Single Supply op+Missing DC Bias Current Return when AC Coup

[malli_1729]

Hi!

i don't know whether i can post this here or not. But, i am posting help me out.
Please Bare with me, because for understanding i am eloberating....
this is regarding the single supply op amp with an issue Missing DC Bias Return current when Input is AC Coupled.
i am just displaying some clips and matter Analog Dialogue from Analog devices
and attaching those figures.
"
In Figure 1, a
capacitor is connected in series with the noninverting (+) input of
an op amp to ac couple it, an easy way to block dc voltages that are
associated with the input voltage (VIN)."
"
What actually happens is that the input bias currents will flow
through the coupling capacitor, charging it, until the common-mode
voltage rating of the amplifier’s input circuit is exceeded or the output
is driven into limits. "

The capacitor blocks the DC.So, no DC Voltage from Vin can enter Op amp. it is right in First para.
But in the next para they have mentioned Input bias currents from Op amp will flow through coupling capacitor and develope voltage.
Here only i am confused.
Up to i know generally Input Bias currents, as they are for bias generally are DC , so, coupling capacitor should not allow it..But here they mentioned it wil charge the capacitor.

i think i struck some where in basics regarding the capacitor. like
whether capacitor can be charged by DC voltage
whether capacitor can be charged by DC current...
whether DC current and DC Voltage exist independently....

please help me out....

 

[keith1200rs]

I think I understand your confusion. However, if you consider the situation where you connect an input voltage of 5V. Initially we will assume the capacitor has no charge and therefore no voltage across it. If you suddenly connect one end to 5V, the other end must also go to 5V. What happens then is the resistor R1 in your figure 2 will pull the opamp end of the capacitor to 0V. This will effectively charge the capacitor to 5V.

Keith.

 

[malli_1729]

Re: Single Supply op+Missing DC Bias Current Return when AC

i am late....... sorry

i didn't understood it.......
if u don't mind, can you elaborate it....i mean
in the first para, DC is blocked by capacitor.
Next para, the Bias currents(up to i know they are DC So, can't go through capacitor) are charging the capacitor.
Both these Bold statements are opposite to each other.one is telling DC is Blocked by capacitor other is telling DC is able to charge the capacitor.
i can't able to understand...

whether Charging , Discharging and Blocking ,Non-Blocking by capacitor are related internally?????????

let me know if i didn't explained it clearly..

thanking you..

 

[LvW]

Re: Single Supply op+Missing DC Bias Current Return when AC

Hi Malli,

taking into consideration some basics it is not confusing at all.
At first, the opamp dc bias current is a continuous current which is necessary for proper operation of the first opamp stage. This is the task of R1 in Fig. 2.
When in Fig. 1 the input C after switch-on is charged up to 100% there is no dc current anymore. The same applies also to C1 in Fig. 2; however, this does not prevent the dc current flowing into the opamp through R1.
In summary, in both cases the capacitor is charged and there is a certain voltage across it - but the duration of this loading effect is very short (after switch-on) and does not influence the opamp operation - if a continuous bias current is possible.

 

[malli_1729]

Re: Single Supply op+Missing DC Bias Current Return when AC

Thanks for the reply....

i think i understood somewhat.....I want to share it, to know whether it is correct or not.
In both the circuits, we know bias currents flow continously.
In the circuit without R1, the capacitor will charge continously upto some point(i don't exactly upto what point it can charge) which will induce errors in the Output of the opamp.
where as in the circuit with R1 there is always path for the DC currents to flow to GND through R1 . So, i think most of the Bias current will now flow through R1 to GND. So that capacitor C1 will be charged to less extent(i don't know to which extent) which will reduce the errors.
is it right..?????

I think, in my graduation i have studied wrongly regarding capacitor.
as per the equation i = Cdv/dt
for DC voltage i = 0; so, capacitor blocks DC voltage
for DC current V = integral(I/C)dt which will increase linearly(this is wrong case)
Because, if DC voltage exists DC current also exist with it.
So, for DC, Capacitor is Open circuit, Current will be zero and voltage across the capacitor will be DC voltage magnitude(i.e if DC of 5V applied current through Capacitor is zero and one plate will be charged to 5V and another plate will be charged to -5V)...

thanking you,

 

[Audioguru]

The (+) input of the opamp must have a DC reference voltage that is provided by R1.

If you let the input float without a DC reference then the input bias current of the opamp will slowly charge the input capacitor and the input will saturate at the positive or negative supply voltage.

For a single supply opamp circuit the input reference voltage is usually half the supply voltage.