Hi, if i consider a signal with a defined bandwith, how can i choose the right opamp to amplify it? For example, if consider a signal with a maximum frequency of 1kHz i think i should use an opamp with first pole equal or greater to 1kHz: is it right?If so, what is the phase effect on the real signal? All frequencies below 1kHz have different phase induced by opamp non idelities, the output signal is equal to Re(|A|e^jfi), so: what shoud have is an output time delay or the output signal is distorted (because different frequencies have different phase shift)? How this problem is important in real pratical cases? thanks in advance
How to choose the right opamp
Nov 23, 2008
2 Replies
Putting feedback around and amp moves the open-loop pole out to a higher frequency. So in the closed loop case, the position of the pole or break point depends on the gain.
If the natural pole is 10Hz and the open loop gain 100,000 times, then closing the loop to a gain of 100 times would move the 10Hz pole to
100,000/100 X 10Hz = 10 kHz.The phase shift of the new pole position is 45 Deg at that frequency, in this case, 10 kHz. In other words both the frequency response and phase response gets "flattened" out and extended by applying feedback.
Even when the closed loop bandwidth appears sufficient for a particular application, the amplifier's slew rate may limit the amplifier's ability to drive high frequencies at a given voltage. Slew rate is independent of bandwidth but must be considered for a particular application.
Usually an amplifiers bandwidth is expressed as Gain Bandwidth Product. This number is any point along the open loop frequency response curve. In the above example, the the gain was 100,000 times at 10 Hz. so the GBP =
1,000,000. It also implies, assuming a single pole response that unitity open loop gain occurs at 1 MHz.
.
Let's see now, first week of December ... TERM PAPER TIME FOR FALL SEMESTER.
Jim
-- "It is the mark of an educated mind to be able to entertain a thought without accepting it." --Aristotle
Join the Discussion
Have something to add? Share your thoughts — no account required.
Didn't find your answer?
Ask the community — no account required