Positive feedback

Question 1

How do oscillatory outputs occur from amplifiers?

Answer 1

If the loop gain ßAv is greater than 1 at any frequency
-large open loop gain makes this inevitable

Question 2

What is the condition for positive feedback?

Answer 2

A portion of the output is combined in phase with the input

Question 3

Describe what happens to waveforms at different values of loop gain

Answer 3

when ßAv < 1:
-any disturbances from voltage impulses are damped

ßAv > 1:
-the circuit is unstable and there is a chance of distortion occuring

ßAv =1:
-circuit oscillates without distortion. Amplitude may fluctuate with small changes in circuit parameters

Question 4

How can oscillations be prevented?

Answer 4

The gain can be controlled by introducing non-linear components (i.e a lamp)
-non-linear components prevent distortion

Question 5

Describe the components of a wien bridge oscillator

Answer 5

-the wienbridge has a low pass filter and a high pass filter
-uses the feedback from these to produce phase delay

Question 6

How does the wien bridge oscillator work?

Answer 6

-The high and low pass filter on the ciruit produce a 2nd order ƒ
-resonant frequency
-ƒ = 1/2πRC