Inverting OpAmp with resistor on non-inverting input - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: February 10, 2020, 07:27:23 pm
content: Hi,with reference to the INVERTING schematic image below (from page 22 of attached datasheet), there is a resistor on the non-inverting pin to ground.1) Why is that?2) it is also set the ratio of the two gain-setting resistors. Why?Thank you

author: Benta
author_rank: Super Contributor
timestamp: February 10, 2020, 07:38:54 pm
content: It's there to make the two inputs see the same source resistance. This cancels out input DC bias current influence on output offset voltage. Very good practice and one you should note for the future.

author: David Hess
author_rank: Super Contributor
timestamp: February 10, 2020, 08:38:42 pm
content: Benta identified its function; it removes the offset caused by input bias current into unequal source resistance. The resistor may be bypassed with a capacitor to reduce its noise contribution. The resistor value is the *parallel* or Thevenin equivalent resistance of the feedback network at the inverting input.The resistor is not required for operational amplifiers which have vary low input bias current (JFET, CMOS) and for amplifiers which include input bias current compensation (OP-07, LT1001) with one exception; some amplifiers like the LT1028 have input bias current compensation which produces correlated noise currents between the inputs so benefit from matched source impedance to produce lower noise.Sometimes an AC network is placed in series with the non-inverting input in a non-inverting amplifier circuit to cancel out variation in input capacitance which would otherwise contribute to harmonic distortion.

author: graybeard
author_rank: Frequent Contributor
timestamp: February 10, 2020, 08:44:33 pm
content: Quote from: Benta on February 10, 2020, 07:38:54 pmThis is a true statement for bipolar transistor input op-amps since the input bias current is primarily due to base current and id likely to match between the two inputs. However for FET input op-amps the input current is usually due to leakage in the ESD protection diodes which are connected to the positive and negative rails. The leakage currents are typically very low level (in the pA to nA) and usually do not match. In addition that input resistor is a noise source.Thus if the input stage of the op-amp is bipolar, then the non-inverting input resistor is almost (but not always) a good idea. For FET input op-amps it is usually best omitted.

author: Zero999
author_rank: Super Contributor
timestamp: February 10, 2020, 08:52:56 pm
content: Yes, the resistor does introduce extra noise, not only thermal noise, but bipolar op-amps produce considerable current noise, fluctuations in the bias currents, which form voltage noise, when passed through a resistor. The noise can be minimised at AC, by bypassing the resistor via a suitable capacitor.

author: ricko_uk
author_rank: Super Contributor
timestamp: February 10, 2020, 09:34:25 pm
content: Thank you allIs the resistor still needed is the input signal contains only AC (i.e. no DC offset)?

author: Zero999
author_rank: Super Contributor
timestamp: February 10, 2020, 09:38:24 pm
content: Quote from: ricko_uk on February 10, 2020, 09:34:25 pmIt depends on what you're doing.In most applications, I'd say no, since the change in the bias voltages, due to the bias currents should be small enough not to matter. It will only be a problem if you have very high resistor values and have an op-amp with high bias currents.What op-amp are you using? Please post a schematic.

author: David Hess
author_rank: Super Contributor
timestamp: February 10, 2020, 09:51:10 pm
content: Quote from: ricko_uk on February 10, 2020, 09:34:25 pmNo, then the amplifier can be AC coupled to remove any offset.

author: sam[PS]
author_rank: Contributor
timestamp: February 28, 2020, 03:11:36 am
content: Dave made a fundamental friday video on that, check for OPAMP input bias or something similar, it's all explained there.Just my 2cts...