Unity gain buffer for driving large load capacitance

[CherryQ]

Hi all,

I want to design a buffer after the differential Opamp stage to drive large load capacitance. The buffer requires small gain and the same CM level with my input differential signal, so I use an inverter with feedback resistor R. By tuning R, I got almost unity gain, and by tuning the inverter NMOS and PMOS, I got the desired output CM. However, I found one stage of this buffer is not sufficient for my load cap. I am wondering if I can cascode several this kind of buffers. Because the gain drops significantly even though I have unity gain for each buffer.

Any comments will be appreciated.

 

[Old Nick]

Re: Unity gain buffer

For driving a load capacitance, it is the output current drive of the amp that is important. Cascading inverters won't help this. Why don't you just design an op-amp capable of driving this load, then you won't have to worry about tuning resistors (which after layout will be of quite different values to the designed!).

 

[shaq]

Unity gain buffer

Single-stage opamp as a buffer will help you a lot.

 

[rexcheung727]

Re: Unity gain buffer

Hi,

I will not suggest you to use an inverter to be a buffer, it is because
(1) the biasing current for the inverter is difficult to be controlled, i.e. your device cannot tolerate process variation under fabriation
(2) The slew rate cannot be too high as the gain of the inverter cannot be too large due to limited output resistance

So your ideal is good as you suggest cascoding (not cascade) the inverter, which can increase the output resistance of your device and results in higher gain. However, you haven't consided the baising problem.

I suggest you try to use the same bais provided in the differential stage and make a second stage amplifier.

 

[surianova]

Re: Unity gain buffer

Hi all,

I want to design a buffer after the differential Opamp stage to drive large load capacitance. The buffer requires small gain and the same CM level with my input differential signal, so I use an inverter with feedback resistor R. By tuning R, I got almost unity gain, and by tuning the inverter NMOS and PMOS, I got the desired output CM. However, I found one stage of this buffer is not sufficient for my load cap. I am wondering if I can cascode several this kind of buffers. Because the gain drops significantly even though I have unity gain for each buffer.

Any comments will be appreciated.

just use 2 stage opamp is good enough as u only drive capacitive load. With gain of at least 60 dB, high output impedance is ok for driving capacitive load.

 

[rajanarender_suram]

Unity gain buffer

I too sugget a simple 2-stage opamp:::

 

[ramz]

Unity gain buffer

design a souce follower

 

[Sadegh.j]

Re: Unity gain buffer

I would not think that the series combination would be problematic, your gain will most definitely decrease.

Cheers

 

[CherryQ]

Unity gain buffer

Thanks for all the solutions. But for 2 stage amplifier, it normally has >40dB gain. If my output has wide swing, it might saturate after the buffer.

Added after 2 hours 14 minutes:

. I read about a class AB buffer, as in the figure. Compared to class AB source follower, when Vin increase/decrease, one of the output transistor goes off. But for this circuit, I have two questions. 1. When Vin varies, both of the output transistor are in saturation. It doesn’t satisfy the definition of class AB. 2. it’s two cascade source follower, the total gain should be around 1. why did my simulation give -5.2dB? Is this due to the body effect? Any comments will be appreciated.

Added after 3 minutes:



I used Upload image beside quick reply, however, the image wasn't displayed. So I pasted the link.

 

[tekno1]

Unity gain buffer

CherryQ today I left a comment about this buffer at the original post. It is class AB but input/output common mode range is Vt+Vds away from rails.

 

[jecyhale]

Re: Unity gain buffer

I think it will be change with PVT.

I think the better way is use the source follow and a differential pair.