# Is there a better version of an "integrator" Servo? -How can I add offset to this amplifier?

Discussion in 'General Electronics Chat' started by coinmaster, Mar 17, 2016.

1. ### coinmaster Thread Starter Member

Dec 24, 2015
357
6
A lot of my planned projects require the use of some sort of DC offset mechanism but the only DC offset mechanism I'm familiar with is an opamp "integrator".

It tries to make its inverting input the same as its non-invering input the same by outputting whatever voltage is needed to do so.
Except it really doesn't.
The output bias does not make the inverting input match the non-inverting input out of the box. First you must adjust the supply voltage in order to adjust the output offset, usually with a trimmer, until you find the "zero" point. Only then will the inverting input match the non-inverting input.

So in other words, it does not match the two inputs without some fine-tuning first.

Normally this isn't that big of a problem but it makes things very difficult for one of the projects I'm working on. I need something that will act like an integrator without the needed adjustment.

Is there some better circuit out there for DC offset?

2. ### DickCappels Moderator

Aug 21, 2008
2,774
671
The integrator you drew will eventually saturate because there is no DC feedback. Putting a resistor in parallel will allow a DC input to control a DC voltage on the output.

Yes there is another way to add an offset to an electronic circuit. Actually there are many ways to create an offset, the best method depending upon the application circuit. If you can describe, or even better yet, post the circuit that needs the offset, you will probably get plenty of good suggestions.

3. ### coinmaster Thread Starter Member

Dec 24, 2015
357
6

So I need the "output" to be at 0v. But I also intend to adjust V1 and V2 between a wide range of voltages. The tubes are basically resistors and the servo adjust the bias of the top tube so the impedance of both tubes is such that there is 0v between them.

The problem is the integrator needs to have its negative supply adjusted until it actually shows 0v at the output, then it keeps it there as the tubes age and whatever. But if I change the B+ and B- of the tubes the servo will not adjust itself to maintain the 0v output.

For example if I use a 240v plate bias on the top 6080 tube then I would need -150v bias on the grid but I would need about 300v negative supply voltage on the opamp to get the -150v I want. But if I bring that 240v plate bias down to 75v with a 300v negative supply voltage on the opamp it will not adjust itself to the -20v grid bias needed for 0v at the output until I fiddle with the supply voltage.

So I need something that will find the "zero" point regardless of what I do with the tube biases.

Last edited: Mar 17, 2016

Aug 21, 2008
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671
5. ### coinmaster Thread Starter Member

Dec 24, 2015
357
6
I'm not sure how that summing amplifier is relevant. What I want is something that will maintain a DC offset by itself without me needing to fiddle with trimmers and voltages.