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OPA860: Calculation Gain of OPA860 as a Forward Amplifier (Fig.48 in datasheet)

Prodigy 110 points

Replies: 4

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Part Number: OPA860

Hi all 

I am going to use OPA860 as a Forward amplifier (Common E configuration). However, I am not clear about its calculation (in Figure 48 in data sheet). In fact, I am using Oscilloscope for obtaining the Vo waveform, and Oscilloscope have 50Ohm input as indicated in bellow figure. 

So, the RL = RC//RIN = 500//50 = 45.45 Ohm 

So, gm = 1/(102*10^-3) = 9.8Ohm 

Consequently, Gain = 45.45/(9.8+78) = 0.52. 

But in the above figure, Gain = 5. I am really not clear about how it can be 5. 

Could anyone please give me an explanation for this difference ? 

Thank you.

  • Hello user3925438,

    This calculation is meant for a load of 500 Ohms. You will notice in the electrical characteristics on page 3 of the datasheet that RL = 500. Realistically, as you will see when doing the math the gain won't be exactly 5V/V but slightly greater as can be seen by calculations and the sim shown below:

    user3925438.TSC

    Best,

    Hasan Babiker

    If this reply has answered your question, please click the "This resolved my issue" button.

  • Hi,

    it's very simple:

    Kai

  • In reply to Hasan Babiker31:

    thanks so much for your feedback. Now i am clear
  • In reply to user3925438:

    thanks so much for your feedback. Now i am totally clear

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