Buffer to drive a large capacitive load?

[Octago]

In my circuits, I want to design a buffer in order to drive a large capacitive load

For example, a source follower is often used in this case as a buffer

But the source follower always introduces a voltage drop between the two stages, which is not desired in my case.

So are there any buffers which can on one hand drive a large capacitive load, but on the other hand not introduce any DC voltage drop?

 

[raduga_in]

Pushpull Class AB buffers can be an option ..

Raduga

 

[Octago]

thanks, I just did a quick search of Pushpull class AB buffers in Internet, it seems like such buffer consists of opamp and the other stages, and the design can be quite complex

Are there any simple topology of Pushpull class AB buffer? My goal is only to drive a large capacitive load.

Pushpull Class AB buffers can be an option ..

Raduga

 

[Old Nick]

In my circuits, I want to design a buffer in order to drive a large capacitive load

For example, a source follower is often used in this case as a buffer

But the source follower always introduces a voltage drop between the two stages, which is not desired in my case.

So are there any buffers which can on one hand drive a large capacitive load, but on the other hand not introduce any DC voltage drop?

A simple OTA (long tailed pair), with -ve feedback is what I normally use, far superior linearity to a SF aswell.

 

[spminn]

I think a CMOS inverter with a feedback resistor between input and output should do the job. It would have enough current drive due to the addition of gm of the pMOS and nMOS and the resistor should set the output bias to be the same as the input bias.

 

[leohart]

In my circuits, I want to design a buffer in order to drive a large capacitive load

For example, a source follower is often used in this case as a buffer

But the source follower always introduces a voltage drop between the two stages, which is not desired in my case.

So are there any buffers which can on one hand drive a large capacitive load, but on the other hand not introduce any DC voltage drop?

A simple OTA (long tailed pair), with -ve feedback is what I normally use, far superior linearity to a SF aswell.

what is -ve feedback pls?

Added after 2 minutes:

I think a CMOS inverter with a feedback resistor between input and output should do the job. It would have enough current drive due to the addition of gm of the pMOS and nMOS and the resistor should set the output bias to be the same as the input bias.

can this one work? it has gain only in a small portion near the middle of input voltage, and what is the purpose of the feedback resistor?

 

[spminn]

As I said, the resistor can be placed to set the output bias to be equal to the input bias. Secondly, the gain of any amplifier is always within a small range of input. The range of this input can be shifted by changing the ratio of the pMOS to nMOS width.

 

[Old Nick]

In my circuits, I want to design a buffer in order to drive a large capacitive load

For example, a source follower is often used in this case as a buffer

But the source follower always introduces a voltage drop between the two stages, which is not desired in my case.

So are there any buffers which can on one hand drive a large capacitive load, but on the other hand not introduce any DC voltage drop?

A simple OTA (long tailed pair), with -ve feedback is what I normally use, far superior linearity to a SF aswell.

what is -ve feedback pls?

Added after 2 minutes:

I think a CMOS inverter with a feedback resistor between input and output should do the job. It would have enough current drive due to the addition of gm of the pMOS and nMOS and the resistor should set the output bias to be the same as the input bias.

can this one work? it has gain only in a small portion near the middle of input voltage, and what is the purpose of the feedback resistor?

-ve feedback, is the shortened way of writing 'negative feedback'. Feeding the outpu to the inverting input. This configures the ota as a voltage follower.

 

[Octago]

thanks

But if I use a OTA with unity feedback as the buffer, will the problem of oscillation be critical?

 

[Old Nick]

thanks

But if I use a OTA with unity feedback as the buffer, will the problem of oscillation be critical?

I use them all the time, and not one of them has oscillated, ever. If you're feeding it with square waves then I wouldn't put my mortage on it not oscillating a bit, but if you're worried about it do a few sims.

 

[Octago]

thanks very much again.

it seems like OTA with unity gain as the voltage follower is quite often used. I don't have much practical experience on operational amplifier design.

Are there any reference schematic or datasheet for this circuit?

thanks

But if I use a OTA with unity feedback as the buffer, will the problem of oscillation be critical?

I use them all the time, and not one of them has oscillated, ever. If you're feeding it with square waves then I wouldn't put my mortage on it not oscillating a bit, but if you're worried about it do a few sims.

 

[Old Nick]

I can't actually give you any exact details of the ones we use, but they are basically a standard long-tailled pair (differential amplifier) with full -ve feedback.

I've attached a schematic of a 5uA buffer, you may need to adjust the sizes of components to suit your drive needs and process etc. just choose sensible overdrive voltages etc, for your transistors. If you have the space though, use a 2-stage op-amp (with -ve feedback), we always use these when drivinng the pads of a chip - biased at several mA.

 

[jecyhale]

You can rebias the DC level by a couple capacitor and a resistor if the frequecny is very high.
And if the frequency is not very high, it will not care the large capacitor.
So can you tell us the frequency and the capacitor value?

 

[Octago]

The frequency is no more than several KHz
the capacitive load needed to drive will not be more than 50pF

You can rebias the DC level by a couple capacitor and a resistor if the frequecny is very high.
And if the frequency is not very high, it will not care the large capacitor.
So can you tell us the frequency and the capacitor value?

 

[jecyhale]

The frequency is no more than several KHz
the capacitive load needed to drive will not be more than 50pF

You can rebias the DC level by a couple capacitor and a resistor if the frequecny is very high.
And if the frequency is not very high, it will not care the large capacitor.
So can you tell us the frequency and the capacitor value?

R=1/(jwc).
if the w=2pi*10k and c=50pf, so R is equal to 300M.
So I think 50pF is a very small capacitor for a frequency less than 10K.
isn't it?

 

[tia_design]

How big the capacitor load is. I think Class AB is a potentialchoice

 

[Octago]

not more than 10pF

How big the capacitor load is. I think Class AB is a potentialchoice