant current source to keep the current the same ?
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Old 3rd July 2008, 06:03 AM   #3
YFW is offline YFW  United States
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You should put negative feedback into the circuit to reduce the THD.
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Old 3rd July 2008, 11:48 AM   #4
PMA is offline PMA  Europe
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You have to make a choice of better input topology.

Cheers,
PM
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Old 3rd July 2008, 12:52 PM   #5
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Default Do not inject, do not plug signal bigger than the transistor bias


and adjust your impedance using dummy resistances into the input as loads to match your pré amplifier...keep the signal inside the limits and distortion will be something you will not even listen.

Of course measurements instruments will see distortions and will read them..... but you will not perceive...so... not your problem...the problem belongs to the Measurement instrument...it may be disturbed or bothered...you will remain very happy listening good sound.

Big secret... and for free... adjust your non distorted level to 75 percent of your volume.... you will have room to increase low level from your audio source and will not have too much risk operating confortable audition levels.

Distortions will always exists....and do not bother about the amplifiers...the speaker will be distorting 100 percent more than the amplifiers...so.... the amplifier is something that is not the real problem to create worries.

Watch the speaker, that dam thing...the air pump... awfull mechanics... very ancient thing.... the guilty is there!

Give a shot into your 6 volts RMS output pré amplifier...not needed, problem creator... the audio sources already has level enougth to drive any power amplifier to the maximum level of power..so....pré amplifiers is one more thing to create problems, as it is inside the audio chain...use potenciometers to control the input volume and be happy my friend.

regards,

Carlos
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Old 3rd July 2008, 12:56 PM   #6
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Default Re: Do not inject, do not plug signal bigger than the transistor bias

Quote:
Originally posted by destroyer X

Watch the speaker, that dam thing...the air pump... awfull mechanics... very ancient thing.... the guilty is there!

regards,

Carlos

People spend ages getting the amp right then send the signal to a speaker with a wildy fluctuating frequency curve.
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Old 3rd July 2008, 01:02 PM   #7
ttan98 is offline ttan98  Australia
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Default Re: Re: Do not inject, do not plug signal bigger than the transistor bias

Quote:
Originally posted by nigelwright7557



People spend ages getting the amp right then send the signal to a speaker with a wildy fluctuating frequency curve.

Looks like to don't visit the loudspeaker forum often enough...
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Old 3rd July 2008, 01:06 PM   #8
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Default Re: Re: Re: Do not inject, do not plug signal bigger than the transistor bias

Quote:
Originally posted by ttan98



Looks like to don't visit the loudspeaker forum often enough...

Not at all, I just read the loudspeaker specification pdf's !!!
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Old 3rd July 2008, 01:54 PM   #9
lineup is offline lineup  Sweden
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Default Re: How to get the less distortion with BJT?

Quote:
Originally posted by dododio
Hello,
- low frequencies preamplifier
- with a differential structure
- (40dB,
- input voltage swing: from 0.01V to 0.1V and
- a good THD over the entire bandwith(audio)).
I simulate my circuits in ORCAD Pspice.

- a simple diff pair with
- current sink
- followed by a class A amplifier DC coulped with the previous stage (for the feedback)

i can't lower the thd specialy when the input voltage rise and i would like to get a good thd over the dynamic of the signal.
When Vce swing Ic swing too so béta swing too=>distortion,
I heard somewhere that cascode could minimize the third order distortion is that right?, is there any other solution?
I also heard about cross coupled amplifier...
If somebody can bring some theory about linearisation of the bjt diff amp...
Sincerely,
dd

When I know too little, I do not want to join in doing speculations.

I have several questions, for clarification, if you can not post a Drawing / Schematic,
to let us see what is going on.
------------------------

1. Power supply? Voltage?
I suppose dual, negative + postive with 0 Volt in middle.

2. 40 dB .. what?
If you by this mean x100 v-gain, maybe

3. LOAD?
What kind of load you intend to drive with preamp?
1 kohm .. or at worst case 5 or 10 kohm

4. What output max voltage you need across this load?
Using 0.01 Vrms / 0.1 Vrms .. at gain x100 = 1 - 10 Vrms Output!!!
This means a peak voltage out = 1.414 x 10 = +- 14.14 Volt.
Would require, in my book, at least a +- 20 Volt supply.

Preferably, a well regulated supply.
This is what good preamplifier deserves.

5. Not many devices fed from a preamp need more than 1- 2 Vrms input.
As hinted by destroyer X, already.

----------------------

I look forward to get mor info.
Then I would give you some good advices, how to do.
And how to lower your THD a bit. For good audio sound ( maybe even Hi-Fi ).
As the others posters here have good knowledge to advice, as well.


Regards, Lineup
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Old 3rd July 2008, 02:21 PM   #10
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Default Re: Re: How to get the less distortion with BJT?

Quote:
Originally posted by lineup



When I know too little, I do not want to join in doing speculations.


Regards, Lineup
And how I am supposed to learn if I dont join in and ask questions !!!!!

I have been in electronics for 32 years so I hardly a novice !

You need to take a long hard look at a speaker datasheet before having a dig at me.

The Fane Collossus has more curves than Pamela Anderson !!!
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