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.design10 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


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

Started by George Herold in sci.electronics.design5 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


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.design10 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.design7 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.design7 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.design10 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 ...


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.design5 months 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.design3 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...


Opamp current noise

Started by George Herold in sci.electronics.design5 years ago 20 replies

Opamp current noise. I got a call the other day from someone seeing =91too much=92 warm up drift in the Johnson noise voltage from our Noise...

Opamp current noise. I got a call the other day from someone seeing =91too much=92 warm up drift in the Johnson noise voltage from our Noise fundamentals instrument. http://www.teachspin.com/instruments/noise/index.shtml I had them send it back to me. (A few hundred dollar learning experience) After much head banging, it turns out that the voltage noise of the first stage opamp increa...


Does the noise density function increase if an OpAmp goes into slew rate limit?

Started by RobertMacy in sci.electronics.design3 years ago 61 replies

Trying to sort out circuit simulation artifact from something that may be real. Does the noise come up [because of the inability of the...

Trying to sort out circuit simulation artifact from something that may be real. Does the noise come up [because of the inability of the OpAmp to reduce] when the OpAmp is driven too fast at too large a signal level? Circuit is a simple LT1028 in an 'inverting' configuration with power of +/-12Vdc. GNDed non-inverting input. Rinput = 1k. Rfdbk = 1k. Cfdbk = 20pF. Load is a simple ...


NFB Rolloff, capacitorless

Started by Anonymous in sci.electronics.design3 years ago 6 replies

Suppose I have a very simple little class B amp, just an opamp driving a small output pair for a watt or 2, and nfb resistors. Zero output pair...

Suppose I have a very simple little class B amp, just an opamp driving a small output pair for a watt or 2, and nfb resistors. Zero output pair bias, lots of crossover distortion. Now... I'm thinking I can skip a nfb rolloff capacitor by using the opamp's own internal cap to give the rolloff, eg for a 1MHz gbw opamp, if we run it at gain 20, its gain is down in the gutter above af a


input bias current of a current-mode opamp

Started by John Larkin in sci.electronics.design3 years ago 6 replies

We think we've observed bias current drift on some THS3062 current-mode opamps, on the non-inverting input, roughly 10 uA in 6 months,...

We think we've observed bias current drift on some THS3062 current-mode opamps, on the non-inverting input, roughly 10 uA in 6 months, systematic in one direction across multiple amps. A regular opamp has, say, an NPN differential pair, identical transistors run at relatively low emitter and base currents. A current-mode opamp has PNP and NPN transistor bases on the ni input, with fa...