Op amp biasing resistor

[Miyagi_83]

Hello, everyone.
I've been reading teemuk's book and I came across some info that I'm trying to wrap my head around. On pages 210 and 211, he writes about operational amplifiers, their gain, configurations etc. He states that the resistor tethering the non-inverting input to reference voltage should, ideally, be equal to the parallel combination of the gain-setting resistors.
Now, I've seen a number of diagrams where that Vref resistor is simply a high-value one, probably to set the input impedance high, if I understand that correctly.

So, is it wrong to tie the non-inverting input to Vref with, say, 1M or 470k? What's the tradeoff?
EDIT: Teemuk writes that the value is important "To prevent the bias currents of the input from interfering
with the offset voltage", but what does it mean in layman's terms?
Thanks in advance.
M.

[Loudthud]

The plus and minus inputs of an opamp are not perfect. If they were, you could ground both inputs and the output would go to zero Volts (assuming a bipolar power supply). In a practical opamp, you would have to apply a small offset Voltage to one input to get the output to zero Volts. This offset Voltage will change at different temperatures. In addition, the plus and minus inputs need a certain current to operate. If the resistance at the two inputs is not equal, this input current will cause unequal Voltages to appear at the inputs adding to the offset Voltage needed to bring the output to zero Volts.

Input offset Voltage is not a problem in most audio circuits, it just means the output will drift around by a few milliVolts. There is usually a capacitor in the feedback network that reduces the Voltage gain to One at DC. So the offset Voltage will have a gain of One at the output of the opamp.

If the circuit requires that no capacitor can be used, then the offset Voltage will be amplified by the gain set by the feedback network. In this case, additional offset Voltage at the input can be caused by different resistances in the plus and minus inputs as input current changes over temperature.

An LM1448 type opamp has enough input current that resistances in the 1 MegOhm range can affect operation. A TLO72 type opamp with JFET inputs have such low input current, that you won't see a problem until circuit resistance are very very high.

[Miyagi_83]

Ok, now it's clear. Thank you, Loudthud.