OpAmp Input Resistance

Started by panfilero in sci.electronics.design7 years ago 7 replies

When does the really high input resistance of the OpAmp go away? ..... I remember the 3 OpAmp Instrumentation amplifier.... and I believe...

When does the really high input resistance of the OpAmp go away? ..... I remember the 3 OpAmp Instrumentation amplifier.... and I believe I recall the final stage OpAmp being a differential OpAmp, and that it had a low input impedance, so there's the other 2 OpAmps as buffers feeding into the differential OpAmp.... is that right? so, I was wondering... for example in a regular inverting ...


Opamp w/ Vsupply >36V

Started by George Herold in sci.electronics.design4 years ago 57 replies

Opamp Vsupply > 36V Hi all, I=92ve got a circuit with that uses an opa2134. I find myself pus= hing the 36V supply limit and I=92m a little...

Opamp Vsupply > 36V Hi all, I=92ve got a circuit with that uses an opa2134. I find myself pus= hing the 36V supply limit and I=92m a little worried. So I went looking f= or a similar opamp but with a bit more headroom. =20 So specs might read, Dual opamp unity gain stable=20 4(+) Mhz BW (8MHz or more would be nicer) 10 V/us slew (again more is better) 20mA current (positive, again


slow down an opamp (compensation cap.)

Started by George Herold in sci.electronics.design6 months ago 15 replies

Well first off, I'm afraid this is a lazy post, 'cause I should be able to find the answer in my opamp books at work tomorrow. Anyway for...

Well first off, I'm afraid this is a lazy post, 'cause I should be able to find the answer in my opamp books at work tomorrow. Anyway for my 60 Hz pick up problem, I've slowed down the opamp driving the fet by hanging a bigger compensating cap on it. There's a pic of the schematic here. Standard opamp driving a fet V-> I. (feedback R is 10 k not 1 k shown.) https://www.dropbox.com/sh


How to choose the right opamp

Started by Anonymous in sci.electronics.design9 years ago 9 replies

Hi, if i consider a signal with a defined bandwith, how can i choose the right opamp to amplify it? For example, if consider a signal with a...

Hi, if i consider a signal with a defined bandwith, how can i choose the right opamp to amplify it? For example, if consider a signal with a maximum frequency of 1kHz i think i should use an opamp with first pole equal or greater to 1kHz: is it right?If so, what is the phase effect on the real signal? All frequencies below 1kHz have different phase induced by opamp non idelities, the output...


"A new opamp's here, a new opamp's here!"

Started by George Herold in sci.electronics.design4 years ago 20 replies

(Title said in the voice of Steve Martin) Seriously this looks like a nice HV opamp. http://www.linear.com/product/LTC6090* (well if you...

(Title said in the voice of Steve Martin) Seriously this looks like a nice HV opamp. http://www.linear.com/product/LTC6090* (well if you download the spec sheet the open loop phase does a dipsy doodle at a few hundred kHz.) I can replace some expensive Apex parts. (as soon as someone has stock.) George H. *as seen on EDN


OPAMP

Started by ECS.MSc.SOC in sci.electronics.design6 years ago 5 replies

hi all what is the meaning of dynamic performance of an opamp in following sentence? "The dynamic performance of the OpAmp must be suitable for...

hi all what is the meaning of dynamic performance of an opamp in following sentence? "The dynamic performance of the OpAmp must be suitable for full scale sinusoidal outputs up to 100kHz" regards


Two opamp differential amp.

Started by George Herold in sci.electronics.design5 years ago 8 replies

I reinvented the two opamp differential amp yesterday. And then found it in AofE* (2nd Ed. figure 7.36). Though I drew it differently. ...

I reinvented the two opamp differential amp yesterday. And then found it in AofE* (2nd Ed. figure 7.36). Though I drew it differently. +-R2--+--R1--+ | | |\U1 | +--R2--+ gnd +-|-\ | | |\U2 | | > -+-R1-+-|-\ | V(-)-----|+/ | > -+--out |/ V(+)---|+/ |/ It=92s a natural addition to the single opamp difference amp


Input of unpowered OpAmp

Started by panfilero in sci.electronics.design4 years ago 7 replies

I had a voltage divider which was suppose to drop down a voltage to 3V conn= ected to the input of an OpAmp. And it worked fine. But I noticed,...

I had a voltage divider which was suppose to drop down a voltage to 3V conn= ected to the input of an OpAmp. And it worked fine. But I noticed, when I= powered off the OpAmp, that 3V dropped down to 1.7V, I wasn't expecting th= is. What do you think would cause this? My guess is some input protection = circuitry, maybe a couple diode drops or something?


power opamp?

Started by John Larkin in sci.electronics.design9 years ago 46 replies

What's a good medium-power opamp? I need to source/sink at least 60 mA, with say +-7.5 volt supplies, and swing +-6 maybe. No...

What's a good medium-power opamp? I need to source/sink at least 60 mA, with say +-7.5 volt supplies, and swing +-6 maybe. No particular requirements otherwise. I suppose I could boost a cheap SOT-23 opamp with a couple of cheap SOT-23 emitter followers.... v+ | c \...


opamp circuit offsets

Started by John Larkin in sci.electronics.design3 months ago 102 replies

I have a fairly bizarre 4-opamp circuit with differential output, and I need to calculate the worst-case DC offset. The math is a...

I have a fairly bizarre 4-opamp circuit with differential output, and I need to calculate the worst-case DC offset. The math is a nuisance, and I'll have to repeat it if I change anything. So, Spice. https://dl.dropboxusercontent.com/u/53724080/Circuits/Amps/E2_Offsets.jpg The issue is now how to program the four opamp DC offset sources. One way is to use four asynchronous sine waves, a...


