Ac coupling and offset voltage in amplifier

In summary, the output voltage due to bias currents is always equal to the feedback resistor value multiplied by the input current.
  • #1
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Hey all,

Just a question concerning the image I have provided. If I have an offset of 0.1 mV at the non inverting input, how exactly does coupling take effect in this set up? If I input an ac signal, it will pass through the cap, but will the op-amp not still experience the offset voltage?

smOvE.png
 
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  • #2
Ignoring input bias current for the moment, a 0.1V offset at the - input means there is a 0.1mv output. That dc offset will always be there. A signal input through C2 will have that DC signal imposed on it.

Think of it this way. The opamp will output whatever is required to maintain 0.1V between + and -. Input bias currents cause a voltage drop across R2, but the effect is the same.
 
  • #3
Is it not possible to utilize bypass capacitors such that the offset does not offset the input ac signal?
 
  • #4
No bypass caps don't work that way. meBigGuy is right. Another way to think of it is you have a 0.1mV DC voltage source in front of one of your op amp inputs. How is a bypass cap going to do anything about that?
 
  • #5
That's what I figured. There is an example in my book, however, that neglects the offset voltage that is inherent to that particular op-amp. For example, they had a nominal gain of 1000, an offset voltage of 0.03 V, and they asked the maximum input signal that would result in an ouput of 10V peak amplitude. They answered with 0.01V, which does not accout for the dc offset. I found this odd considering the dc offset was given at the beginning of the example. Just thought I would ask in case I was missing something. Thanks.
 
  • #6
No worries. Offsets typically set the real-world limit to the gain you can get out of a closed-loop voltage amplifier. You can do things to cancel them but that adds complexity, of course.
 
  • #7
Perfect, thanks again!
 
  • #8
sandy.bridge said:
For example, they had a nominal gain of 1000, an offset voltage of 0.03 V, and they asked the maximum input signal that would result in an ouput of 10V peak amplitude. They answered with 0.01V, which does not accout for the dc offset.
Maybe the OP-AMP was powered by rails of ± 45V?
 
  • #9
Here, I will display the question at hand word for word. "A non-inverting amplifier with a closed-loop gain of 1000 is designed using an op-amp having an input offset voltage of 5 mV and output saturation levels of +/-13V. What is the maximum amplitude of the sine wave that can be applied at the input without capacitive coupling and with it?" The figure with capacitive coupling was provided above already.

The answers provided by both the textbook and my professor were 8 mV without coupling, 13 mV with it.

Another question regarding the output voltage due to bias currents for the integrator with feedback resistor [itex]R_f[/itex]: my textbook states that the output voltage due to the bias currents is [itex]v_o=R_fI_{OS}[/itex], however, if non-inverting input is grounded, the only current contributing to the output voltage is the bias current entering the inverting node. Hence, [itex]v_o=R_fI_{B^-}[/itex]. What exactly am I missing here?
 
Last edited:

1. What is AC coupling in amplifiers?

AC coupling refers to the process of removing the DC component from an input signal in an amplifier. This is achieved by using a capacitor in series with the input signal, which blocks the DC component and allows only the AC component to pass through to the amplifier.

2. Why is AC coupling necessary in amplifiers?

AC coupling is necessary in amplifiers because most signals in electronic circuits have a DC component, which can cause distortion or saturation in the amplifier. By removing the DC component, the amplifier can accurately amplify the AC signal without any interference from the DC component.

3. How does AC coupling affect the frequency response of an amplifier?

AC coupling can affect the frequency response of an amplifier by introducing a high-pass filter effect. This means that low-frequency signals will be attenuated or blocked, while high-frequency signals will pass through unaffected. The cutoff frequency of this high-pass filter is determined by the value of the coupling capacitor.

4. What is offset voltage in amplifiers?

Offset voltage refers to a small DC voltage that is present at the input of an amplifier, even when there is no input signal. This voltage can cause the output of the amplifier to be offset from the desired level, resulting in distortion or inaccurate amplification.

5. How does offset voltage affect the performance of an amplifier?

Offset voltage can affect the performance of an amplifier by causing distortion in the output signal and reducing the accuracy of the amplification. It can also impact the stability of the amplifier, as the offset voltage can change over time and temperature, leading to changes in the output signal. To minimize the effects of offset voltage, amplifiers often have built-in offset voltage compensation circuits.

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