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Modifying this design: the increase of voltage range

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Member level 5
Oct 14, 2003
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Modifying this Design

Hi everybody,

Share some solid idea to increase the voltage output range in this design. I mean instead of getting 0 to 4.096V ............... I need to modify it for 0 to 15V.

Any comments??


Modifying this Design

you should :
- increase supply voltage to opamp (check that it supports more than +-15 V supply and what voltage on supply should be set to get 15 output without saturation , if necessary replace it with
suitable one )
- decrease R2 to 10 kOm
- tune opamp output by R3 to 15 V when max output set in DAC.

Re: Modifying this Design

Can u pls explain how one can do this calculation for the values u mentioned?

I mean incresing supply voltage makes the sense. But how can one calculate the other parameters to set?


Modifying this Design

above i roughly estimated R2 values but more less strict calculation could be done as below :

opamp gain when working in non inverting mode = 1 + R4/(R2 + R3) . Now lets calculate everything for trimmer resistance in middle position when R3 = R3/2 to make sure that we will have a oportunity to change voltage down and upwards . The gain will be R4/(R2 + R3/2) = 3.29 . Assuming that DAC provides for max output 4.096 V , we need to increase gain 15/4.096 times =~ 3.66 times so new gain is to be 3.66*3.29 =~ 12.04

the negative feedback coefficient could be calculated as :

12.04 - 1 = R4/(R2 + R3/2)

if R4 is to be unchanged

R2 + R3/2 = R4/11.04 = 9.05 kOm

As gain increases you can decrease value of R3 down to 3 kOm then can use R2 = 7.5 kOm and R3 = 3 kOm


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Re: Modifying this Design

Thanks a lot for ur help!!

Pls also see the following attachement. If I used the same DAC in this way ...... I mean having refrence voltage of 15V and then taking Vo as mentioned in the formula. I am just not able to find out that will this circuit be able to give around 300mA???

As the DAC can go max. to 50mA ?????

Any idea how can I get around 300mA at output.

Note: I saw OP77 datasheet but couldn't find any current ratings at output ..!!


Modifying this Design

the simplest way is to add voltage follower on BJT directly to opamp output and taking feedback from its emitter. But that requires low ohmic resistor and dissipates a lot power

or possible to add push-pull like here :
Of course this circuit works best when biased for class A.

there are also other ways like here :
**broken link removed**

For practical circuit look to "Opamp for everyone" TI's application note under clause called something like - increase opamp otput voltage and current . (clause 14) . The doc should be available here or from inet .

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