Problem with opamp gain stability

[kiranswaps]

opamp gain stability

dear all
i have designed a analog amplifier using op07 . the input voltage is millivolts from..0-100mV and the gain of the circuit is 8.74 . when i apply the input the gain of the circuit increases slowly and takes upto 5 - 10 min to reach the full value. it doesnt shoot up directly to that value.. can anybody please explain me wht might be the problem?
m attaching the daigram along with this...

 

[sachinagg77]

Re: opamp gain stability

As per my understanding, you want to know why the rise time is too long for your circuit.

There can be two reasons for that:

1. The load capacitor for your circuit is too big
2. The slew rate of your op-amp is too small

Please check your load capacitor and design the circuit to get sufficient slew rate.

I hope this would be of some help.
With Regards

 

[kwkam]

opamp gain stability

It should be due to thermal effect of the transistor. When BJT heat up, beta will increase. And gain will increase.

 

[kiranswaps]

opamp gain stability

My gain isn't increasing more than the maximum value. And secondly there isn't any transistor in the circuit.

 

[invent]

Re: opamp gain stability

The transistor refered to may be the transistor in the opamp ic.

Added after 16 minutes:

btw, is your signal ac or dc, sine wave or square wave?

 

[laglead]

Re: opamp gain stability

Do you mean gain or output signal "shoot up" to that value?

 

[kiranswaps]

opamp gain stability

"invent" my signal is pure DC.
"laglead" actually i measure output which takes time to reach it's actual desired value.

 

[invent]

opamp gain stability

I have asked around but no one really knows. Most suggest that the resistors value maybe too big, or input too small.
Tell us how you did it when you eventually get it to work.

 

[timo67]

opamp gain stability

Normally you shoud have a gain of 8.8 an not 8.74
The behave of your amplifier is anormal
try to change your op amp by a 741 for example
and dont take in acount the output offset voltage when you measure you gain!
dont choose a high frequency (1KHz for example)
Put your scope in AC coupling
good luck!

 

[pixel]

Re: opamp gain stability

What if you put small capacitance in feedback loop in paralel with R?

 

[selina_wch]

opamp gain stability

I agree with sachinagg. The loading of your AMP is heavy and the current of your AMP is a bit small. Maybe the amp need to be redesigned.

 

[maharshi_qis]

Re: opamp gain stability

2 things that will decide the time for op-amp response are
............slew rate
............ capacitance.
If ur slew rate is too low then ur op-amp takes long time to reach full value.
Even if capacitance is too high which is due to Load transistors size and load,opamp will take long time.
So to incease slew rate increase current not by increasing transistor sizes but by increasing overdrives.then capacitance decreases and curr increases.so slew rate increases.

 

[satyasiva]

opamp gain stability

u have to consider the SR of OPamp.

 

[layes2]

opamp gain stability

check pm and gm for stability

 

[Mad2Max]

Re: opamp gain stability

I think that the suggestions coming from sachinagg77 are really fine!
Max