Two OpAmp connected in series

[engr_joni_ee]

Hi,

I have signal which comes from an OpAmp which has done amplification. I would like to add an offset to this signal which means I need to use a summing amplifier whose one input should be amplified signal and the other input has to be the offset signal.

How is the impedance related in this case ? when the output of one OpAmp has to go to the other OpAmp ? Do I need to terminate the output of first OpAmp with 50 Ohm and then connect to the second OpAmp ?

 

[KlausST]

Hi,

the only useful information is "50 Ohms".

Don´t you have any other informations for us?
Like
* why 50 Ohms? What signal is it? Where does it come from and where does it go to?
* signal frequency range
* amplitudes
* voltages
* wire length
* what OPAMP
...

Klaus

 

[engr_joni_ee]

- 1kHz sin wave signal 1 V peak to peak with 0 DC offset is connected to AD844 which amplify it to 5 times.
- All the wires are between 10 to 15 cm in length on Breadboard.
- The input signal is just a test signal which is coming from a signal generator whose output impedance is 50 Ohm.
- The amplified signal (1kHz sign wave 5 V peak to peak 0 DC offset) has to be connected to another AD844 to add offset of 2.5 V so that the output signal become 0 V to 5 V.
- Do I need to terminate the first OpAmp signal with 50 Ohm before it goes to second OpAmp ?

 

[KlausST]

Hi,

All your new informations call against 50 Ohms.

* termination is useful when the wavelength of the signal frequency is shorter than (or comes close to) the cable length.
With 1kHz you may expect a wavelength of 250km...
* AD844 is not suitable to drive 50 Ohms. May I ask why you use a that expensive and overly fast OPAMP?

****
Application:
You only need a second OPAMP when you are interested in the DC voltage of the original signal.
Otherwise just use a good capacitor and two resistors for 2.5V DC biasing.

****
--> It´s absolutly not useful to use 50 Ohms termination. No benefit.

Klaus

 

[BradtheRad]

Although certain situations might need two op amps to achieve a similar job, I'm pretty sure your entire task is achievable with a single transistor. 1) Amplify 1VAC by several gain, and 2) output rides a DC voltage.