The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
Subject: Re: Audio noise in diff amps
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Wed, 18 Dec 2002 07:55:40 GMT
NNTP-Posting-Date: Wed, 18 Dec 2002 08:55:40 MET
Phil Allison wrote:
> "Ban" wrote in message
>> when you look at the schematic of a differential amp, there are
>> besides the input resistors also 1 feedback resistor from output to
>> inv. input and 1 resistor from non-inv. input to gnd.
>> These resistors form now the input impedance and they might be
>> bigger than the 22k, in case there is a gain. Lets assume they have
>> 100k for 13dB gain. When you leave the inputs open, the effective
>> input resistance is now 100k for each input.
>> When you connect the 22k to gnd, the effective input resistance is
>> only 18k. The noise is thus 7.5dB lower, quite a difference.
> ** No way. With open cct input the gain is unity and the source
> impedance 22K.
> Even with 0dB gain the
>> input resistances are halfed and this results in a 3dB improvement.
> ** No. The gain increases and the noise goes up.
You're right Phil, I should have better thought about the _output_ noise.
Got up too early. :-(
Still it will be good to ground the pins to prevent some high freq. noise to
be picked up and demodulated by the input stage.
One question to the OP. Why didn't you measure the noise with the resistors
connected or earthed?
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup