Confused with 0TA3080

[Tony Hardie-Bick]

On 20/01/13 21:04, Marc Nostromo [M-.-n] wrote:
>> if you remove R7, the voltage at pins 2 and 3 of the first ota whould be
>> half the supply voltage. if you put R7 beck in circuit, the dc level
>> shouldn't change much.
>
> So, If I have R10,R9 connected, I get half voltage. i.e. 2.5v
>
> However as soon as I connect them to the ota, the voltage goes to 3.6.
> Pin 2 doesn't really matter. Pin 3 makes the voltage change (R7 is
> disconnected).

pin 3 is a diode to ground (base-emitter junction of an npn input 
transistor in
the internal curcuit from the CA3080 datasheet), so connecting R10.R9 to 
pin 3
should, if anything, reduce the voltage, but you are seeing an increase.

i'm not there in person, and can see i'm tired from mistakes i've made
earlier, but this does look suspicious

Tony (HB)