A quick question on differential op amps - Page 1
Forum section: electronics / beginners

author: Etesla
author_rank: Regular Contributor
timestamp: February 09, 2019, 05:39:14 am
content: I have a question about differential amplifiers. Say I have an op amp in the standard differential op amp configuration (like this:https://www.electronics-tutorials.ws/opamp/opamp_5.html), where all the resistors are say, 10K (gain of -1). I power the op amp with +/- 15 volts. How far outside of that range of +/- 15V am I allowed to go on the inputs of the circuit (V1 and V2 in the webpage example)? Can I measure the difference between 100 and 101 volts, or will this hurt the amplifier? I'm not asking about specialty devices here, just a 'standard' op amp like the 741 or something like that. If this is not allowed, what parameters in the data sheet should I be looking at to determine how far out of the range I can go? Thanks in advance!

author: awallin
author_rank: Frequent Contributor
timestamp: February 09, 2019, 07:19:12 am
content: unless it's a special op-amp you can't go much outside the supply rails V-/V+ on the +IN and -IN pins.One approach is an attenuating voltage divider in front of the inputs - if you need to measure 100V.some purpose-made voltage/current-sensing amplifier might have an 'isolated' input stage and be capable of handling common-mode voltage beyond the DC-rails on the input pins... but not a standard op-amp AFAIK.

author: not1xor1
author_rank: Frequent Contributor
timestamp: February 09, 2019, 08:31:03 am
content: Quote from: Etesla on February 09, 2019, 05:39:14 amIf you have a rail-to-rail input opamp you can't exceed +/-30V as the resistors would divide the input voltage by 2.If you need higher voltages search "high common mode voltage differential amplifier". Those usually have a 5th resistor to reduce the voltage at the opamp inputs.

author: rstofer
author_rank: Super Contributor
timestamp: February 09, 2019, 11:01:06 pm
content: Pick an op amp and read the datasheet. We talked about this last week and the op amp chosen for discussion could handle 30+ volts.https://www.eevblog.com/forum/beginners/will-an-op-amp-be-damaged-if-input-voltage-is-higher-than-supply/The uA741 can not handle much in the way of overvoltage. The way I read it, the maximum input voltage is +-15V with a maximum Vcc of +-18V.The maximum differential voltage is also +-15Vhttp://www.ti.com/lit/ds/symlink/ua741.pdfPage 5There are specialty op amps but at very high voltages, they are going to be scarce.

author: David Hess
author_rank: Super Contributor
timestamp: February 09, 2019, 11:04:10 pm
content: Quote from: Etesla on February 09, 2019, 05:39:14 amThat really should be called an instrumentation or difference amplifier because differential amplifier has an existing and very different meaning which includes differential outputs.QuoteThere are two requirements:1. The input common mode range of the operational amplifier may not be exceeded.2. The output must not saturate.The divider ratios set the gain but the R2/(R2+R4) divider sets the common mode input voltage so at lower gains and especially gains below 1, the input common mode range increases above the supply voltage. A 741 running on +/-15 volts could have an input common mode range of +/-10 volts but operating as a 4 resistor instrumentation amplifier with a gain of 1/10, has an input common mode range of +/-100 volts. At a gain of 10, the input common mode range is more like 10 volts.

author: Zero999
author_rank: Super Contributor
timestamp: February 10, 2019, 12:07:21 am
content: What are you using the differential amplifier for?

author: alex.martinez
author_rank: Contributor
timestamp: February 10, 2019, 07:39:38 pm
content: Take into account that the input of the opamp is a differential pair. They have an associated maximum input depending on the model, and you can find it under the common mode voltage range input. It usually goes from values of 0 to Vcc - x for the opamp to remain stable.I would guess that if you want to obtain the subtraction of the signal you mention, a good approach is to use a voltage divider resistor config, to scale the voltage down by a known factor.