Feedbacks Guide

The thing i had the hardest time understanding in school was feedbacks. Positive,negative,frequency dependant,frequency determining, negative feedback for current for voltage etc. They are so many that you have a hard time keeping up. I will try to cover most feedbacks and give you a few tips to keep in mind, kinda as a rule of thumb. 1.Negative feedbacks - they're used in amplifiers mostly to increase fidelity, set bandwidth boundaries and temperaturely stabilise the active element. In the given schematic i see 6 negative feedbacks and 2 positive (one of which is known as a bootstrap). I'll get to the positive later. The first two feedback you must notice is the 2x 330ohm resistors at the emitter of the transistor. They limit the overall gain of the transistor and provide more stability. Whenever you see an emitter resistor keep in mind it acts as an emitter degeneration negative feedback.The third and fourth negative feedbacks are the 22k and 8.2k voltage dividers. They provide both by voltage and by AC negative feedback that further reduces the gain and provides even more stability. AC feedback means that you neutralise some of the input signal by applying some of the inverted output signal (sum of opposite vectors is 0). This reduces the overall gain. The fifth negative feedback is the 100pF capacitor. It is a bandwidth setting negative feedback. The capacitor being a frequency dependant component it provides a strong negative feedback above a certain frequency thus shaping the upper frequency response of the amplifier. These feedbacks are applied to restrict the bandwidth to somewhat decrease harmonic distortions. The last sixth negative feedback is explicitly AC negative feedback. The 48k and 100uF RC circuit translates some of the inverted output to further neutralise the input finally reducing the overall gain to 5.2 times (4.8dB). 2. Positive feedback - typically used in oscillators to determine the frequency and jump start the oscillations. In amplifiers unintenden positive feedback are highly unwanted except a few specific ones. A bootstrap feedback is the 100uF capacitor between the 2 emitters. It is usually used when you need to increase the input impedance of a given stage and it actually provides more fidelity to the signal. It doesn't cause the amplifier to self oscillate because an emitter follower has a Ku<1. The second positive feedback is not exactly a positive feedback but it provides a gain boost when needed and is used in conjuction with an emitter degeneration resistor to provide the higher gain that we inadvertently decreased a lot to ensure fidelity. It comprises the 100ohm resistor and the 220uF capacitor.

published 10 years ago