Author Topic: For The Experts: Is Opamp I.C.M.R. and Bandwidth Related?  (Read 4135 times)


Re: For The Experts: Is Opamp I.C.M.R. and Bandwidth Related?
« Reply #20 on: May 14, 2012, 11:16:35 PM »
After a bit more reading I found that the TL0xx series opamps DO suffer phase reversal if the common-mode input gets too close to the negative rail. So if you are using a 9V supply and Vref=4.5V then your signal is swinging down to 3.5V and up to 5.5V. That 3.5V downward swing might be enough to cause problems in UGFB configuration. According to the datasheet the max output voltage for a 9V supply is about 6V P-P (not very good!) and in UGFB configuration the common-mode input voltage is (ideally) equal to the output voltage (meaning since the . If the output clips negative and you get phase reversal from too low CM input, this will typically causes the signal to slam into the positive rail and either latch up or recover (the TL0xx stuff is supposed to be latch-up free though, which they are IME). Sounds like you might be experiencing this, I don't think I have ever though..

I believe that is exactly what I was doing. I remember I had the Op Amp biased directly from the transistor, most likely a 10k off the collector. The first one was probably getting full swing, and the second one I blew must've still been getting enough voltage to blow. Could've been a Bazz Fuss for all I remember. I've used 358's and 2904's like that with no problems, but I'm guessing they are less problematic since the outputs can swing all the way to ground. Or are they prone to this issue as well? (LM358 (2904) has definitely become my favorite little op amp).
Give a man Fuzz, and he'll jam for a day... teach a man how to make a Fuzz and he'll never jam again!

Cliff Schecht

Re: For The Experts: Is Opamp I.C.M.R. and Bandwidth Related?
« Reply #21 on: May 15, 2012, 02:58:08 AM »
I always look for the schematic diagrams that many manufacturers include in their datasheets. They usually show at least all of the important transistors that make up the gain stages and protection circuitry (biasing is represented as a current source for simplicity when drawing the schematics). This will tell you quite a bit about how the op amp will perform when put under certain "stressful" conditions, be it operating as a unity gain buffer or driving large currents. Some op amps are slightly overcompensated to ensure a good phase margin (one of the criterion for amplifier stability) as a UGB but this has an effect on the transient performance. More compensation means a slower transient response.

Doing a bit of research through a book like Baker's text on op amps at first may seem daunting, but there is a lot of more general information on different op amp topologies and their pros/cons. In his later books he delves into three stage op amps and crazy casocoding techniques that we don't really encounter with general purpose op amps (this is more for ultra low voltage CMOS design in sub micron processes :icon_redface:) but the basic information about general purpose two stage op amps is still there. He also delves into specifics when dealing with JFET and BJT based inputs (or entire amplifiers based on these devices if there is a performance benefit) as well as using PNP/P-Channel vs NPN/N-Channel based inputs (depends on if the common mode needs to extend to the positive rail or negative rail) or even amplifiers that use both P channel and N channel inputs in parallel. I have done the latter in a high speed op amp design and was really excited that I got near rail to rail performance when the amplifier was configured as a UGB, even on the bench!
« Last Edit: May 15, 2012, 03:04:11 AM by Cliff Schecht »

Paul Marossy

Re: For The Experts: Is Opamp I.C.M.R. and Bandwidth Related?
« Reply #22 on: May 15, 2012, 09:49:42 AM »
Edit: Ha! Just reread the thread and saw Paul said what I was saying already (about the phase reversal stuff). Also I was wrong on the supply voltage, Paul is using 12V which means he should have enough headroom to not be clipping. It sounds like the DC is off somewhere, maybe a leaky coupling cap? Maybe your keyboard is putting out DC? If you really do have the op amp hooked up correctly (not that it's hard :P) then the circuit should work from the get-go. If it doesn't, something else is wrong. I've used TL0xx's as unity gain buffers a thousand times and rarely had a problem that wasn't either user error or something else in the chain causing issues (faulty cables, bad components, etc..).

Yeah, that is what was puzzling the heck out of me is that it should have worked with no problems right from the start. I mean it's pretty hard to screw up something THAT simple. It could be that there is some DC coming from the keyboard, maybe a small amount which is enough to whack out the opamp. If there is, you certainly can't hear it. I can play the keyboard thru an amp, no problems. Nothing that sounds like DC on the input. I can use headphones (which signal comes from the same source in the keyboard and has to go through the same capacitor from the output opamp as the line out does), no problem.

In any case, I built a new circuit with a gain of 10 last weekend and it works much better. It's been working every time so far. I'm still using the LM353 though. Maybe I have a bad batch of TL072s or something. I'm not even going there with this project!
« Last Edit: May 15, 2012, 09:57:35 AM by Paul Marossy »