TS9222 Impedance analysis

[KR.5]

Hi team

I am using TS9222 ( two stage inverting buffer) op-amp as a microphone pre amplifier. it will acts as a voltage follower. Resistor set in the inverting input and feedback is 47K. capacitor in the feedback is 100pF. this is repeating twice(two stage).  In the non inverting input it is pulled to vcc/2. My aim is to measure the input and output impedance of this amplifier using bode 100 tool. 

Please share the impedance analysis procedure if you have( bode 100). 

Any other method or tool recommended ?. Can i try the same in ST simulation tool?

I used impedance analysis simple method also. I tried output impedance analysis method, I am getting distorted output (expecting half time input voltage without any distortion) if the load impedance becomes less than 1.6K for 2.5V+ 1Vpeak( offset+ ac) 1KHz sine wave input. This is not good way to use max power transfer theorem since voltage is clipping below 1.6k. I am getting 15K as the input impedance in the same way. 

Thanks

[LCE]

Output voltage clipping with a load of 1k6 with that opamp (80 mA output drive) sounds like something's wrong:
maybe there's something wrong with your circuit, or your input signal?

Checked and buffered the supply (which is what?) with at least 100 nF?
2.5 V DC offset at input correct? Do you see this DC at the output?

Input impedance of an inverting opamp circuit should be = R = 47k.

Even though your description is good - a picture would be better!

[KR.5]

attached reference image for one stage.. it is repeating for next also.. please share the input and output impedance measurement procedure 

I am getting 2.5V DC offset at the output.

[LCE]

Let's have a look at the DC.

1st stage: (2.5V * -1) + (3.75V * 2) = -2.5V + 7.5V = 5V

2nd stage: (5V * -1) + (3.75V * 2) = -5V + 7.5V = 2.5V

So that is correct.

So even with the sine applied, the signal is always within the RRIO range.

Maximum load current for the last stage is (2.5V + 1V) / 1k6 < 3mA, so that should not be a problem either.

Input impedance is 47k.

Maybe reduce the 100pF feedback (maybe it's higher).

And you didn't say anything about supply buffering. put there at least 100nF to VCC close to the opamp.

Edit: is the signal after the first stage clean or distorted?

[KR.5]

Yes 100nF and 10uF cap available at the VCC of opamp. I haven't probed 1st stage output.. My aim here is to measure the input and output impedance for the given circuit.. May be i am wrong while doing measurement..

Please suggest your method of analysis to do the measurement

[LCE]

For the inverting amplifier, if you put any serial resistor in front of the first input resistor, you are changing the gain, also affecting the DC values, so that might be a problem.

Anyway, if I'm not wrong, the input impedance of an inverting opamp is in your case R4 = 47k.

For the output impedance, when trying to measure AC impedance, you should AC couple the output with a BIG capacitor (no cheap ceramics!).

But you should at first find out why the signal is distorted.

[KR.5]

yes correct Impedance should be 47k( inv terminal resistor). but its not in actual.. may the reason is below..

addition of series resistor as per input impedance in  impedance analysis  method, gain is look to be shifted.

1st stage: (2.5V * (-47K/(47K+15K)) + (3.75V * (1+47K/(47K+15K) )= -1.89V+ 6.59V = 4.7V

2nd stage: (4.7V*(- 47K/(47K+15K)) + (3.75V * (1+47K/(47K+15K)) = -3.56V + 7.5V = 3.03V

Since the offset is now 3V, it may be possible that ac signal will get distorted as per my calculation and

next question is how can i measure the input impedance?

for the output impedance measurement- distortion- i am not sure why distortion occurs

[LCE]

I'm pretty sure that the inverting opamp's impedance is equal to the first resistor, in your case 47k.

If you want to confirm that without changing your DC levels, you could "split" the 47k into 2 resistors and measure "inbetween", but not so sure about that.
On the other hand, your DC levels with the additional 15k still seem to be okay if your input signal is only 1Vpeak.

So first of all find out why there's distortion.
This should not occur in such a simple circuit with that signal, opamp, load. 
Is your source signal clean?
What does that distortion look like? Do you have scope screenshots? Input, after 1st stage, output.