[SOLVED] Regarding op-amp configuration and circuit fucnctioning

[ashugtiwari]

Regarding op-amp configs

Hi All,
While designing op-amp based circuit, the ratio of feedback resistance decides gain, so RF=10K, R1=1K and RF=100K, R1=10K wil give tha same gain and input output resistance then is there any differennce of using any one pair over other and what is its implecation on circuit functioning. How to decide these values.

The cicuit in figure is voltage follower, but what is the significance of 100K resistance.
Please reply. Thanks in Advance.

Ashutosh

 

[Kral]

Re: Regarding op-amp configs

ashugtiwari,
Lower resistance values provide a circuit that is less susceptible to capacitively coupled noise. Also, high values of resistance result in larger errors due to input bias current. But, if you connect the non inverting input to common through a resistor that is equal to the resistance that the inverting input "sees", then the bias current error is eliminated.
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The resistor, along with the input capacitor, forms a high pass filter, which attenuates input frequencies below a value determined by the capacitor and resistor values, The frequency at which the attenuation is 3db is fc = 1/(2*pi*R*C) Hz. The attenuation increases at a rate equal to 6db/octave, or 20db/decade.
Regards,
Kral

 

[LvW]

Re: Regarding op-amp configs

It is correct that the 100k resistor acts as a high pass (together with the coupling C), however the PURPOSE and the TASK of this resistor is simply to establish and fix a bias point since the input transistor needs a bias current (that means a galvanic connection to ground). Otherwise the opamp didn´t work properly.

 

[Kral]

Re: Regarding op-amp configs

You are correct!

 

[ashugtiwari]

Re: Regarding op-amp configs

Thanks for reply. Actually, I am new to op-amp design.
Keeping aside the fitering aspect of the circuit, if the 100K is replaced with 10K what difference will it make to the bas current and all other parameteres eg. input & output impedence, offsets, performannce etc.
I am designing variable gain amplifier as in figure, do i need to keep Rin and if I have to what value should decide.

 

[Kral]

Re: Regarding op-amp configs

ashugtiwari,
The resistor has no effect on bias current, but it does affect the output error due to bias current. If the resistances "seen" by the Inv and NI inputs are equal, then there will be no error due to the resistance values. Let Rinv and Rni be the resistance from the Inv and NI inputs respectively. Let Ib equal the bias current. Let Rf be the resistance between the output and the NI input (determined by the pot setting.) Then the output error voltage due to bias current is given by:
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Verror = IbRf[1-(RNI/RInv)]
From this equation, it is apparent that the error is zero if RNI=RInv.
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If the impedance of the coupling capacitor is negligible, then the input impedance of the circuit is equal to the value of the resistor connected to the NI input.
.
The resistor has no effect on the output impedance.
The resistor has no effect on the errors due to input offset voltage or offset current.
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At low frequencies, except for the error due to bias current mentioned above, the resistor will have no effect on performance. At high freqencies, lower resistance values will minimize the effects of coupling due to stray capacitances.
Regards,
Kral