how to set the dc operating point of op amps with feedback

[henrywent]

dc operating point

Hi all,
as we know, op amps need to be biased at some specific dc levels in order to work properly. but when the op amp is used in closed loop configution, the feedback elements (resistors, capacitors) may chage the original open loop dc bias we have designed,our op amps may not work properly. How to solve this problem, or how to set the dc operating point for circuits with feedback loop?
thanks in advance!

 

[LvW]

amplifier dc operating point

Quote:
as we know, op amps need to be biased at some specific dc levels in order to work properly

Yes, to be specific: for a dual symmetric supply the ideal bias voltages are zero volts at both inputs as well as at the output. Problem: Without feedback the offset voltage is multiplied by the open loop dc gain (1E5...1E6). Thus, the output cannot be around zero volts; most probably the amplifier is in saturation.

Quote:
but when the op amp is used in closed loop configution, the feedback elements (resistors, capacitors) may chage the original open loop dc bias we have designed,our op amps may not work properly.

The opposite is true. Due to resistive feedback the gain is decreased down to (1...1000, for example) and the output will remain in the vicinity of zero volts.
However, ff the feedback element is a capacitor you have an integrator circuit which indeed has a bias problem.

How to solve this problem, or how to set the dc operating point for circuits with feedback loop?

Use resistive feedback leading to a closed loop gain of lower than 1000, and your circuit will have no severe bias problems.

 

[henrywent]

what is dc operating point

thank u LvW,
but let's say the dc bias voltage for the inputs of the open loop op amp is 2v(for a 5v single voltage supply) and the output dc voltage is 3v for a specific application, when we add resistors to form a close loop amplifer, because the input dc level and output dc level are not equal the feedback resistors will draw some current from the op amp and change the dc operating conditions of the original open loop op amp. Will this cause some bias problems to the op amp? please help, thank you!

 

[LvW]

op amp adding bias

thank u LvW,
but let's say the dc bias voltage for the inputs of the open loop op amp is 2v(for a 5v single voltage supply) and the output dc voltage is 3v for a specific application, when we add resistors to form a close loop amplifer, because the input dc level and output dc level are not equal the feedback resistors will draw some current from the op amp and change the dc operating conditions of the original open loop op amp. Will this cause some bias problems to the op amp? please help, thank you!

Your requirements sound a bit uncommon - single supply 5 volts, and 2 volts input bias with 3 volts dc at the output. Nevertheless, you can design it !
Your circuitry must have a gain of +3/2=+1.5. That requires a voltage divider in the feedback path with two resistors R and 2R. Thus with 2 volts dc at the pos. input you get 3 volts at the output. At the same time you have a signal gain of only 1.5.
If you want another ac gain (above a certain corner frequency) you have to use another R-C-series path in parallel to the resistor 2R between the neg. input and ground. Thus, the ac gain is increased without touching the bias point.
And don`t worry about dc currents. If you use resistors in the mid kohms range, their influence is negligible.

 

[henrywent]

dc operating point explained

so the feedback resistors do affect the dc bias of the open loop op amp? but we can make their impact negligible by choose their values larger, is that right?

 

[LvW]

what is dc operating point?

so the feedback resistors do affect the dc bias of the open loop op amp? but we can make their impact negligible by choose their values larger, is that right?

1.) If you have resistive feedback you cannot speak about "dc bias of the open loop opamp" since the loop is closed and you only have a dc bias of the closed loop system.
2.) The influence of the feedback resistors is negligible only if their values are SMALL (only then the voltage developped by dc bias currents is small).