After this differential amplifier, is a buffer needed?

Hello All,

A differential amplifier is implemented in my circuit( please see the picture in the attachment). The amplifier shown in the attachment is composed of a differential-input single-ended transconductance output stage and a source follower as buffer.

1, In my application, this amplifier has to drive a capacitive load (not more than 100pF), is it possible to just use this amplifier without source follower(use node A as output) or should I also include the buffer(use node B as output)?

By the way, the papers I read on operational amplifier or differential amplifier and so on always include a buffer stage; But in my simulation, I often find the circuits can work well and drive the following stage without any buffer. So I am wondering what is the essential advantage of a buffer? Or when should I use a buffer?

2, If I use A as output, than A should be connected with Vnn (as the red connection in the picture). Is it the correct cofiguration for negative unity gain feedback? The feedback ciruit sometimes has the problem of stability, for this circuit, how likely is it going to oscillate?

3. In order to increase the gain and the output voltage upper level, how can I optimize the transistor parameters (W/L)(M1-M4), (W/L)(M5-M6) of this amplifier?

thanks very much!

As a first misunderstanding: The output stange isn't a source follower, it's an inverting amplifier with considerable voltage gain instead. Using A as the output would turn your amplifier in an OTA with limit output voltage range. May be suitable for your application or not...

Please reconsider.

Yes, the output buffer is a common source amplifier, I have written something wrong. Thanks

I would like to have the output voltage range up to the supply voltage VDD, and I've simulated the amplifier A without buffer, the output is limited up to 1.8-1.9V

If the buffer is also included, could this increase the output voltage upper level? After adding a buffer, is there any additional problem related to oscillation?

FvM said:

As a first misunderstanding: The output stange isn't a source follower, it's an inverting amplifier with considerable voltage gain instead. Using A as the output would turn your amplifier in an OTA with limit output voltage range. May be suitable for your application or not...

Please reconsider.

Click to expand...

When operatig the output stage with capacitive load, stability issues may be observed, I think, they should be controllable by the miller compensation network. But I'm not an IC design exper with intuitive knowledge of typical parameters that can be expected.