Simulating a Voltage follower Op-amp in LTSpice, what am I doing wrong?

Thread Starter

Sycko

Joined Jul 11, 2019
23
Hello everyone! o/

I've been trying to simulate a voltage follower/buffer using an ADA4860 op-amp so that the signal at its output is the same as the input (unity gain (G=1)

upload_2019-7-11_15-29-45.png

The input signal to the op-amp is called Vout_2 and it comes off a 10:1 voltage divider and goes into the Op amps input (non-inverting configuration)

The input signal to the op-amp is 1.5V AC with a frequency 13.56MHz.

When I simulate it, I expect Vout_2 to equal Vout_3 (1.5V AC).
Something is going very wrong there... At the output of the (Vout_3) Op-amp I get about -3.16V DC :eek:

upload_2019-7-11_15-33-10.png


Any Ideas what I'm doing wrong?
 

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BobTPH

Joined Jun 5, 2013
2,057
Take out the capacitor, for one thing. There has to be a DC path to ground from the input.

Also the resistors are way too high. There is a minimum current, the offset bias current needed for each input.


On the other hand, at that high a frequency, what you get will depend on the high frequency characteristics of the opamp. A standard opamp would be no good at all as the have unity gain at about 1MHz.

I don’t know the specs of the one you are using.

Bob
 
Last edited:

ericgibbs

Joined Jan 29, 2010
8,886
hi S,
Welcome to AAC.
Check your +/-5V supply sources.????
ie: the -V
E

Update: I seem to get different results on different Runs.??? I will check.

EDIT:
The input cap must be removed and a resistive path from the NI input to 0V must be connected.

I am having a problem with my system. [recent upgrade]
When driving the OPA with 13megHz test source, R and C removed, I had 1V in and 1V out, then .... nonsense .
 
Last edited:

Thread Starter

Sycko

Joined Jul 11, 2019
23
Hi Bob and Eric, thanks for the replies :)

upload_2019-7-11_16-7-45.png
I simplified the circuit a bit and removed the unneccesary coil. The sim file's attatched in case anyone wants to have a play :p

The Op-amp I'm using, the ADA4860 is a high frequency Op-amp that should work for frequencies above 50MHz with no issues!
(I attatched the datasheet for it)

What do you mean by a DC path to ground? I've added a 50k and ground to the input of the amplifier and removed the capacitor, still getting wierd results.. :(

And my voltage supplies look allright I think. So this mistery still illudes me
 

Attachments

BobTPH

Joined Jun 5, 2013
2,057
It is not an ideal op amp. There needs to be a certain amount of input current, the input bias current, available to the inputs and the there will always be an input offset voltage. You are seeing the effects of this because your signal is small with large resistors in the path. See my results when I reduce the voltage divider to 1:1 and remove the extra resistor on the input:

upload_2019-7-11_11-21-52.png
 

Thread Starter

Sycko

Joined Jul 11, 2019
23
Thanks again you two!

I retested it with Eric's setup and also tried your suggestion Bob and it worked to perfection! :D
Since I'm only using 13.56MHz I'm totally fine with the bode plot response of the Op-amp.
What I never considered was the input current, I guess it was being destroied by the two super large Mega Ohm resistors at the voltage divider, whoops!

I changed the potential divider to 9k and 1k to maintain the 10:1 ratio and it finally did work as expected!

Leaving the final results here for anyone else who runs into this issue :D
upload_2019-7-11_16-33-39.png


Thanks again you twoo, I learnt something new today ;)
 

Audioguru

Joined Dec 20, 2007
11,251
The datasheet for this very high speed opamp says it is unstable if it has your voltage gain of only 1 and a feedback resistance less than 550 ohms. Your feedback resistor is a piece of wire.
It oscillates if its output feeds the tiny amount of capacitance to ground of just about anything.
The datasheet also says how to properly decouple the power supply with capacitors. Your circuit has none.

Why does your new circuit have R5 and R7 in parallel? Only the 1k low resistance of R5 is needed.
 

Thread Starter

Sycko

Joined Jul 11, 2019
23
Yeah I forgot about the current return path earlier Eric, but its all good now!

Oh and hi Audioguru,

Yes I realize that extra parell resistor does nothing when in paralell with the 1Meg one now that you pointed out, so I removed it!

Hmm I missed that important detail from the datasheet
I'll fix that too,

Thanks for all the help everyone
 
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