Calculating opamp 741 input impedance

[chris_vtech]

working on this new assignment. OP_AMP 741

Input pin got 20Kohm to a non-inverting pin 3 and pin 2 is a inverting pin feed back 10 Kohm tie to output of pin 6. Need advise of the input impedance?
10, 20, 30, 31.

 

[pourfardm]

Re: OP-AMP 741

For calculating opamp input impedance you are not allowed to use voltmeter if your feed back increase input impedence by D=1+Ab factor because voltmeter has input impedence near your op-amp and will not work properly

so apply a another stage of op-amp behind your current stage op-amp with appropriate feedback resisitor( i suggest 0.1of it) which sample your input signal and increase its magniyude then you can apply voltmeter now for this stage of your circuit and measure the magnituded input signal and calculate the input resistor of second stage now (do not forget to multiply the input impedence of stage 1 to calculate the input impedence of stage 2

 

[Borber]

Re: OP-AMP 741

Your circuit looks like follower with 10kohm feedback resistor and 20k tied to input.
What kind of advise do you need.
You may calculate input impedance using data about input resistance and input capacitance from datasheet. It has real part about 2Gohm up to 100Hz and some negative imaginary part.

 

[Kral]

Re: OP-AMP 741

chris_vtech,
Good rule of thumb for low frequency input impedance:
Let Rol = the open loop input resistance specified by manufacturer (Known).
Rcl = closed loop input resistance (To be calculated)
Aol = open loop gain of Op Amp (Known from spec sheet)
Acl = closed loop gain of Op Amp (Known =1)
Rcl = Rol(Aol/Acl)
~
You can add your 20K input resistor to this if you like, but it is an insignificant contributor.
Regards,
Kral

 

[Gats]

Re: OP-AMP 741

is thr any diagram to be included so we can undestand better ? thanks

 

[Davood Amerion]

OP-AMP 741

Rin= 20K ohm + (op-amp input resistance)

 

[Borber]

Re: OP-AMP 741

Look Kral's post:
Rcl = Rol*Aol