[sdiy] Spare op-amp useful in gain stage?

[Roman Sowa]

Opamp noise is referenced to its input, so whether it's amplified by 
30dB and then abother 30dB, or by whole 60dB in one go, overal noise 
should remain the same.
Distortion also increases with gain, so that's another reason to split 
it to 2 lower gain stages.
OTOH there are application when it's better to have one high gain stage.
There's no single rule for everything.

Roman

W dniu 2016-01-28 o 19:41, Tom Wiltshire pisze:
> Plus the second amp is going to be amplifying the noise from the first. Is that better or worse than having one op-amp with very high-gain amplifying it's own noise?
>
> On 28 Jan 2016, at 17:37, Quincas Moreira <quincas at gmail.com> wrote:
>
>> I would expect signal to noise ratio to be worse, but distortion to be
>> less using Roman's two opamp idea.  Probably not by much, but each
>> opamp stage would add a tiny bit of noise...  Am I wrong?  I usually
>> go this route when I need high gain, two stages....
>>
>>
>>
>> On Thu, Jan 28, 2016 at 7:43 AM, Michael Zacherl
>> <sdiy-mz01 at blauwurf.info> wrote:
>>>
>>> On 28.Jan 2016, at 13:36 , Roman Sowa <modular at go2.pl> wrote:
>>>
>>>> In high gain application it's better to split to 2 opamps, this way you get much higher bandwidth than when done on single opamp.
>>>> For example if 60dB gain on one opamp would have 3dB bandwidth ending at about 10kHz, while 30+30dB on 2 opamps of the same kind it jumps over 300kHz.
>>>
>>> hm .. so what’s the impact on the SNR?
>>>
>>>> W dniu 2016-01-28 o 13:03, Mattias Rickardsson pisze:
>>>>> Hi!
>>>>>
>>>>> I was in a discussion about what to do with a spare OP-amp. These
>>>>> fellows come in duals and quads, and sometimes the quantities don't
>>>>> end up perfectly and you stand there with an extra unused op-amp.
>>>>>
>>>>> Is there any clever way of using two op-amps instead of one in, say, a
>>>>> noisy high-gain stage in order to reach lower noise or something like
>>>>> that?
>>>>>
>>>>> Douglas Self's small signal book suggests paralleling two gain stages
>>>>> ("multipath amplifiers"), having their outputs tied together with 10
>>>>> ohms to the destination, winning 3 dB SNR. Are there other clever ways
>>>>> of using a spare op-amp, particularly in a stage with high gain?
>>>
>>>
>>> --
>>> http://mz.klingt.org
>>>
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>>
>>
>>
>> --
>> Quincas Moreira
>> Test Pilot at VBrazil Modular
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