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Old 29th June 2007, 11:11 AM   #11
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no. max is 18 volts. In the finaly thing both the opamp and the output stage would run on +/- 16V.
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Old 29th June 2007, 11:28 AM   #12
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Quote:
Originally posted by happyboy


Hi,
Sorry for the misconception, but the LT1001 is just there for me to do simulations. I intend to use AD826 in the final thing. Would it be a problem with the 826?
Yes, it would be a problem. AD826 has very high input currents (as do many fast opamps) - upto 10 uA ! In your circuit it would introduce far too much offset. Easy way of getting rid of it without changing opamp or circuit would be to put a coupling capacitor (say 10uF) on the input before R3 and reducing R3 to 18K to balance DC resistance as seen by the inputs.

However I would be careful in using AD826 or similar opamp in a circuit like this. Fast opamps do require much attention to details, including power supply arrangement, layout, feedback resistor values etc. In your circuit 100K and 22K in the FB chain is far too high resistance for AD826.

I would use for example OPA134 to start with in a circuit like this and with little experience of fast opamps.

Cheers

Alex
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Old 29th June 2007, 11:55 AM   #13
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Quote:
Originally posted by x-pro


Yes, it would be a problem. AD826 has very high input currents (as do many fast opamps) - upto 10 uA ! In your circuit it would introduce far too much offset. Easy way of getting rid of it without changing opamp or circuit would be to put a coupling capacitor (say 10uF) on the input before R3 and reducing R3 to 18K to balance DC resistance as seen by the inputs.

However I would be careful in using AD826 or similar opamp in a circuit like this. Fast opamps do require much attention to details, including power supply arrangement, layout, feedback resistor values etc. In your circuit 100K and 22K in the FB chain is far too high resistance for AD826.

I would use for example OPA134 to start with in a circuit like this and with little experience of fast opamps.

Cheers

Alex
Hmm in that case i think i will just use a NE5532 Opamp. Am i right to say that it will not have much DC offset cause the input current is 200nA?

Thanks!
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Old 29th June 2007, 12:41 PM   #14
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Quote:
Originally posted by happyboy


Hmm in that case i think i will just use a NE5532 Opamp. Am i right to say that it will not have much DC offset cause the input current is 200nA?

Thanks!
If you will use a capacitor on the input and change R3 to 18K the offset wouldn't be a problem. To calculate the additional input offset due to input currents you need to multiply input ("bias") current of the opamp to the DC resistance connected to it. In your case 18K*200nA=3.6mV. To reduce the output offset you should make sure that the offsets on both inputs are the same, in this case the output offset (excluding the initial offset of the opamp itself) should be 0. Any difference would be amplified by the gain of the circuit - in your circuit it is about 6. Even if your non-inverting input is grounded the output offset due to the input current would be around 3.6*6=21.6mV which is acceptable for a power amplifier output.

However I would advise you to use the input capacitor as you may have some DC offset coming from your source - this would be amplified accordingly and may become dangerously high for the speaker.

Cheers

Alex
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Old 29th June 2007, 01:45 PM   #15
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Quote:
Originally posted by x-pro

However I would be careful in using AD826 or similar opamp in a circuit like this. Fast opamps do require much attention to details, including power supply arrangement, layout, feedback resistor values etc. In your circuit 100K and 22K in the FB chain is far too high resistance for AD826.
Pardon my lack of knowledge but is there some way to know if the resistors in the feedback chain is suitable?

Thanks!
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Old 29th June 2007, 01:51 PM   #16
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It comes down to the input current requirements of the op-amp and how much noise you can tolerate, traded with the dissipation in the resistors.
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Old 29th June 2007, 02:10 PM   #17
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Quote:
Originally posted by happyboy
Pardon my lack of knowledge but is there some way to know if the resistors in the feedback chain is suitable?
As a quick rule of thumb - the parasitic properties of the components preferably should not interfere with the workings of a circuit. These parasitics mostly consist from small capacitances (acros a resistor, on the input of an opamp etc.) Larger the value of the resistors used in the circuit - lower the frequency where such unwanted influence would occur. Generally it is advisable to keep parasitic RC parameters out of the working range for a circuit. For example: AD826 has got the unity gain frequency around 30-50 MHz and 1.5 pF input capacitance. Cut-off frequency for a feedback resistor and the input capacitance would be sensible to keep much (2-5 times) higher than the unity gain frequency. For 1.5 pF you'll need a value of approx. 1K resistor to get a cut-off frequency of 100 MHz. For a feedback resistor value 22K, cut-off frequency would be in the area of 5MHz and that could create a potential stability problem. There are ways of dealing with such difficulties however it is usually not an easy task

Cheers

Alex
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Old 29th June 2007, 02:10 PM   #18
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Quote:
Originally posted by richie00boy
It comes down to the input current requirements of the op-amp and how much noise you can tolerate, traded with the dissipation in the resistors.
Hmm, is there any example that i could take reference to? or is there something like a formula? Thanks!
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Old 29th June 2007, 02:23 PM   #19
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Quote:
Originally posted by richie00boy
It comes down to the input current requirements of the op-amp and how much noise you can tolerate, traded with the dissipation in the resistors.
These are generally secondary effects - I was concerned first about stability of a circuit with a fast opamp. You may add to your list maximum available current from the opamp output as low values of feedback resistors also create a load for the opamp.

Cheers

Alex
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Old 29th June 2007, 02:37 PM   #20
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Quote:
Originally posted by x-pro


As a quick rule of thumb - the parasitic properties of the components preferably should not interfere with the workings of a circuit. These parasitics mostly consist from small capacitances (acros a resistor, on the input of an opamp etc.) Larger the value of the resistors used in the circuit - lower the frequency where such unwanted influence would occur. Generally it is advisable to keep parasitic RC parameters out of the working range for a circuit. For example: AD826 has got the unity gain frequency around 30-50 MHz and 1.5 pF input capacitance. Cut-off frequency for a feedback resistor and the input capacitance would be sensible to keep much (2-5 times) higher than the unity gain frequency. For 1.5 pF you'll need a value of approx. 1K resistor to get a cut-off frequency of 100 MHz. For a feedback resistor value 22K, cut-off frequency would be in the area of 5MHz and that could create a potential stability problem. There are ways of dealing with such difficulties however it is usually not an easy task

Cheers

Alex
What i understand from the example is that i must use a feedback resistor that when matched with the input capacitance of the Opamp must be out of the unity gain bandwidth of the opamp? If what i understand is right, then how do i calculate what value of a resistor to use with different input capacitance of different opamp?
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