Can an opamp be used to divide two signals?

[BkraM]

Hi all,

I'm trying to to find the ratio of two sinus shaped signals in the same phase.
From the two AC signals I want a DC voltage representing the amplitude ratio of the AC signals.
First thought is to use an opamp with a variable gain (one of the two signals) but i'm getting a bit stuck on how to actually do this.
Can anyone hint me towards a solution?

Thanks,

 

[crutschow]

A standard op amp won't work for that. The easiest solution is to use an analog multiplier/divider.

 

[Ian Rogers]

A standard op amp won't work for that. The easiest solution is to use an analog multiplier/divider.

I totally respect your postings Carl... But Multipliers and dividers are made up from op-amps surely....I need educating...

 

[crutschow]

.......... But Multipliers and dividers are made up from op-amps surely....I need educating...

A multiplier or divider needs a non-linear element and op amps can't provide that by themselves since they are basically linear. It can be done in a limited way by adding a diode or other non-linear element to the op amp feedback loop, but those generally are only accurate over a narrow input voltage range. For accurate multiplication over a wide range down to DC a Gilbert cell is typically used in most multiplier ICs.

 

[Ian Rogers]

Educated...... Thanks !!

I've been looking at examples and I now see the problem....

 

[BkraM]

I was hoping to build it with a simple opamp solution, but apparently not as straightforward as i hoped.
Time for a rethink of my concept..

Thanks for the help.

 

[moffy]

There is a trick that if you put a multiplier into the feedback loop of an op amp, with the output of the op amp being one multiplicands and the other being one of the voltages. If the other voltage is fed into the inverting input (the output of the multiplier and input voltage have to go through resistors, to the inverting input) you end up with a divider, with Vo = -V1/V2*R1/R2. Usefull if you have access to a multiplier but not a divider. Don't know if it helps but it is neat.

 

[ccurtis]

You can exploit the fact that the time to charge a capacitor is inversely proportional to a constant current used to charge it.

One implementation could be a 555 in astable operation mode, except that the timing capacitor is charged by a linear voltage controlled current source instead of through the traditional resistor tied to a fixed voltage. The voltage controlling the current source is the denominator. The other voltage is the Control Voltage terminal of the 555. The output pulse from the 555 will have an ON time proportional to the Control Voltage (numerator) but inversely proportional to the voltage controlling the current source (denominator). The small OFF time will be fixed by the time it takes the 555 to discharge the capacitor. The average value of the resulting pulse train is proportional to the numerator/denominator times the amplitude of the pulse train, such as would be obtained by integrating the pulse train.

Of course, this idea only works for steady-state input voltage, so that your input sinusoids will have to be peak detected, or something.

Other inverse relationships such a freq=1/T might be useful also.