Couple EE questions...

Magnus Danielson magda at it.kth.se
Tue Oct 22 01:28:14 CEST 1996


>   Hi guys,

Hi!

>   I'm re-designing some stuff and had a couple simple theory questions...
> 
>   1. When you have a cap in parallel in the feedback loop of an inverting
>   op-amp to limit self-oscillation, it creates a LP filter - yes? What is
>   is the formula for the cut frequency - I assume it's a function of the
>   cap value and Rfb or Rin. What's the roll-off of this filter?

The transfer function of a inverting opamp with feedback capacitor is

	Vout     Rfb      1
	---- = - --- -----------
	Vin      Rin 1 + sRfbCfb

where the steady state cutoff frequency fc is

	           1
	fc = ------------
	     2*PI*Rfb*Cfb

It is a simple 1 pole lowpass filter. The lowfrequency gain of this inverter is

	      Rfb
	A = - ---
	      Rin

This all assumes high input impedance of the op-amp, infinite internal gain
and no op-amp poles and zeros in the passband. The fc is usually selected to be
well below the dominant pole of the op-amp.

>   2. When adding an inverting op-amp to the end of a signal chain simply to
>   re-invert a signal (gain = 1), does it make any difference what resisters
>   you use? Obviously, you wouldn't want to use an 8ohm resistor into a
>   virtual ground, but will 2 10K resistors behave in exactly the same way
>   as 2 100K? Is there *any* trade-off?

It does make a diffrence, not only from the obvious heat problems but also from
the noise level. By makeing wise selection of the resistors you can balance the
termal noise and the current noise. You can teoretically find a optimal source
resistance to each input of an opamp, if you stick close to this value (which
is op-amp type dependent) you will minimize the noise figure. You migth want to
stick in a resistor between the (+) input of the opamp and the ground when
doing an inverter for this reason among others.

So, if you want to care about your noise you should pick them with some care,
but being 10k-ish is probably more rigth for most standard opamps than 8 ohm...

This among other things you can find out from "The Art of Electronics" by Paul 
Horowitz and Winfield Hill (ISBN 0-521-37095-7) which I strongly recommends.
It is the book I use mostly when I do Electronics since it is quite oriented
towards practical design and real components.

Magnus



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