# What is the input impedance of a differential amplifier?

Discussion in 'Electronic Basics' started by [email protected], May 19, 2005.

1. ### Guest

It seems like a simple question, but I've gotten drastically different
responses from different people. How do you calculate the differential
input impedance for an amplifier like this:

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar6.html

What if you keep your source between the two inputs, but ground one?
Does the impedance now change?

If you connected two separate sources from ground to each input (or a
single common-mode source?), what input impedances do they see?

And so on. I have been very confused with people telling me I'm not
taking "negative impedance" into account and such.

Is it possible to measure the input impedance with a standard ohmmeter
or RCL meter? (As long as the amp doesn't clip, of course.)

2. ### colinGuest

the non inverting input just sees 2 100k resistors in sreries asuming the op
amp impedance can be neglected so is just purly 200k.

the inverting input just sees the first 100k as the op amp tries to keep the
end of that resistor at half the potential of the non inverting input,
however a change in voltage on just the non inverting input cuases a change
in curent on the inverting input.

Colin =^.^=

3. ### BanGuest

Differential input impedance is 200k on the +in and 66.66k on the -in
common mode input impedance is equal and 200k on both inputs.
Let us have +Vin= V1 and -Vin= V2
You can find that out by first looking at the +in, where the level is
1/2Vin. The opamp steers its output in such a way that there is the same
voltage on the -in. This means the voltage across R1 is 1.5Vin. since the
resistor is 100k this corresponds to R= 100k/1.5 =66.66666k
Now do the same with V1 and V2 both +Vin to obtain the common mode input
impedance.
This is taking the opamp as ideal, which is pretty much what you can also
measure.
I think you should measure the current from a fixed voltage source. The
ohms range it is pretty meaningless and might damage the meter depending on
the supply voltage of ther opamp. The additional current can lead to
unexpected output voltages.

5. ### BanGuest  