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'[EE]: Voltage follower at 5VDC'
2001\07\18@043903 by Jeszs

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Hi everyone,

I have got an audio signal (0.5V AC on top of a 4 V DC level). I need to feed the AC part of this signal into a telephone transformer. When I do so directly (or through a cap to block the DC) the information is distorted.

I need a voltage follower to adapt the impedances. Two problems: first, the only power suply available is 5VDC; second, I have searched on the web for op amplifiers or buffers fed at this level but when I come to the shop they are never available.

Could you recommend a 'generally available' op amplifier or buffer working at 5VDC?

Thanks in advance!

--------------------
Jesús Gonzalo
INSA, Ingeniería y Servicios Aeroespaciales, SA
Paseo Pintor Rosales 34
Tel: +34-915489149
28008 Madrid (SPAIN)
--------------------

2001\07\18@130627 by Mike Hardwick

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>Could you recommend a 'generally available' op amplifier or buffer working
at 5VDC?

Anything wrong with using a simple transistor emitter-follower?

Mike Hardwick
Decade Engineering

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2001\07\19@033659 by Vasile Surducan

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Jesz, any common operational amplifier will work ok at +5V. Take for example the oldest 741.
You have only to do appropriate polarisation: + AO input must be polarised
at +2.5V using a resistor network ( 10k to 47k for the resistors are
good).
Negative feedback reaction through a capacitor from the output to -
operational input. The structure I've described will be an inverted
operational amplifier. V0 = - (Z2/r1) * vi where r1 is the input resistor
from AC source to - AO input, z2 is the feedback reaction ( capacitor +
r2 resistor) Take capacitor reactance 10 times less than resistor values and
the gain will be: v0/vi = -r2/r1 The output will have ac signal straddle to a +2.5DC so don't forget to use
a decoupling capacitorwhen you'll drive the transformer.
Vasile




On Wed, 18 Jul 2001, Jeszs wrote:

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2001\07\19@040745 by Jeszs

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Dear Vasile,

Thank you very much for your clear explanation. I have build it and it seems not to work (some sound is heard but not pure tones) with the 741. Looking at the datasheet, I found for that chip:
VccMin: 10 V
VccMax: 36 V
VinMin: 2 from V- rail
VinMax: -2 from V+ rail
VoutMin: 1 from V- rail
VoutMax: -1 from V+ rail

I have changed to the LM324 and everything is working wonderful. The only problem is that it is a Quad chip (14 pins). Does anybody know an equivalent single version (8 pins)?

Thank you in advance!

-----------------------
Jesús Gonzalo
-----------------------


{Original Message removed}

2001\07\19@090741 by Spehro Pefhany

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At 10:15 AM 7/19/01 +0200, you wrote:
>Dear Vasile,
>
>Thank you very much for your clear explanation. I have build it and it
seems not to work (some sound is heard but not pure tones) with the 741.
Looking at the datasheet, I found for that chip:
>VccMin: 10 V
>VccMax: 36 V
>VinMin: 2 from V- rail
>VinMax: -2 from V+ rail
>VoutMin: 1 from V- rail
>VoutMax: -1 from V+ rail
>
>I have changed to the LM324 and everything is working wonderful. The only
problem is that it is a Quad chip (14 pins). Does anybody know an
equivalent single version (8 pins)?

You can use the dual LM358, but the cost is actually about the same per
chip as the LM324, so unless you are
tight for space, you could do worse than to standardize on the LM324 in DIP
or SO-14.

Best regards,
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.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
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2001\07\19@185810 by Russell McMahon

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> I have changed to the LM324 and everything is working wonderful. The only
problem is that it is a Quad chip (14 pins). Does anybody know an equivalent
single version (8 pins)?

The LM358 is a dual version of the LM324 and is in an 8 pin package.



RM

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2001\07\21@024620 by Peter L. Peres

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The 741 does not work on 5V single ended supplies, and it does not even
come close to swing even 70% of the supply at 7V which is ist minimum
supply. The LM358 or 324 will work at 5V but only swing to +3.5V at the
output. It is inexpensive and second sourced. There are rail to rail
opamps that will do this right, for example from Microchip. The 741 was
obsolete before I started even thinking about using it. And it's a really
crappy opamp, but not as bad as the error amps in a TL494 ;-).

If the follower need not be linear you can use a CMOS gate directly. You
can also use a simple emitter follower or whatever else suits your needs.

Peter

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2001\07\21@024641 by Peter L. Peres

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>Thank you very much for your clear explanation. I have build it and it
>seems not to work (some sound is heard but not pure tones) with the 741.

Ahh, tones. So it's for sound. The 'classic' 5V sound amp is RC(A)4558
which is an improved 358 version.

Peter

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2001\07\21@025840 by Spehro Pefhany

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At 08:55 AM 7/20/01 +0300, you wrote:
>>Thank you very much for your clear explanation. I have build it and it
>>seems not to work (some sound is heard but not pure tones) with the 741.
>
>Ahh, tones. So it's for sound. The 'classic' 5V sound amp is RC(A)4558
>which is an improved 358 version.

The 4558 is an improved dual 741 rather than a single-supply type.
In particular, the output won't swing nearly as close to the negative
supply rail as the LM358 will.

Best regards,
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