Single Rail Op Amp Input

Thread Starter

D1990

Joined Jan 7, 2012
2
Hi there!

I have been studying the schematic attached with much confusion. At the input stage I am seeing VB applying 4.5v DC offset to the Op Amp through a 1M resistor.

Surely the voltage drop across this resistor would mean the Op amp wouldn't receive 4.5v bias?

I Understand the 1M resistor sends the input to a default state when nothing is connected and I understand that the op amp needs 4.5v DC to allow full swing between 9 and 0v, but the way it is applied in this schem is confusing me.

Can anyone help?
 

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praondevou

Joined Jul 9, 2011
2,935
The input bias current is typically 150nA for this Opamp RC4558, so yes there will be a voltage drop over the 1M resistor. (max is 800nA though)

If you wanted less voltage drop you could use Opamps with FET inputs.
 

crutschow

Joined Mar 14, 2008
23,136
Base upon praondevou's bias current values, the drop across the 1M resistor would vary from a typical 0.15V to a maximum of 0.8V which is probably not enough to significantly affect the op amp operation.
 

Audioguru

Joined Dec 20, 2007
11,251
The schematic uses opamps that were not too bad 30 years ago. There are much better opamps today.
 

Thread Starter

D1990

Joined Jan 7, 2012
2
Oh ok, I think I understand.

So the only current to enter the op amp is the stated bias current, there for the voltage dropped across R2 is V=IR. How does R1 affect this?
 

Audioguru

Joined Dec 20, 2007
11,251
R2 does not affect the DC. R1, the input impedance of the opamp and R2 attenuate the input signal slightly.
 

Adjuster

Joined Dec 26, 2010
2,147
Oh ok, I think I understand.

So the only current to enter the op amp is the stated bias current, there for the voltage dropped across R2 is V=IR. How does R1 affect this?
R1 affects the bias voltage it because it is in the bias current path - it is connected to VB!

R2 is in the signal path, connected only to a capacitor leading to the input socket. Capacitors block DC current.
 
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