The overall effect of the global negative feedback on frequency response, output impedance, gain reduction, and distortion reduction will be the same with either form of feedback. B. increases the input impedance and bandwidth. When the open loop gain is large, the negative input of the op-amp is a virtual ground and so the input impedance is just equal to R. This is very different from the non-inverting case where the input impedance is proportional to A for large A. Effect of Feedback on Impedance In practice , the system is bound be connected to an external circuit. . This is a cascode amplifier. Solid state on the other hand has impedances that range from 5-250k. The feedback in which the feedback energy i.e., either voltage or current is out of phase with the input and thus opposes it, is called as negative feedback. Sometimes. Answer: Option B I have heard that there is a negative feedback here but I don't see where is the feedback so I can apply it to calculate output impedance. Linearity improvements : using negative feedback, which lowers input AND output impedance but reduces voltage gain by using R ratios; Bandwidth (fixed GBW) Signal inversion (collector) or not (emitter) If you want to maximize any of the above (1~4) then there must be compromises in some others. For example an ideal voltage amplifier would have a very high input impedance and a Formulas for the input and output impedance for an inverting amplifier are derived in H&H Section 4.26. Rout = rds1 (1 + loop gain) ≈ rds1*gm2*rds2. Voltage feedback in series with the input (e.g. In negative feedback, the amplifier introduces a phase shift of 180 o into the circuit while the feedback network is so designed that it produces no phase shift or zero phase shift. It is quite interesting and intuitive: Consider this op-amp in negative feedback. As in the approach to input impedance, the effect of negative feedback on output impedance can be obtained by analysis of the equivalent circuit. Most tube amplifiers have a very input impedance. 4. 3. Thus, negative feedback has not only eliminated the distortion but also removed the offset (caused by the transistors’ base-to-emitter voltage drop) between control and output. I would like to calculate output impedance of this circuit using negative feedback theory. Decreasing Output Impedance. Now how does this happen? . the grid-to-ground resistor of this example). Hope someone could help me out. Similarly, feedback from the output current tries to maintain that current, increasing the output impedance. Thank you. Za = (rpRl)/(rp+Rl+µKRl) Here the amplifier gain is given by: A = - µRl/(rp + Rl) Thus the shunt impedance is reduced by a factor (1-AK). D. does not affect impedance or bandwidth. 2. Since its in unity gain configuration, output will be ideally equal to input. to the cathode of the input tube) increases the input impedance (but does not affect resistors not included in the feedback, e.g. Controlling Input & Output Impedance with NFB The way that negative feedback is derived from the output of the amplifier and applied to the input can be used to modify the amplifier’s input and output impedances so that impedance matching is maximised. C. decreases the output impedance and bandwidth. In general, an amplifier should have high input impedance and low output impedance. Negative feedback _____ A. increases the input and output impedances. Things become more subtle here because the input voltage v in must be held constant while we see how v out varies with i out.The easiest way to do this is with the partial derivative Consider the figure shown. The working of the system depends on the input and output impedance. Improving Impedance . I understand the effects of an output to input impedance miss-match, but what I don’t understand is why there’s such wide range in (especially input) impedances.
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