EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: eev_carl on August 09, 2018, 01:23:05 pm

Hi,
I was following a W2AEW video on measuring output impedance but I can't seem to get a good value for Z for my circuit. I think it may be a problem with how I'm measuring things or a power supply setting.
In the schematic below, I have a 741 in an inverting configuration. Vin is a 35.4mVrms sine wave of 100Hz. I'm measuring Vout by putting my red +V MM lead on pin 6 and the black COM on the ground of the power supply. The open circuit Vout is 339mVrms. The seems in line with the circuit's function (10x gain).
When I put a decade resistor in parallel with the MM (connected to Pin 6 and ground), I get values that are all over the place. My Zo calculations range from 92ohms  530ohms. I've read that the 741 has 75 ohms. I'm not varying the frequency at all, so I'm confused as to why I can't just dial up an RL and get a value close to 75.
My Klein MM700 has a frequency range of 50400Hz.
Thanks,
Carl

The output impedance has nothing to do with the minimum load it can drive. Impedance is AC, resistance is usually DC. Your load is DC not AC.
The output impedance is the output resistance divided by (the open loop gain at the frequency used minus the set gain), much less than 1 ohm but about 1 ohm with the slow 741 with a gain of 10.
The datasheet shows that its output is overloaded with a resistance less than 2k ohms or 10mA.
The design of a 741 opamp is 50 years old (!) and it was designed to use only a positive and negative 15V supply.

I get values that are all over the place.
Perhaps your decade resistor adds capacitance, causing oscillations.

When do EE students learn of transfer functions and Bode plots and stuff ? I have a bunch of EE books but not actual text books. I'm too lazy tho, I need more focus.
I finally made a working Wien bridge oscillator. I had tried with a LM358 before and didn't know it's freq. limit's were stopping it from working.
So yesterday I used a LM741 and it worked right away. Wish I knew more of the math besides RC filters tho, used in them.

In the schematic below, I have a 741 in an inverting configuration.
Hi Carl,
1) Please show the complete schematics, including power supply and other components (i.e. capacitors on the supply rails etc).
2) The way you are trying to measure the output impedance is wrong for this circuit. The output impedance is quite low and if you "measure" it by increasing the load, the output will be overloaded well before you get a meaningful result.
3) The correct way to measure the output impedance of an amplifier like this one is to supply a small AC current which would not overload the output (say, 1mA RMS) directly to the output (and ground the input), for example, using 10V RMS signal from a generator and 10K resistor. The AC voltage you measure on the output pin will be proportional (1mV/Ohm for 1mA current) to the output impedance providing that impedance is much smaller than 10K. Unfortunately, you need a sensitive AC millivoltmeter or at least an oscilloscope capable of 12 mV/div gain to measure this low level AC signal. Or you can use another amplifier  say, x100 gain and AC coupled, connected to the output of your 741 and than even your multimeter may be able to measure the amplified voltage reasonably accurately.
Cheers
Alex

Is this link mentioning that the output resistance is 75 ohms for the 741 is relevant? I'm keeping my frequency constant.
ftp://ftp.ni.com/pub/devzone/tut/labb6.pdf

1) Please show the complete schematics, including power supply and other components (i.e. capacitors on the supply rails etc).
I attached a picture with the voltage source wired in and the equipment listed. (Will be uptospeed with Eagle soon.)
I don't think my gear is sensitive enough to measure 3) by itself. My scope goes down to 10mV and MM to 1mV. If I add a 100x amp, won't that affect the output impedance calculation of the 741 stage?
Thanks,
Carl

I don't think my gear is sensitive enough to measure 3) by itself. My scope goes down to 10mV and MM to 1mV. If I add a 100x amp, won't that affect the output impedance calculation of the 741 stage?
Your 100X amp's input impedance will be 1000s of times higher than the 741's output impedance so won't effect it at all, the error in the X100 will be very much higher.

Is this link mentioning that the output resistance is 75 ohms for the 741 is relevant? I'm keeping my frequency constant.
ftp://ftp.ni.com/pub/devzone/tut/labb6.pdf
"Sort of relevant"
As audioguru was already trying to explain:
75 ohms output impedance is with zero feedback. After you apply the feedback loop the output impedance is output impedance divided by feedback factor.
741 has open loop gain of something like 200 000 and you are using 10x gain circuit leaving you with 20 000x feedback factor at close to DC > your circuits output impedance is 75/20000 = abouts 0.004 ohms. (pretty difficult to measure)

Is this link mentioning that the output resistance is 75 ohms for the 741 is relevant? I'm keeping my frequency constant.
ftp://ftp.ni.com/pub/devzone/tut/labb6.pdf (http://ftp://ftp.ni.com/pub/devzone/tut/labb6.pdf)
The value specified on the data sheet is the typical open loop output impedance. Once the LM741 is connected in an amplifier circuit, its output impedance drops dramatically, due to negative feedback and becomes dependant on the gain and frequency, as well as the openloop output imedance.
https://www.colorado.edu/physics/phys3330/phys3330_sp12/phys3330_sp12/Lab_Manual_files/Exp_4_Spring12.pdf (https://www.colorado.edu/physics/phys3330/phys3330_sp12/phys3330_sp12/Lab_Manual_files/Exp_4_Spring12.pdf)

Thanks everyone. I'll do the Colorado lab tonight!

Is this link mentioning that the output resistance is 75 ohms for the 741 is relevant? I'm keeping my frequency constant.
Since the open loop gain of the old 741 opamp is about 8000 at 100Hz and the gain is set to 10, then the output impedance is about 1 ohm.
The output impedance is about 75 ohms at DC with no negative feedback and at a high frequency where there is little open loop gain, because then there is no negative feedback to reduce it.
My calculation for the output impedance at 100Hz might be wrong. I have never calculated or measured the output impedance of an opamp, but I know that negative feedback reduces the output impedance of an amplifier.