Input And Output Impedance Of An Opamp Circuit and Impedance Matching

Hi everybody,

I am trying to design opamp circuits, but I couldn't fully understand how to find input and output impedance of an Opamp circuit.

When the configuration is Non-Inverting, input impedance of the circuit is High-Impedance and for impedance matching I may use a resistor with parallel to opamp +Vin . For example I use the configuration below for 50Ohm input impedance matching. Also, I use serial resistor for output impedance matching because circuit's output impedance is zero I guess :-?



And when the configuration is inverting, circuit's input impedance is Rg right ? So for input impedance matching I must choose Rg appropriately right ? For example, for 50ohm impedance matching, I must choose Rg 50 Ohm right ? Also I find output impedance for this circuit is 0 Ohm, am I right ? :-?

Lastly, I found input impedance of a differential Opamp circuit is 2*Rg. So if my first opamp is AD8015 : https://www.analog.com/media/en/technical-documentation/data-sheets/AD8015.pdf , then for input impedance matching do I have to use something like the below ?



Also for differential opamp circuit, I found output impedance of the circuit is zero ohm. However, I am not sure if my method is true or false.

I don't see how the differential amplifier circuit could be implemented with AD8015, which is no general purpose OP-Amp. It e.g. has no freely connectable non-inverting input.

Regarding input impedance, please consider that the differential amplifier configuration according to the schematic has different input impedances for + and - input, also the impedance of the inverting input depends on the signal applied to the non-inverting input. It's effectively not suited for any kind of impedance matching or applications that require a balanced load, e.g. when amplifying the output signal of a sensor in bridge configuration.

Most op amps can't drive 50-ohm loads to more than
1V amplitude. "Zero output impedance" is an assumption,
for small signal analysis in feedback configuration, not a
general reality.

Likewise infinite input impedance, although relative to
transmission lines where you'd care, it's plenty close
enough.

Are you overthinking the op amp impedance when
you really just need to idealize it and work the board
level circuit design?

Normally you don't need to impedance match with the low frequencies that op amps typically operate at.
Why do you think you need to match impedances?

FvM said:

I don't see how the differential amplifier circuit could be implemented with AD8015, which is no general purpose OP-Amp. It e.g. has no freely connectable non-inverting input.

Regarding input impedance, please consider that the differential amplifier configuration according to the schematic has different input impedances for + and - input, also the impedance of the inverting input depends on the signal applied to the non-inverting input. It's effectively not suited for any kind of impedance matching or applications that require a balanced load, e.g. when amplifying the output signal of a sensor in bridge configuration.

Click to expand...

I connect AD8015 outputs to the OPA695 inputs so I convert differential line to single ended and using OPA695 I can set the gain. Here my circuit :



- - - Updated - - -

crutschow said:

Normally you don't need to impedance match with the low frequencies that op amps typically operate at.
Why do you think you need to match impedances?

Click to expand...

Some of the projects I am working on, operates up to 1 GHz . So I thought , impedance matching is critical. :-?

strahd_von_zarovich said:

Some of the projects I am working on, operates up to 1 GHz . So I thought , impedance matching is critical. :-?

Click to expand...

At those frequencies it is.
You're using op amps at those frequencies??

Connecting AD8015 output differentially makes sense if you have a differential load or are driving a differential transmission line. Otherwise you would rather connect one AD8015 output to your single ended load instead of using another OP for differential to single-ended conversion.

crutschow said:

At those frequencies it is.
You're using op amps at those frequencies??

Click to expand...

Well, some of signals I want to amplify has 5ns rise and fall time. :-|

That ought to be challenging for many op amps,
but at 5nS rise and fall and 1nS/foot, are any of
your traces long enough that they should be treated
as Tlines (and if so, are they indeed laid out as
controlled impedance lines such that you could
match)?

strahd_von_zarovich said:

Well, some of signals I want to amplify has 5ns rise and fall time. :-|

Click to expand...

That doesn't translate to 1GHz bandwidth.
It's closer to about 70MHz.