Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Op Amp input impedance

Status
Not open for further replies.
Sorry guys, Im really struggling now, but...

Would the impedance of the source make a difference? In the right hand drawing (post 17) if it were a high impedance source the resistor would do nothing, but what if it were a low impedance source?

Also, is it correct to assume that a coil would be of low impedance?

Thanks

Tom
 
A 9k ohms input resistor is far too low for a modern high impedance opamp. A tuned coil has a much higher impedance.
The tiny capacitor across the feedback resistors stops the small amount of peaking caused by stray capacitance to ground at the (-) input of the opamp without them.
 
Sorry guys, Im really struggling now, but...
Would the impedance of the source make a difference? In the right hand drawing (post 17) if it were a high impedance source the resistor would do nothing, but what if it were a low impedance source?
Also, is it correct to assume that a coil would be of low impedance?
Thanks
Tom
Tom, forget the right-hand drawing in post#17. It does not work.
Here is my recommendation: At first, concentrate to the biasing problem only. Don't confuse yourself with other circuit properties like noise, input impedance levels, overshoot problems, bandwidth (audio, RF,..)...
 
Thanks Nigel



Is it possible to calculate the roll off point?

Also what happens if the resistor is omitted? Will the op-amp oscillate or create noise? (or smoke!!)

without the resistor it becomes an integrator (if you're talking about leaving a cap in it's place) or an op amp operating open loop with nothing there, in which case the output will likely latch to a rail, and nothing comes out except DC.
 
it might help if we knew more about the coil. so far all we know is that it is a coil, and the OP is using it to pick up a 1khz signal with it, and he's trying to drive a pair of headphones, and very likely he wants to use a single supply voltage.... we don't even know what op amp he wants to use
 
Last edited:
Ok, so just focusing on the input bias resistor, that must go on the (+) input to Vcc/2, this is here to create an even input impedance for both inputs, to reduce the error caused by input bias current. =>The impedance must be equal on both inputs.

Now in the left hand side of post 17 The (+) input sees an impedance of the blank resistor, whilst the (-) input sees an impedance of R1||R2. =>The blank resistor must equal R1||R2 to minimize the error effects of the input bias current.

The problem with the right side of post 17 was that no current, or very little, would flow, causing problems. => Wont work :( => Given up!!!

With the coil I have, I want high input impedance amp as to not load the coil and dampen the resonance. By putting a 10M ohm bias resistor in the values of R1 and R2 have to be exceptionally large to offer equal resistance and would lead to noise and other bad things. Without the bias resistor the amp would be open to having an input bias current error, which would result in a DC offset (not sure), but it would result in a high input impedance without the need to have large, additional, resistors.

The new problem I have is which is more important, high input impedance or no input bias current errors.
 
please describe he coil....
 
Since you do not want to load down your tuned LC source, you want a high gain and you do not want high value noisy negative feedback resistors then use a low noise audio opamp with Jfet inputs. I have used TL071 single, TL072 dual and TL074 quad low noise audio opamps for most audio projects. They have Jfet inputs with an extremely high input impedance and no input bias current.

Connect one end of your input coil to the (+) input of the opamp and connect its other end to half the supply voltage for an input voltage reference and a resistor is not needed. Use any value you want (values not too low and not too high) for the negative feedback resistors.
 
Thank-you Audioguru, I shall do that. :)

Just out of curiosity, what does the bias current and associated error voltages do? Is it just a small DC offset at the output?

PS I just had a look at the TL071 Datasheet, can the offset pins be left floating?
 
Last edited:
What does the bias current and associated error voltages do?
Bias current in the resistor that sets the DC reference voltage for the (+) input of an opamp causes a DC voltage at the input that might be amplified by the opamp circuit. Then the output voltage will be wrong.
The bias currents for each input might not be exactly the same which causes an offset voltage that also might be amplified by the opamp circuit. Then the output voltage also will be wrong.
If both inputs of an opamp have exactly the same DC voltage then any DC input offset voltage also might be amplified by the opamp circuit. Then the output voltage also will be wrong.

PS I just had a look at the TL071 Datasheet, can the offset pins be left floating?
ALL single opamps have offset null pins. Yes if you do not need to null the offset voltage then leave the pins floating.
 
so at 1khz, Xl is 1.13k, and 140nf is required to resonate at 1khz. a 10k to 50k input impedance won't load it down very much
 
Tom_pay,

Ok, so just focusing on the input bias resistor, that must go on the (+) input to Vcc/2, this is here to create an even input impedance for both inputs, to reduce the error caused by input bias current. =>The impedance must be equal on both inputs.

I think you need some theory on how the compensation resistor works. Using the nomenclature in the attachment, setting R2 to the parallel combination of R1 and R3 will reduce the offset to the lowest value. If you want zero offset, then you will have to use other offset techniques. The compensation resistor R2 will not significantly affect the input impedance because the input impedance of the opamp is already very high. The idea is to minimize the offset, not to equalize the impedance of both inputs. As far as I can see, you have only one input.

Ratch
 
Thanks Ratchit,

I thinks it is slowly coming together now, Ill still have to read those pages again a few more times though :)

Thanks Everyone

Tom
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top