measuring rise time and fall time

[elli]

hi
does anybody khow how we can measure the rise-time of the output voltage of the open-loop op_Amp
with different input voltages,(we know in this condition that this op_amp has a high gain that the output is saturated)?

 

[FvM]

Rise and fall time of an operational amplifier is a large signal quantity and has to be measured with a respectively large output voltage, but you'll usually avoid output saturation. It has to be measured with a defined gain setting and output voltage.

You'll wire the OP e.g. for G = +1 (or any reasonable value) and apply a square wave input signal that achieves large but unsaturated output signal. Then determine the slew rate from the output waveform. Datasheets are often specifying measurement setups or are showing respective waveforms.

 

[elli]

THANKS,
but i meant to measure the rise-time with just the open-loop given op-amp!
so i design circuits for this op-amp to find the open-loop dc gain and open-loop frequency and the slew rate.
now i have this parameters, but still i don't know how to do this?

 

[FvM]

An amplifier isn't operated in open loop, slew rate in open loop is no useful specification.

But if you want to operate the amplifier as a comparator, what's the problem to measure a response time and rise-/fall time or calculate a slew rate as a function of input overdrive?

 

[KlausST]

Hi,

if you look into datasheets where the rise time and fall time is tested, then you see that it is done with two thresholds.
Often those are at 10% and 90% of the expected levels.

So i´d connect two comparators at the output of the DUT and measure the time between the two comparator events.
tis is the rise/fall time

Subtract the two comparator threshold levels and get dU
Measure the time difference to get dt, then calculte the rise/fall rate dU/dt in V/us.


Klaus

 

[elli]

An amplifier isn't operated in open loop, slew rate in open loop is no useful specification.

But if you want to operate the amplifier as a comparator, what's the problem to measure a response time and rise-/fall time or calculate a slew rate as a function of input overdrive?

yeah, i know this, and i think our instructor want to teach us that if we use the op-amp in open-loop then we have a comparator, but this is not a good idea, and this will be proved via simulating the circuit with a pulse and see that the output in lower input pulses is not like a pulse and we have a noticible rise time.(this is becouse of the slew rate limitations or frequency bandwidth)
so he wanted us to give this open loop op-ampwith +-5m, +-10v input voltage,(while the positive terminal is grounded) and measure the rise time and my problem is exactly here... dont know how to calculate this, in paper???my numbers dont match with the simulation results!

 

[Audioguru]

Probably your simulator was not taught that anybody would use an opamp open loop then it gives the wrong results when you blast an input signal and it saturates its output. An opamp is designed with very high voltage gain so that it works accurately (very low distortion) when it has negative feedback. Since the many parts in the opamp cause phase shift then the negative feedback becomes positive feedback at high frequencies that causes oscillation. Therefore most opamps use a "frequency compensation capacitor" inside that cuts high frequencies so that at the frequency where negative feedback becomes positive feedback then the voltage gain is less than 1 to prevent oscillation. This capacitor slows down its slew rate (output rise and fall times).

 

[KlausST]

Hi,

+-10v input voltage,

Some OPAMPs you will kill with that input voltage ... within milliseconds or less.
Be sure it is within the OPAMP´s specification.

Klaus

 

[FvM]

Beyond useful amplifier specifications, it's true that general purpose OPs with miller compensation have an almost constant slew rate set by the input stage saturated output current and the miller capacitance.

I don't understand what's the calculation problem. Presumed you get a trapezoidal output waveform between lower und upper saturation point, slew rate (dV/dt) and rise-/fall-time (time between 10 and 90% level) can be easily derived, isn't it?

 

[elli]

Hi,



Some OPAMPs you will kill with that input voltage ... within milliseconds or less.
Be sure it is within the OPAMP´s specification.

Klaus

It is, thanks

 

[D.A.(Tony)Stewart]

THANKS,
but i meant to measure the rise-time with just the open-loop given op-amp!
so i design circuits for this op-amp to find the open-loop dc gain and open-loop frequency and the slew rate.
now i have this parameters, but still i don't know how to do this?

Slew Rate has no significance in open loop response of the Op Amp since it is designed as an integrator with the internal capacitance controlling the bandwidth around 10 Hz in the linear region. Thus what you see on the output is the linear gain of > 1e6 of your input signal.

Slew rate is always performed with a gain from 1 to 10 or as specified by the OEM MFG with 50 Ohm input, 500 Ohm output CL and Rf/Rin =Av values. The standard times are either 10~90% for analog relevance or 25~75% for digital relevance as guaranteed logic levels are often in this range and method must be included in results to be considered valid.

the reason Slew Rate is only done in closed loop tests, as it only a function of the output stage and load, but can be affected by loop stability and so always done with standard values as chosen by MFG for repeatability and validity. When adding capacitance to the std load CL, you can then determine your expected change in rise time due to current limits of the driver.

 

[elli]

Beyond useful amplifier specifications, it's true that general purpose OPs with miller compensation have an almost constant slew rate set by the input stage saturated output current and the miller capacitance.

I don't understand what's the calculation problem. Presumed you get a trapezoidal output waveform between lower und upper saturation point, slew rate (dV/dt) and rise-/fall-time (time between 10 and 90% level) can be easily derived, isn't it?

i get different rise times when the input voltage is +-10v and +-5mv in simulation.
how you calculate this without "simulation"?:bang:

 

[FvM]

i get different rise times when the input voltage is +-10v and +-5mv in simulation.

May be. We should look at the OP topology.

5 mV is probably not driving the input amplifier into saturation. 10 V may achieve unexpected effects in some input stages.

 

[crutschow]

There is no reason to measure the open-loop slew rate of an op amp as the slew rate does not vary significantly with close-loop gain.

 

[KlausST]

Hi,

how you calculate this without "simulation"?

usually with the help of the datasheet. But we don´t know what OPAMP type you use.


Klaus

 

[Audioguru]

The datasheet of a lousy old 741 opamp says it has a typical slew rate of 0.5V per micro-second.
A TL07x audio opamp has a typical slew rate of 13V per micro-second.

The datasheet tells you the slew rate, then you look at the maximum voltage swing which is also on the datasheet and use simple arithmetic to calculate the rise time and fall time.