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What is the purpose of the series resistor and external cap?

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jordan76

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Buffer compensation?

HI

As it can be seen from the figure, it is a unit gain follower in essence. In addition to that, a small series resistor and a big external capacitance has been added.

What is the purpose of the series resistor and external cap? Some say it may improve phase margin. But how to analysize it? Thanks!

regards,
jordan76
 

MK28

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Buffer design!!

hello there,

I have to design a source follower buffer for driving 50Ω load, its for wide band system, therefore, i've to use feedback system to get the broad bandwidth! But my buffer design is not so good, do you have any good materials about 'buffer design'........please?

MK28
 

alenhan

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Buffer compensation?

Hi, jordan 76
the res. and the cap. are for compensation the phase
margin, the designer want the dominate pole at the
last stage, to improve the stability.
 

jordan76

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Re: Buffer compensation?

alenhan said:
Hi, jordan 76
the res. and the cap. are for compensation the phase
margin, the designer want the dominate pole at the
last stage, to improve the stability.

Hi alenhan,

If so, then what is the use of the small series resistor?

Actually I want to know how to analyze the circuit's stability since
it is closed loop instead of open loop
. Any inputs about that?

Hi MK28,

If you can give more specific details, maybe I can give you some useful informaiton.

regards,
jordan76
 

mists

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Buffer compensation?

i think the small series resistor is a damping resistor to compesate bind wire inductor.
 

Btrend

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Re: Buffer compensation?

1. it is more likely a 1st order low pass filter, and it also generate a low freq. pole (~1/(r1*C)) which is also served as shunt feedback network to ur buffer.
2. the series resitor (16 ohm) must be the purpose of ESD protection.
 

jordan76

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Re: Buffer compensation?

mists said:
i think the small series resistor is a damping resistor to compesate bind wire inductor.

Hi mists,

Could you please explain a bit more about your point? Thanks!

regards,
jordan76

Added after 5 minutes:

Btrend said:
1. it is more likely a 1st order low pass filter, and it also generate a low freq. pole (~1/(r1*C)) which is also served as shunt feedback network to ur buffer.
2. the series resitor (16 ohm) must be the purpose of ESD protection.

Hi Btrend,

For your first point, I 100% agree!

For your second point, I asked some ESD experts and they said 16 Ohm might be too small for ESD protection, several hundreds Ohms more common.

Do you think the combination of series resistor and external cap will help improve the stability? If so, how to analyze it? Thanks!

regards,
jordan76
 

Btrend

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Re: Buffer compensation?

1. stability problem only come from closed loop, the ckt u asked has only a closed loop which is from buffer input to buffer output.
2. except the shunt feedback, there are no other feedback
3. u are right, it's true that 16 ohm is small for ESD purpose, but u can't use too high value, cause that will produce attenuation to your output, the gain is r1/(r1+Rs).
4. U can break the loop at the bufer output & in , and run ac simulation. to find out the effect of Rs to the phase margin.
 

jordan76

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Re: Buffer compensation?

Hi Fom,

Really good article except the figures are a bit blurry:) Thanks!

Also I find the following link:
"OP AMPS DRIVING CAPACITIVE LOADS"
https://www.analog.com/library/analogDialogue/archives/31-2/appleng.html
And an applet for demonstrating stability effects in opamp buffers:
**broken link removed**

Below are some useful excerpts from it:
Out-of-loop compensation: Another way to stabilize an op amp for capacitive load drive is by adding a resistor, RX, between the op amp's output terminal and the load capacitance, as shown below.Though apparently outside the feedback loop, it acts with the load capacitor to introduce a zero into the transfer function of the feedback network, thereby reducing the loop phase shift at high frequencies. To ensure stability, the value of RX should be such that the added zero (fZ) is at least a decade below the closed loop bandwidth of the op amp circuit.

regards,
jordan76
 

beckchm

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Re: Buffer compensation?

but buffer has wide band.
 

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