LT Spice opamp a liittle weird

Started by John Larkin in sci.electronics.design3 months ago 21 replies

I was wondering about the power dissipation of an opamp, driving a sine wave into a mainly inductive load. See below. The first few...

I was wondering about the power dissipation of an opamp, driving a sine wave into a mainly inductive load. See below. The first few cycles current limit or something. I'm using UniversalOpamp2 with current limit 2 amps and rail=0. This opamp isn't documented in the HELP, or much anywhere else. Peak current is under 700 mA, so why is it clipping? Higher supply voltage helps. ...


don't need no opamp

Started by John Larkin in sci.electronics.design6 months ago 5 replies

If all you want to do is, say, pick resistors to get the right gain and such, you don't need an opamp model in Spice. An 'e' thing works just...

If all you want to do is, say, pick resistors to get the right gain and such, you don't need an opamp model in Spice. An 'e' thing works just like an opamp but doesn't need power supplies. This one does what I want to do, using only resistor values already on my board. It's a nuisance to find a combination that works, but Spice sure beats doing this with a calculator. I find myself Spicing ...


opamp differential amplifier high frequency/loading effect

Started by kris...@hotmail.com in sci.electronics.design10 years ago 5 replies

Hi, I have a simple difference amplifier based on an opamp. I have few issues related to its deisgn and analysis. I have to design it for a...

Hi, I have a simple difference amplifier based on an opamp. I have few issues related to its deisgn and analysis. I have to design it for a differential gain of 10. The gain is decided by the ratio of the resistors used. R2/R1=R4/R3=10. Normally, what is the guidelines to select range of resistor values? Is this information provided in the specific opamp datasheet? Is there some literature ...


Precision current source

Started by George Herold in sci.electronics.design2 years ago 11 replies

Hi all, I've using a voltage reference-> Resistor-> opamp feedback to make a current source. Like figure 10...

Hi all, I've using a voltage reference-> Resistor-> opamp feedback to make a current source. Like figure 10 here. http://www.ti.com/lit/ds/symlink/ref102.pdf The reference is the REF102, but I'm using an opa134 for the opamp. (Currents down to 10 nA so I wanted a fet opamp.) Now someone wants to use it where noise might be more of an issue, so I figured the first thing was to measur


capacitive loading an opamp

Started by John Larkin in sci.electronics.design6 years ago 85 replies

A few opamps are specifically spec's to drive any capacitive load, like LM8261 for instance. And sometimes you'll see an opamp that...

A few opamps are specifically spec's to drive any capacitive load, like LM8261 for instance. And sometimes you'll see an opamp that is loaded with a big cap to ground, in an appnote for example. If an opamp has some internal compensation node, midway through, that's the dominant pole, adding a cap on the output makes a second pole in the loop, potentially unstable. A huge cap might be s...


Opamp (output) over voltage protection

Started by George Herold in sci.electronics.design4 weeks ago 10 replies

Hi all, So I've got an opamp driving a remote FET. The current incarnation of the circuit has students able to change (screw up) the...

Hi all, So I've got an opamp driving a remote FET. The current incarnation of the circuit has students able to change (screw up) the connections. There's a ~60V power supply available so I'd like to make it so that the opamp survives having it's output connected to 60 V. Since it's driving a Fet (slowly) there is not much current and I'm sticking in two series depletion Fets (LND150's) ...


Apex HV opamp issues.

Started by George Herold in sci.electronics.design2 years ago 50 replies

Hi guys (and dolls), So if you read in JL's TI opamp thread, I am having noise issues in our laser diode. I turns out (I think) that the extra...

Hi guys (and dolls), So if you read in JL's TI opamp thread, I am having noise issues in our laser diode. I turns out (I think) that the extra noise is coming from my Piezo drive. The original circuit used a PA141 from Apex. (I can't find a schematic on-line, but I have a hard copy.) Several years ago this HV opamp went away and the recommended replacement from Apex was the PA314 http...


static precaution, fet opamp

Started by Anonymous in sci.electronics.design2 years ago 7 replies

I have a minimal opamp circuit that uses a random very low current opamp, such as TL062 (but could be anything low current). This thing is going...

I have a minimal opamp circuit that uses a random very low current opamp, such as TL062 (but could be anything low current). This thing is going to be used in uncontrolled conditions in the 3rd world, and needs to be relatively reliable. The outside world goes to -in via 68k + 47nF. Question is what level of extra static protection do I need. What I've added so far is a diode fro


Class B distortion

Started by Anonymous in sci.electronics.design3 years ago 48 replies

Something I thought I knew, but am left puzzling... Take a simple class B output pair driven directly by an opamp. Result is na= sty crossover...

Something I thought I knew, but am left puzzling... Take a simple class B output pair driven directly by an opamp. Result is na= sty crossover distortion. Now, the nfb is taken from the circuit's output t= erminal, not from the opamp's output, so surely an ideal opamp should corre= ct for the 0.6v Vbe drops. Question is why doesn't it? The other question is what strategies exist to tackle...


unstable opamp ringing oscillating and such

Started by panfilero in sci.electronics.design7 years ago 18 replies

can anyone in here explain why a OpAmp could oscillate or become unstable if there are 2 poles before it reaches 0dB on a plot of gain vs...

can anyone in here explain why a OpAmp could oscillate or become unstable if there are 2 poles before it reaches 0dB on a plot of gain vs frequency? Here's what I've gathered thus far... there's a cap internal to the op- amp, that cap is there for some reason and it puts the dominant pole in the op-amp's transfer function. That pole also puts a 90deg phase shift on the output of the opamp...