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Op-Amp with a load resistor..Plz help


shaiqbashir
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Hi Guys!

well!

im just a bit confused here.


i want to ask you a question:

just have a look at the fig below which i have posted. You can see a load resistor "RL". my question is that whenever i studied this inverting configuration of opAmp, there is no load resistor given for it. So, now when i have to find the equation of its gain , with a load resistor, do u think that it will change from the one without a load resistor.

i know the gain equation which will come without load resistor. but what about tha equation with load resistor.


now do u think that with the load resistor RL, this gain equation will change?

thanks in advance:


opamp_1.JPG

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Hi Shaiq,
Your circuit doesn't need R3 and R4. Connect the output to R2.
If the load resistance is high enough that the opamp can drive it, then the voltage gain isn't affected by it. When the opamp's output begins to drop, the negative feedback is reduced by about the same amount, then the 200,000 times internal gain of the opamp makes-up the difference.

The output voltage of the opamp does drop a tiny amount when loaded, and you can calcualte it:
1) The output resistance of an opamp is about 75 ohms. The 200,000 times internal gain of the opamp reduces the output resistance when negative feedback is used, to about 75/200,000= 0.38milli-ohms at DC and with a closed-loop gain of unity.

2) The lowest load resistance for most opamps is 2k, so the voltage divider of the 38 milli-ohms to the 2k load is a ratio of 1:5.3 million.

3) So if the unloaded output voltage is 10.0V then it is 9.999999...V when loaded with 2k.

The voltage drop across the opamp's output resistance increases at frequencies above about 10Hz where its gain is lower.
The voltage drop across the opamp's output resistance is increased when the circuit has a closed-loop gain of more than unity.
With R2 connected to the output of the opamp, it is also part of the load.

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