well im using an op amp in a circuits class and we connected some resistors on a breadboard and were asked to find the gain at different frequencies ( we used a function generator as the power source with a sine wave setting i dont know if that helps) and as i get to frequences of about 100kHz my gain is no longer close to the value i calculated using vin/vout why is this???
please help
2007-10-18 12:39:47 · 3 answers · asked by bob mark 2 in Science & Mathematics ➔ Engineering
well im using an oscilloscope to graph the out put and when it is at low frequencies it shows me a good sine wave but at high frequencies i get triangular waves why is this...
2007-10-19 02:43:05 · update #1
my TA said it had somthing to do with hysteresis . i look this up and found nothing in the book or on line that was useful...
2007-10-19 02:44:41 · update #2
Your calculation is for an ideal op amp, or for use at much lower frequency than the upper limit.
Look at the gain bandwidth of the op amp. As the frequency increases, gain of the op amp will decrease. Configure the op amp for maximum gain and measure the frequency vs gain figures again. Multiply frequency x gain and you should notice it equals the gain bandwidth figure.
2007-10-18 12:52:24 · answer #1 · answered by Warren914 6 · 2⤊ 0⤋
Probably due to all manner of stray capacitive reactance on your breadboard. Especially if it's in the feedback loop. Putting a capacitor in the loop (in parallel with the feedback resistor) gives you a single pole filter with a 3 db point of f=1/2ÏRC. Or it may be that the gain of the op-amp istelf is starting to roll off. Look up the specs on your device and see if there's a published 'gain-bandwidth' curve for it. I know that the '741' type op-amps start drooping around 100 kHz or so, even at unity gain.
Doug
2007-10-18 12:54:06 · answer #2 · answered by doug_donaghue 7 · 2⤊ 0⤋
You've approached and/or exceeded the op-amps gain/bandwidth product.
Basically an op-amp has a large but finine amount of gain at low and zero frequencies. As the signal frequency increases, the available gain (due to factors such as stray capacitance, slew rate and gain/bandwidth product) decreases therefore your op-amp's output diminishes.
Check out National Semiconductor's high speed op-amps and compare them to the one you are using.
http://www.national.com/products/tree/table.do?treeId=5
Also do some reading from the mind of Bob Pease, the man is a genius:
http://www.national.com/rap/
2007-10-18 14:19:33 · answer #3 · answered by Anonymous · 0⤊ 0⤋