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7. Feedback Network.
Amplifiers use current or voltage as input or outputs, you can check the amplifier classification. The Oil Tanker Fuzz has a
negative feedback called shunt-series feedback (Current Controlled Current Source CCCS). Part of the output current is
taken from Q2 drain and introduced as current in Q1 base, so the feedback resistor R2 is shunt connected with the input
and in series connected with the output
Why using feedback?
In amplifier design the degenerative (negative) feedback is used to:
•
Desensitize the gain: make the gain value less sensitive to transistors (i.e. component variation caused by
temperature).
•
Reduce nonlinear distortion: make the gain constant.
•
Reduce the noise: minimizing the contribution to the output of unwanted electrical signals.
•
Control the input/output impedance: raising or lowering their values.
•
Extend the bandwidth of the amplifier.
The properties above are obtained at the expense of a reduction of gain. As a rule of thumb with more feedback, there is
less global gain, following the formula:
A
FB
=
A
OL
/ (1 +
B
FB
A
OL
)
Where
AFB = Total current gain of the amp in the closed loop.
AOL = Current gain of the amp in open loop
B = Feedback constant (not to be confused with the transistors β parameter).
When the fuzz control increases the attack, the 22uF C2 cap will gradually shunt the negative feedback to the ground,
thus letting the circuit operate with more gain (right image below).
Where
•
AFB = Total current gain of the amp in the closed loop.
•
AOL = Current gain of the amp in open loop
•
B = Feedback constant (not to be confused with the transistors β parameter).
