4 Replies Latest reply on Sep 29, 2014 6:16 AM by DaKn_263916

# Only positive value output from voltage follower. What's wrong with it? Thank you!

Hello everyboby,

I'm trying to use voltage follower in my system. But I find that no matter what input signal inputs to voltage follower, only positive part can be output from voltage follower.  I'm confusing that if I set the voltage follower in wrong way or if there are some other setting of the board.

Here is the detail of my experiment:

I just used a voltage follower whose input+ is port 0[2] and output is port 0[1].

I also set the power of voltage follower to 'High power'.

In my code, I add 'Opamp_1_Start();' into for loop.

the input signal is sine wave with 1kHz, 1 Vpp and 0 offset.

But the output of voltage follower only have the positive-value part shown in attached picture.

I really want to know what's wrong with it? Thank you so much!

• ###### 1. Re: Only positive value output from voltage follower. What's wrong with it? Thank you!

The electronics inside the PSOC can only work between the Vdd and Vss values. Typically Vss is 0V.

Therefore you are seeing what I would expect: when the input voltage goes below Vss then the circuit simply cannot follow it.

If you want to follow the entire waveform and it is 1Vp-p then you will need to bias the waveform so that it varies between 0V and 1V. You might try using a capacitor in series and then a couple of resistors of the same value between Vdd and the input pin, and the input pin and Vss which will pu tthe bias point at Vdd/2.

(By the way, I have little expereince with the PSOC devices and there may be a better way to do the biasing).

Susan

• ###### 2. Re: Only positive value output from voltage follower. What's wrong with it? Thank you!

Thank you susan,

It's a good idea to change the bias. I've found that when I change the offset to 1 v, the output will be the same as input.

• ###### 3. Re: Only positive value output from voltage follower. What's wrong with it? Thank you!

Attached are some approaches to generating offset voltage to bias

circuits.

Regards, Dana.

• ###### 4. Re: Only positive value output from voltage follower. What's wrong with it? Thank you!

Thank you, Dana:)