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INA2141: Op-Amp design

Part Number: INA2141
Other Parts Discussed in Thread: OPA172

Hi,

Please refer below requirements

Input Voltage: 0.2 to 3V

Output voltage: 0 to 11V

Output Current: 12mA

Vs: 12V or 1.8V or 3.3V can be provided

Please let me know how to implement this in circuit level.

Also please let me know what are the other Op-amps which will be suitable for my requirement.

  • Nagendra,

    The INA2141 will not meet your application requirements because the gain is fixed at 10 V/V, or 100 V/V, the input common-mode range does not extend down to 0.2 V with a single supply, the output doesn't swing down to 0 V with a single supply, and the output can't deliver 12 mA of output current. The INA2141 is intended to be used with a dual (+/-) supply, not a single supply.

    Is the output level supposed to be 0 V with 0.2 V applied to the input, and  11 V with 3 V applied to the input? What are your tying to accomplish with the circuit? The more information you can provide about the application requirements, the more likely we will be able to assist you.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Thanks Thomas.

    Yes. The output level should be 0V with 0.2V applied at input and 11V output with 3V applied at input.
    With this op-amp's output voltage I need to control an amplifier Gain.
  • Hi Negendra,

    There is a bit of a conflict in your description of what the amplifier needs to do. If it is set up for an output of 0 to 11 V with an input of 0.2 V to 3 V, that will require a gain of +3.9 V/V, with an offset of about -0.8 V. That is one condition. But then you mention "I need to control an amplifier gain," which implies setting a different gain. That different from the first condition. Please explain what you intend.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Hi Thomas,

    Sorry for the confusion. The amplifier I was mentioning is about the RF amplifier.
    With a single supply to the Op-amp and with input voltages of 0.2 to 3V, can we have an output voltage of Op-amp of 0 to 11V? The output voltage of this Op-amp is used to control an RF amplifier gain.

    Also please let me know if any op-amp output goes from 0 to 11V with a 1.8V or 3.3V single supply with Op-amp o/p current > 12mA.
  • Hello Nagendra,

    In order for the output to be able to achieve a level of 11 V the supply voltage cannot be 1.8 V, or 3.3 V, but instead must be higher than 11 V. Therefore, you will have to provide a supply level more along the line sof 12 V, or 15 V, to acheive that output level. Also, op amp outputs do not swing to exactly 0 V on the low end, but instead something more on the order of tens or hundreds of millivolts above 0 V when powered by a single supply. Op amps powered by dual supplies (+/-) can achieve 0 V and negative output voltage levels as well.

    I have provided an example of a circuit using the OPA172 op amp that provdes a nearly 0 V to +11 V output, for a +0.2 V to +3.0 V input. The OPA172 can easily provide the 12 mA output current. The 480 mV reference voltage used to provide the necessary offset would be hard to come by; however, changes could be made to the circuit to use a higher, standard reference voltage such as 1.25 V, or 2.5 V. I wanted to keep the circuit pretty basic to illustrate the design idea.

    Here's my TINA Spice model if you wish to try some simulations.

    OPA172_0_to 11Vout_01.TSC

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Hi Thomas,

    As you were referring to 480mV offset voltage, can I provide this 480mV offset from a resistor divider to 12V op-amp supply?

  • Hello Nagendra,

    The resisitve divider would be loaded by the current flowing through the input resistor and that would introduce a dc error. Also, the impedance of the resistive divider enters into the gain equations making the determination of the gain through the two inputs more compex. It would be better to use a standard voltage reference since 840 mV is not a standard reference voltage value. It would be best to use a standard voltage reference such as 2.5 V, or another, and then change the gain in the circuit that is associated with producing the approximate -0.786 V that is needed to offset the output to a 0 to +11 V range.

    I went though the calculations for a 2.5 V reference voltage and came up with the values shown in the TINA Spice diagram shown below. Do note that the voltage offset and bias currents of the op amp will introduce small errors just as they do in any circuit. Therefore some adjustment to the circuit values may be necessary to optimize performance

    I've included my TINA Spice file so that you can experiment with a different reference voltage, or whatever else you would like to try.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

    OPA172_0_to 11Vout_02.TSC  

  • Hi Thomas,

    Thanks for the TINA Spice circuit file. It was very helpful. I could know the output voltage at any point in the circuit given by you.

    Just mentioning again the requirements here, input voltage = 0.2 to 2.7V, output voltage = 0 to 11V, Output current = 12mA with  Vs=12.5 V can be provided. The output voltage 0 to 11V is used to control an RF amplifier Gain. 

    Please provide me some technical notes on how to do these kind of calculations.

    Could you please let me know what are the gain and offset calculations to be used with any formulas required, if I need to change the input and output voltage requirements?

    Thanks

    Nagendra

  • Hello Negranda,

    Please see the attached pdf. It provides the calculations I made to determine the component values used in the circuit. I hope you don't mind that they were done simply using pencil and paper.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

    OPA172_amp_with_dc_offset.pdf