IC op amp input resistance or impedance measurement

[ParkerMike]

Troubleshooting boards that are inoperative

To test each stages IC inputs and output incircuit

I tried using my Digital meter and set it to ohm to measure the IC op amps input

The IC op amps input pins should be in the meg ohms from the the datasheets say for a 741 op amp

out of circuit input pin test:
To measure the IC op amps input you place the red probe on input#1 and the black probe on input#2 of the IC 741, it should measure in meg ohms

you're measuring both inputs at the same time putting the meter probes across both inputs to measure the input resistance which should be in the meg ohms right?

In circuit test:
When measuring the IC op amps inputs in circuit will not be in the meg ohms?

If the 741 IC op amp inputs are good , in circuit test the inputs will measure in the meg ohm or not?

if the 741 IC op amp inputs are damaged, in circuit test the inputs will measure from 0 to 1k?

out of circuit output pin test:
I tried measuring the output pins resistance of a 741 from what the datasheets say

I put my red probe on the output pin of a 741, but where do i put the black probe ? how do you measure the output resistance of a IC op amp?

 

[esp1]

hi Mike,
Its almost impossible to get a 'meaningful' reading of an OPA input impedance when the OPA is connected to the rest of the circuit.

Depending on your ohm meter, when measuring meg-ohms, it may be applying 15Volts to the circuits nodes, which may forward bias any semiconductors and so you will get a lower resistance value than you expect.

If its possible to apply the DC operating power to the PCB, [ ie: non power shorting fault], measuring the voltage of the nodes around the OPA and on the PCB is IMO the easiest way to locate a faulty component.

E

 

[Audioguru]

The input resistance of an opamp is the resistance from one input to the circuit ground. It is not the resistance between the inputs. It is almost impossible to measure because the test upsets the input bias voltage. You can measure the input bias current then use Ohm's Law to calculate the resistance.

You do not use an ohm meter to measure an opamp's output resistance. Instead you feed the input an AC or DC signal and measure the output voltage of the opamp. Then add a series resistor from its output to ground and vary the resistor value until the output voltage is half, then the output resistance of the opamp is the value of the resistor. Look on an oscilloscope to make sure the output is not limiting the current which would happen if your input level is too high.

I have worked with thousands of opamp circuits and have NEVER seen a bad input or output. I have seen a few bad opamps, one got very hot. A bad opamp is found by measuring input and output levels, not resistances. If the input level is loaded down by an opamp then the input of the opamp is probably bad. If the input level is correct but the output level is not then the opamp or its feedback parts is/are bad.

 

[ParkerMike]

The input resistance of an opamp is the resistance from one input to the circuit ground

I tried this in circuit test and you just measure 100K to 800K , the datasheets for a 741 that each input pin should be high impedance in the megs

I turn the circuit off and measure the input of the 741 op amp and reference it to ground, i don't get high impedance or in the meg ohms

Same with the output pin of a 741, it should be very low impedance of what the datasheets

If the input level is correct but the output level is not then the opamp or its feedback parts is/are bad.

I found an op amp that would only be bad when the output was loaded

The op amps gain checked good, but when loaded it was inoperative

What is happening to this op amp?

It is almost impossible to measure because the test upsets the input bias voltage.

So the DVM meter upsets the input bias current and voltage?

What sets the input bias current and voltage is a resistor to ground on the non-inverting pin

Instead you feed the input an AC or DC signal and measure the output voltage of the opamp. Then add a series resistor from its output to ground and vary the resistor value until the output voltage is half, then the output resistance of the opamp is the value of the resistor.

Will the AC signal test and DC signal test be at the same voltage the op amps output be at the same resistor value when the output voltage is in half?

Either way if i inject an AC voltage or a DC voltage, the op amps output resistance value will be at the same for either an AC voltage or DC voltage? will the AC and DC voltages be the same also? or different

I'm thinking that the output resistance value will be difference when the op amps output is in half , compared to either it's input is an AC voltage or Dc voltage right?

Because an op amps amplifies a AC voltage differently compared to amplifying a DC voltage right?

so the output impedance changes depending if it's an AC input voltage or an DC input voltage

The output impedance is dynamic depending if the input is an AC input voltage or an DC input voltage

 

[Audioguru]

Why do you think you can measure the input resistance of an IC that is full of transistors when it is not powered??
The IC must be powered and the input voltage must allow the opamp to work normally.

The datasheet for an LM741C shows that its typical input resistance is 2M ohms but its minimum is 300k ohms.
The typical output resistance is 75 ohms.

Why do you want to measure input and output resistance?

 

[ParkerMike]

Why do you think you can measure the input resistance of an IC that is full of transistors when it is not powered??

the datasheets never said that it has to be powered, it says the input and output impedance resistance on the datasheets

Why does it have to be powered to measure the input resistance? even the datasheets don't say that

Why do you want to measure input and output resistance?

To check if the inputs and outputs are not blown or damaged

Because I have noticed that some Op-amps when unloaded work and the gain is good but when loaded the op amps is inoperative

 

[Audioguru]

The input of an opamp is the base of a transistor that draws a small input bias current when it is powered. The current causes the input resistance. Without current then there is no resistance. but your ohm-meter might cause too much input current and show a resistance that is much lower than when the opamp is powered and biased properly. The current from the ohm-meter might damage an input.

The datasheet for most opamps says that the maximum allowed input voltage is the supply voltage. But when unpowered the supply voltage is zero so you must not connect an ohm-meter to the input because an ohm-meter applies voltage.

The input resistance is from one input to ground. If the supply has plus and minus then the ground is between them but you do not have a power supply when it is not powered so there is no ground.

If an input or output is blown then the opamp simply does not work properly and measuring the input or output resistance does nothing but waste time.

 

[ParkerMike]

The current causes the input resistance.

I don't understand how current cause input resistance

but your ohm-meter might cause too much input current and show a resistance that is much lower than when the opamp is powered and biased properly.

Yes true it does

But when unpowered the supply voltage is zero so you must not connect an ohm-meter to the input because an ohm-meter applies voltage.

I thought a load draws what ever voltage or current that it needs

The Op amps input internal transistor is the load and it will draw the current and voltage it wants

If a DVM meter is outputting more current and voltage, the load internally inside the Op amps input pin should just draw want it needs without damaging right?

 

[Audioguru]

I don't understand how current cause input resistance

If there is no input bias current then the input resistance is infinity. Look at the input resistance of a TL081 opamp that has Jfets on its inputs that have no input bias current. Its input resistance is typically
10 trillion ohms due to stray leakage current.

I thought a load draws what ever voltage or current that it needs

A conducting diode (even an LED) or a base-emitter junction is not a load. Instead it is almost a dead short.

The Op amps input internal transistor is the load and it will draw the current and voltage it wants

Only when the opamp is powered and biased properly.

If a DVM meter is outputting more current and voltage, the load internally inside the Op amps input pin should just draw want it needs without damaging right?

You cannot measure the input resistance with an ohm-meter of a powered opamp and a non-powered opamp. Instead you must measure its input current when it is powered then calculate its input resistance.

 

[ParkerMike]

ok thanks for the help and your time on this