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op amp troubleshooting

J

jim

Jan 1, 1970
0
Can anyone help here? As a newbie,I'm experimenting with the 741 op
amp just to familiarize myself with its operation and it's acting
rather peculiarly. The supply pins read at +/- ( +/- 9V) appropriatly
and I'm running it as an inverting amp with feedback resistor of 10K
and my input resistor of 1K and my signal into this. My + input is
grounded
as is pin8. Both offsets are open. So my gain should be 10. The first
problem is that I'm getting a triangular wave when my input is
anything higher than around 50mV rms. I'm showing gain,not anywhere
near 10. If I feed it 10mV rms I get a nice gain back with a well
formed sine wave,but as I progressively go up ,it changes to a
triangular wave,and the gain dumps.,no clipping noted. This is all at
100khz,the lowest my generator will go. I'm working on those
breadboards from radio shack. Any ideas on whats going on? Thanks.I
just went through several bad 741"s,nothing on the output pin and I
thought I was going crazy until I got one of the older model 741's and
at least I'm getting signal with gain but the waveform has me
flumoxed. Any help is appreciated. jim
 
J

John Larkin

Jan 1, 1970
0
Can anyone help here? As a newbie,I'm experimenting with the 741 op
amp just to familiarize myself with its operation and it's acting
rather peculiarly. The supply pins read at +/- ( +/- 9V) appropriatly
and I'm running it as an inverting amp with feedback resistor of 10K
and my input resistor of 1K and my signal into this. My + input is
grounded
as is pin8. Both offsets are open. So my gain should be 10. The first
problem is that I'm getting a triangular wave when my input is
anything higher than around 50mV rms. I'm showing gain,not anywhere
near 10. If I feed it 10mV rms I get a nice gain back with a well
formed sine wave,but as I progressively go up ,it changes to a
triangular wave,and the gain dumps.,no clipping noted. This is all at
100khz,the lowest my generator will go. I'm working on those
breadboards from radio shack. Any ideas on whats going on? Thanks.I
just went through several bad 741"s,nothing on the output pin and I
thought I was going crazy until I got one of the older model 741's and
at least I'm getting signal with gain but the waveform has me
flumoxed. Any help is appreciated. jim

The 741 is slew rate limiting; its output can't slam fast enough to
keep up with the ideal output waveform at 100 KHz. Check the 'slew
rate' spec. Also keep in mind that its gain-bandwidth product is only
about 1 MHz, so it can't deliver a correct gain of 10 at 100 KHz. You
need faster opamps or a slower signal generator!

John
 
J

jim

Jan 1, 1970
0
The 741 is slew rate limiting; its output can't slam fast enough to
keep up with the ideal output waveform at 100 KHz. Check the 'slew
rate' spec. Also keep in mind that its gain-bandwidth product is only
about 1 MHz, so it can't deliver a correct gain of 10 at 100 KHz. You
need faster opamps or a slower signal generator!

John
Yep,Thanks,I kinda of was thinking along those lines but I guess I was
expecting clipping rather than the triangular wave. I was also worried
about the effect of stray capacitance the breadboards are noted to
have. jm
 
P

Paul Blitz

Jan 1, 1970
0
Yup, it's the gain-bandwidth limit that's upsetting things!

I gave up using 741's ages ago, and now use the TL071 / TL072 / TL074
(single / dual / quad) range. They are just as cheap (probably cheaper),
very well behaved, and perform a LOT better than the 741 does.

Paul
 
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