ua741
in [Basics]
|
From: George Herold on 17 Oct 2009 23:40 cc wrote: > On Oct 16, 7:57 pm, "Bob Eld" <nsmontas...(a)yahoo.com> wrote: > > "cc" <pc...(a)comcast.net> wrote in message > > > > news:37514530-3118-4123-bc52-ff93af7461f1(a)i12g2000prg.googlegroups.com.... > > > > > I'm aware of uA741 slew rate issues and limited GBW, but I can't even > > > get above gain of 2x with Vcc of +/- 12v and input sine wave of 5mV > > > peak at only 100Hz (yup, one hundred Hz). What? > > > > Are you applying any feedback? It sounds like you are trying to operate it > > open loop. > > > > At 100 Hz the open loop gain should be about 100 times and, 5mV should give > > about 500mV out, open loop. > > > > However, the DC input offset voltage and offset current could easily drive > > the open loop amp output into one of its rails causing virtually no AC out. > > > > Connect the amp as a gain of 10 with a 1k input resistor and a 10k feedback > > resistor to the neg input, ground the pos input and be sure there is a DC > > path to ground through your source and try again. > > > Thank you all. > It's set up as simple inverter, with (+) pin to ground via same R as > the input R and only 100Hz. > > Still working on it, but I replaced the F/B circuit with only 10ohms > (R) between signal and (-) pin, and 1000k between (-) pin and output, > for closed loop gain 100x and it worked. So it seems that I need low > R at (-) input terminal. You see, I had been using R=1k and the > feedback R=100k. > > > Now, my fcn generator has about 700 o/p impedance. Does that matter? > > > I had similar problem with AD8620 and CA3140...still working on them. cc, Sorry did I understand that correctly, you've got 10 ohms and 1 Meg ohm (1000k) That's a gain of 10^5. Yeah! it aint gonna work at 100 Hz. George H.
From: cc on 18 Oct 2009 13:45 On Oct 16, 6:02 pm, cc <pc...(a)comcast.net> wrote: > I'm aware of uA741 slew rate issues and limited GBW, but I can't even > get above gain of 2x with Vcc of +/- 12v and input sine wave of 5mV > peak at only 100Hz (yup, one hundred Hz). What? OK, guys.....my bad............. I thought I had checked this first when I began using my (cheapo) fcn. generator, but I was getting about 2.5v dc bias on which I was riding 2-5mv signal, so of course i couldn't get much gain with the +/- 12v supply(or 9 depending on my mood that day). All I got was flatline or noise when trying for even 10x gain. Sorry for the fuss, but I'm learning at home.....I'm lucky I found a deal on an old Tek 7633 and I can even see the waveforms at all. This is how we learn.......sometimes with help from others. Lesson, check all the supposedly obvious solutions first. CC
From: Jamie on 18 Oct 2009 15:26 cc wrote: > On Oct 16, 6:02 pm, cc <pc...(a)comcast.net> wrote: > >>I'm aware of uA741 slew rate issues and limited GBW, but I can't even >>get above gain of 2x with Vcc of +/- 12v and input sine wave of 5mV >>peak at only 100Hz (yup, one hundred Hz). What? > > > OK, guys.....my bad............. > > I thought I had checked this first when I began using my (cheapo) fcn. > generator, but I was getting about 2.5v dc bias on which I was riding > 2-5mv signal, so of course i couldn't get much gain with the +/- 12v > supply(or 9 depending on my mood that day). All I got was flatline > or noise when trying for even 10x gain. > > Sorry for the fuss, but I'm learning at home.....I'm lucky I found a > deal on an old Tek 7633 and I can even see the waveforms at all. This > is how we learn.......sometimes with help from others. > > Lesson, check all the supposedly obvious solutions first. > > CC Have you seen that magical smoke yet? One thought, just don't let it escape.
From: default on 19 Oct 2009 09:21
On Sat, 17 Oct 2009 12:12:16 -0400, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_(a)charter.net> wrote: >default wrote: > >> On Fri, 16 Oct 2009 18:02:16 -0700 (PDT), cc <pccmd(a)comcast.net> >> wrote: >> >> >>>I'm aware of uA741 slew rate issues and limited GBW, but I can't even >>>get above gain of 2x with Vcc of +/- 12v and input sine wave of 5mV >>>peak at only 100Hz (yup, one hundred Hz). What? >> >> >> Open loop gain of a 741 would give you a rail to rail square wave on >> the output. You have to be limiting the gain external to the op amp >> or your measuring method or instrument is wrong. >A 741 won't do Rail TO Rail. I think the fact it does square wave >in an open loop is what you're trying to convey in this situation. > > To be honest, I haven't really found a spec anywhere that signifies > what Rail to Rail is. Every chip i've spec'ed as being rail to > rail, never reached it's actual rail voltages in the data sheet. > So some where, some one has decided what constitutes a rail level > output and what does not. Compared to what RR types do that are >currently out there, the 741 does not fall into that category. > > Ditto that. "rail to rail," if specified, doesn't meet full specifications at the rail extremes. figure of speech. -- |