Buffer circuit/product inuqiry with tunable amplitude

[bhl777]

Hi All,
I have an arbitrary waveform generator that can generate square/sine wave signals up to 1MHz, and it has a 2.5V peak-to-peak voltage level (like -1.25V to 1.25V or 0 to 2.5V). I am looking for an off-the-shelf device to work as a buffer to generate the signal that has the same shape but tunable peak-to-peak voltage up to 5V. Since most of my applications only require signals up to 100kHz, I am only looking for a good enough one to meet the case.
- If such a device does exist, would anyone give me the direction? I have tried different keywords, such as voltage-controlled attenuator, but it does not seem to return the exact device I am looking for.
- if there are no off-the-shelf devices that exist, I am planning to build one. But I have some other issues to consider:
(a) I am going to use a potentiometer to achieve the amplitude tuning process, Is there any reliable method I can use to do so?
(b) if the input is a -1V to +1V sine wave, and I want to convert it to a -2.5V to +2.5V, does this mean I have to use a level shifter or an opamp chip that has both +VCC and -VEE. If I only have a single voltage supply input like 5V, does that mean I have to use a charge pump or buck-boost converter to generate -VEE?
Thank you!

 

[KlausST]

Hi,

You need to look for an "amplifier" / OPAMP.
From your post it seems a gain of 2.5 should be sufficient.
If you want to adjust the amplitude, then add a potentiometer in front of the amplifier.

The supply voltage needs to be wider than the expected output voltage .... in both durections.
Standard OPAMPs need about 2.5-3V headroom at each rail.
There are so called "rail to rail output" OPAMPs thad need much lower headroom.

--> what output current do you need?

--> frequency: it depends on waveform. "Sine" is the most simple waveform to amplify, because it has no overtones (contains integer multiples of the fundamental frequency). But any other waveform contains overtones, thus your amplifier needs to be able to amplify tge overtones, too.
Now a square contains overtones up to infinite frequency. So if you want to amplify the waveform with good quality your amplifier needs to be able to amplify much higher frequencies. For a 100kHz square wave maybe use a 10MHz amplifier ... but still the waveform will not be perfect.

--> terminology "shift" ... in the area of signal processing often is used as "DC shift" not to amplify.
* amplify is to "multiply" a signal
* shift is to "add" a signal
Example:
Let's say you have an input signal with it's peaks at -0.5V and +1.5V.
* Amplify by (a factor of) 3 means both peak values become multiplied by 3: to -1.5V and +4.5V
* Shift by 2V means both values become added by 2V : +1.5V .... +3.5V

Do you want to adjust "DC shift", too? (With a second potentiometer?)

Klaus

 

[bhl777]

Hi Klaus,
Thank you so much for your help!
To answer some questions you pointed out:
1. since it is signal level without the need to drive something, I assume any rail-to-rail op-amp can do the work
2. thank you for your clarification and education on the term "shift"! I do not need to adjust the DC shift.

According to what you instructed, I do not even need a pot to adjust the output level. Instead, I can adjust the voltage level using my function generator, within the range of 2.5V peak to peak, then use a fixed gain to do so. Therefore, the design can be like this, is this correct?

if this is correct, do you think I can quickly buy a breakout board of an opamp IC to rework? Thank you!

 

[KlausST]

Hi,

In post#2 I asked 2 questions. They are still valid.

I wonder why in post#1 you twice talked about a potentiometer to adjust the level ... and now say you don´t need it.

***
Basically your sketch is correct.
A clear information about the maximum voltage level and the minimum voltage level also is missing.
For a high quality operstion I additionally recommend:
* using capacitors / filters for the power supply
* using ESD protection at all "user accessible" signals

Klaus

 

[danadakk]

If you want to translate the symmetrical signal (about ground) to a single
ended signal you need to offset that. A simple method is :

Calculator (excel) attached. This would be done at Non Inverting input to your
OpAmp.

Regarding OpAmp. At 1 Mhz, 5V pk - pk, sine, you need an OpAmp with
a slew rate >~ 31 V / uS. That also constrains squarewave Trise/Tfall.....

This is an excellent tool for doing G and offsets for amps :

https://www.ti.com/lit/an/sloa030a/sloa030a.pdf?ts=1643075503201&ref_url=https%253A%252F%252Fduckduckgo.com%252F

Regards, Dana.