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voltage follower design

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chaitanya2k6er

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hi!!
can someone help me out how to design a voltage follower using decompensated opamp?!
i.e. LMP7708. I have seen the application note of LMP7708 but it was bit confusing. I would be glad to receive your suggestions.

Regards,
Chaitanya.
 

A decompensated OP isn't unity gain stable, so you won't use it for a voltage follower. There is a principle option to reduce the loop gain by an additional compensation circuit, but it's not reasonable at all.
 
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    exnol

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chaitanya2k6er said:
hi!!
can someone help me out how to design a voltage follower using decompensated opamp?!
i.e. LMP7708. I have seen the application note of LMP7708 but it was bit confusing. I would be glad to receive your suggestions.
Regards,
Chaitanya.
Click to expand...

I think you can utilize the compensation scheme as shown in fig. 14 of the data sheet.
Another alternative is "feedforward" compensation: Apply negative feedback via RF and Ro and feed the input signal into Ro as well as into the non-inv. input. This gives you a gain of unity and - at the same time - the opportunity to select the proper loop gain for good stability. Try it by simulation!
 

I would expect to hear at least a motivation for using the amplifier at unity gain. Otherwise, it sounds like a stupid idea.
 

Thank you for your replies, nd more over what i heard is lmp7708 is internally tailored for the use of voltage follower with out any external compensation. And it is mentioned in the data sheet under capacitive load section(page no-15) that lmp7708 can be used as voltage follower with high isolation for preventing the capacitive loading. But practically i have observed the oscillations in the voltage follower section. How to over come this??!!
 

what i heard is lmp7708 is internally tailored for the use of voltage follower with out any external compensation
Click to expand...
The datasheet clearly states, that low gain applications (e.g. V=+1) can't be handled without additional external compensation. I don't get the meaning of the term "voltage follower" found in a single place at page 15. I fear, it has been copied erronously from a different datasheet. The only possible reason to use a decompensated OP for a voltage follower intentionally would be to achieve higher slew rate. But you should better use a faster unity gain compensated amplifier in this situation, I think.
 

I think there are no reasons not to use the LMP7708 as a unity gain amplifier. There are no disadvantages connected with the 2nd alternative (feedforward compensation) as mentioned in my former post. In contrast, the other compensation scheme (Fig. 14 in the data sheet) suffers from increased noise properties.
More than that, a unity-gain compensated opamp may exhibit ringing of the step response when used in unity gain configuration. If this is a problem, the feedforward compensation scheme offers the possibility to adjust the loop gain at a value that ensures the best compromize between ringing and short rise time (that depends on the small signal bandwidth).
 

I think there are no reasons not to use the LMP7708 as a unity gain amplifier.
Click to expand...
Figure 14 (designated lead-lag compensation) is reducing the feedback factor and thus also increasing noise above the lead corner frequency. I don't see it as feedforward.
 

FvM said:
Figure 14 (designated lead-lag compensation) is reducing the feedback factor and thus also increasing noise above the lead corner frequency. I don't see it as feedforward.
Click to expand...

Me too.
As mentioned already in my posting, I repeat that the "feed-forward" scheme is alternative No. 2 (input connected to Ro and to the pos. input) - in contrast to alternative No. 1 (lead-lag, Fig. 14) which increases noise.
 

Yes, I misunderstood the suggestion. But I think, you need Rf/Ro > 9 for stability, resulting in a noise gain of 10.
 

FvM said:
Yes, I misunderstood the suggestion. But I think, you need Rf/Ro > 9 for stability, resulting in a noise gain of 10.
Click to expand...

Yes, that´s true. And the unity-gain small signal BW, of course, is determined by the noise gain (and not identical to the GBW of the opamp). That`s the price to be paid for stability.
 

what is the output impedance of a 741 voltage follower circuit?
 

what is the output impedance of a 741 voltage follower circuit?
Click to expand...
Do you mean the closed loop, small signal output impedance?
It's generally frequency dependent, about open loop output impedance * (1/loop gain).

Because the output impedance isn't specified in the datasheet, you can check it in a SPICE AC simulation with a standard 741 model.
 

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