ac coupled inverting amplifier topology (drain resistor) choice - Page 1
Forum section: electronics / projects

author: CopperCone
author_rank: Super Contributor
timestamp: January 17, 2018, 01:19:15 am
content: So, I am wondering what kind of topology I would need to use to AC couple a chopper amplifier, a LT1150.I see some op-amps don't require any kind of drain resistor, some circuits put a R3 to ground on the noninverting terminal, and some put a resistive divider connected to V+ and V- on the noninverting terminal.There is:1)2)http://slideplayer.com/5267265/17/images/42/Figure+2.35+In+an+ac-coupled+amplifier+the+dc+resistance+seen+by+the+inverting+terminal+is+R2%3B+hence+R3+is+chosen+equal+to+R2..jpg3)http://farhek.com/a/c/lm/lm2904-n-general-purpose-op-amps-operational-amplifiers-ac-coupled-inverting-amplifier_ac-amplifier_electric-motor-wiring-connections-capacitor-use-start-sizing-simple-run-connection-_850x445.pngHow do you tell which one will work with a LT1150?Some op amps, for instancehttps://www.maximintegrated.com/en/app-notes/index.mvp/id/781, do not require a bias.Datasheet for op-amp I want to use:http://cds.linear.com/docs/en/datasheet/1150fb.pdfThe bias current is in the PA rangeWill testing these on a breadboard be OK? I'm not sure about leakage.

author: jbb
author_rank: Super Contributor
timestamp: January 17, 2018, 02:35:53 am
content: Before everyone gets into the topology question, why do you want to use a chopper amp (expensive, low DC offset) with AC coupling (don’t care about DC offset)?You mention low bias current. How about using a JFET input amp instead?

author: CopperCone
author_rank: Super Contributor
timestamp: January 17, 2018, 02:48:14 am
content: No, I need to eliminate the DC offset from the previous stage before it goes to the chopper amp. It is a band pass LNA passing a AC signal (audio amplifier).The signal will then be passed without AC coupling to the next stageIt's actually a composite amplifier, that is a LTC1150 that disciplines a LT1010 power output stage driving a low impedance.

author: David Hess
author_rank: Super Contributor
timestamp: January 17, 2018, 05:13:02 am
content: The LTC1150 operates like any other operational amplifier; each input requires a DC return path for its input bias current.Beware that if the corrected amplifier saturates, then the DC correction loop will windup causing a long recovery time. This is especially a problem with chopper stabilized amplifiers.

author: CopperCone
author_rank: Super Contributor
timestamp: January 17, 2018, 01:53:37 pm
content: Can I go with just having R3 as a drain resistor, or do I need R3 and R4 biased to the rails? I would typically go with R3 and R4. I'm not sure if that circuit (looks like sandra smith) is over simplified.

author: David Hess
author_rank: Super Contributor
timestamp: January 17, 2018, 01:58:39 pm
content: Quote from: CopperCone on January 17, 2018, 01:53:37 pmThey are performing the same function. R3 is for cancelling the input offset voltage creates by the input bias current and is not required for parts with input bias current cancellation. R3 and R4 are providing a false ground for biasing purposes. So it just depends on your circuit.

author: CopperCone
author_rank: Super Contributor
timestamp: January 17, 2018, 02:01:44 pm
content: Why would someone pick the higher component count choice over the other ? It also routes easier.Is there any other analysis that should be done? What are the benefits of a false ground? It sounds like a single supply thing.Edit: the only examples of that circuit I find to be with single supplies. I guess that answers my own question. I thought it was between VCC and Vee so I was a bit confused, since you are already at the midpoint.

author: David Hess
author_rank: Super Contributor
timestamp: January 17, 2018, 03:31:01 pm
content: Quote from: CopperCone on January 17, 2018, 02:01:44 pmThe two resistor solution works with a single supply voltage. The one resistor solution works with dual, positive and negative, supply voltages.QuoteSome integrated amplifiers do not require mid-supply biasing or handle it internally. They also may not require an external DC bias current return.

author: CopperCone
author_rank: Super Contributor
timestamp: January 17, 2018, 04:02:10 pm
content: Is there any clues to look for about this self biasing?Maxim just makes it sound like rail-to-rail amplifiers are capable of operating without a bias resistor.

author: David Hess
author_rank: Super Contributor
timestamp: January 17, 2018, 04:26:44 pm
content: Quote from: CopperCone on January 17, 2018, 04:02:10 pmThe datasheet will say and the application notes will show it. Usually this is only found in audio parts.QuoteNo, rail-to-rail amplifiers just have a rail-to-rail input common mode range or output range or both. They still require a DC return path for their input bias current.