# Gain of the amplifier

If we have open loop gain of 2dB or 3dB what happens if we use this in closed loop (except the gain reduction)..i mean how does gain affect on the output of feedback amplifier.

• The question is not clear for me. Why would you put feedback on an amplifier with so little open loop gain? What would you achieve? – Claudio Avi Chami Apr 27 '18 at 9:21
• I remember this was actually one of our interview questions awhile back... most new college graduates in analog field have memorized the equations for ideal op-amp in inverting closed-loop gain configuration, but this is just a special case. At sufficiently high frequency, the open-loop gain A decreases significantly enough that the difference between IN+ and IN- becomes non-negligible. If you write out the equations there will be terms like (1/A) which become negligible and terms like (A-1)/A which become unity as A increases. Actual equation depends on the feedback network. – MarkU Apr 27 '18 at 10:19
• @MarkU I did not understand the term (A-1)/A , how did you get that term? – Balakrishna Apr 27 '18 at 10:32
• The actual effect depends on the feedback network. I wrote the equations for the typical inverting feedback (which you'll recall results in Vo/Vi=-Rf/Ri). Then I assumed finite open-loop gain A, so Vg>gnd, and Vg=(A)(Vo). Write the current equation for Rf and Ri (which have the same current) and substitute Vg=AVo. Solved the equation and got something which becomes Vo/Vi= -Rf/Ri for large enough A: there is a 1/A that drops out to 0 and an (A-1)/A that approaches unity. For a different feedback network circuit, the equation will be different. – MarkU Apr 27 '18 at 20:06

If we have open loop gain of 2dB or 3dB what happens if we use this in closed loop

Rearranging... Y = G(X - Y) therefore Y(1 + G) = G.X

So $\dfrac{Y}{X} = \dfrac{G}{1+G}$

If G is 2 dB (1.26 in real numbers), the net gain is 0.558.

If G was large, the net gain would be close to unity.

• I have said already except the gain reduction how will this low gain(0.558) affect the output response of the block included above – Balakrishna Apr 27 '18 at 9:57
• What do you mean by "output response"? My diagram is clear and theoretical - it is fixed in gain across all frequencies. If you want to include such things as frequency response, slew rate limiting, noise and errors, then you need to do some research and ask a sensible question. – Andy aka Apr 27 '18 at 10:02
• will the lower gain makes output to oscillate? – Balakrishna Apr 27 '18 at 10:06