I have built a headphone amp with an bipolar opamp for voltage gain and another as a buffer. Nothing special I think but it has no input capacitors because I want to test it without.
It works but when I measure DC offset at the output without a source I get 77mV.
When I connect 10k at the input I measure 5mV.
Why is this? Without anything connected at the input I can imagine it picks up mains hum and such but then I would expect more AC than DC at the output.
Can anyone explain this to me?
Thanks,
regards,
Cor
It works but when I measure DC offset at the output without a source I get 77mV.
When I connect 10k at the input I measure 5mV.
Why is this? Without anything connected at the input I can imagine it picks up mains hum and such but then I would expect more AC than DC at the output.
Can anyone explain this to me?
Thanks,
regards,
Cor
What's the input impedance / resistance? It would seem to be on the order of 150k...
Without a source, opamp input bias current will flow through this resistance and create a finite voltage drop. Same goes for the resistance on the other leg, which is likely to be substantially lower. This leaves a net voltage offset, which is then amplified by DC gain - I have a hunch this is greater than unity in your case.
That's about as much as I can say without a schematic.
Without a source, opamp input bias current will flow through this resistance and create a finite voltage drop. Same goes for the resistance on the other leg, which is likely to be substantially lower. This leaves a net voltage offset, which is then amplified by DC gain - I have a hunch this is greater than unity in your case.
That's about as much as I can say without a schematic.
The input impedance is 100k, the other leg has 2k to ground. The gain is 5.1x. So your explanation is probably right.
Usually I measure the DC offset with input connected to a source or the input connected to ground but not this time.
Now I now why this happens, learned something, thanks!
Usually I measure the DC offset with input connected to a source or the input connected to ground but not this time.
Now I now why this happens, learned something, thanks!
Input bias current.
Try to put jfet-based opamp at the input position or compensate source resistance at the inverting leg of the opamp.
It will be no problem because when the amplifier is in use it has a 10k potmeter at the input and it is connected to an input source.
I predict the DC offset will be minimal then. But thank you for your suggestion.
I predict the DC offset will be minimal then. But thank you for your suggestion.
And when you turn that potz you'll pick an output DC.
Place something like AD8065 after potz or choose bjt-input opamp with smaller input bias current.
I more or less already tried that by connecting 10k at the input. In that case DC offset is only 5mV.
> 77mV.
I suppose there is some reason you are not telling the details, such as what Op-Amp.
5532?
I suppose there is some reason you are not telling the details, such as what Op-Amp.
5532?