Q. Explain the effect of feedback on amplifier bandwidth?
The bandwidth of an amplifier is defined as the range of frequencies for which the gain remains constant.The gain -bandwidth product of an op-amp is always a constant.The gain of an op-amp and its bandwidth are inversely propotional to one another.The bandwidth of an op-amp can be increased by providing feedback signal to its input.The product of closed-loop gain and closed-loop bandwidth is same as the product of open-loop gain and open-loop bandwidth.i.e.
AFBWCI = ABWOL
The op-amp 741 has an open-loop gain of 200,000 and a bandwidth of about 5Hz.Therefore,the product of its open-loop gain and bandwidth is
ABWol = 200,000*5Hz = 1MHz.
The open-loop gain -bandwidth product of a 741 is 1MHz.The left-hand side of eqution (1) is the product of closed-loop gain and closed-loop bandwidth.No matter what the values of R1 and RF ,the product of closed-loop gain and closed-loop bandwidth must equal the open-loop gain-bandwidth product,i.e.for 741 the closed-loop gain-bandwidth product must also be 1 MHz.
The open-loop response is shown in Figure. The open -loop gain has a maximum value of 200,000. When the operating frequency increases to 5 Hz ,the open - loop gain is down to 0.707 of its maximum value . The gain keeps dropping off with increasing frequency .After the upper cut-off frequency, fOL,the gain drops by 20dB/decade.The unity - gain frequency is the frequency where the open-loop gain has decreased to unity. In figure funity equals 1MHz
The curve in figure represents the closed-loop response of an op-amp.It can be seen from this figure that the open-loop gain decreases continuously until it approaches the value of closed-loop gain .Then , the closed-loop gain starts to decrease and at fCL,the closed-loop gain is down to 0.707 of its maximum value.Thereafter,both the curves superimpose and decrease to unity at funity.
If the feedback resisters are changed , the closed-loop gain will change to a new value and so will the closed-loop cut-off frequency.But ,because the gain-bandwidth product is constant,the closed-loop curve superimposes the open-loop curve beyond cut-off frequency.