Biasing of LM3900 op amp

[JERD]

I have a supply of LM3900 single supply quad op-amps and want to use one as
a 3 stage audio band pass filter.

I can not seem to sort out the biasing of the +ve input.

Some application notes show it tied to the positive rail via a 1M to 10M
resistor and other application notes 75 k ohms. Wide variation here!

Can someone please give me some guidance on this?

From what I can gather the usual way of biasing the +ve input to half rail
does not apply to this device.

jerd

 

[petrus bitbyter]

"JERD" <notme@bigpond.net.au> schreef in bericht
news:Ss3sm.40974$ze1.6930@news-server.bigpond.net.au...

I have a supply of LM3900 single supply quad op-amps and want to use one as
a 3 stage audio band pass filter.

I can not seem to sort out the biasing of the +ve input.

Some application notes show it tied to the positive rail via a 1M to 10M
resistor and other application notes 75 k ohms. Wide variation here!

Can someone please give me some guidance on this?

From what I can gather the usual way of biasing the +ve input to half rail
does not apply to this device.

jerd

You'll have to look at the datasheet. This amplifiers are not using a long
tailed pair but some kind of current mirror for the inputs. The datasheet -
at least the one from Motorola I use - describes extensively the whys and
hows of biasing.

petrus bitbyter

 

[christofire]

"JERD" <notme@bigpond.net.au> wrote in message
news:Ss3sm.40974$ze1.6930@news-server.bigpond.net.au...

I have a supply of LM3900 single supply quad op-amps and want to use one as
a 3 stage audio band pass filter.

I can not seem to sort out the biasing of the +ve input.

Some application notes show it tied to the positive rail via a 1M to 10M
resistor and other application notes 75 k ohms. Wide variation here!

Can someone please give me some guidance on this?

From what I can gather the usual way of biasing the +ve input to half rail
does not apply to this device.

jerd

Have you looked at the data sheet yet? ... plenty of description and
examples there
http://www.datasheetcatalog.org/datasheet/nationalsemiconductor/DS007936.PDF

Quoting: 'Operation of this amplifier can be best understood by noticing
that input currents are differenced at the inverting-input terminal and this
difference current then flows through the external feedback resistor to
produce the output voltage'. etc. It's all there.

Chris