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

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

• Hi,

it's very simple:

Kai

• In reply to Hasan Babiker31:

thanks so much for your feedback. Now i am clear

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