DIY Guitar Pedals Oil Tanker Fuzz, Инструкция по эксплуатации, Страница: 13 / 29

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4. Input Stage.
The input stage is a Common Emitter NPN amplifier. It provides a high voltage gain with low input impedance and high
output impedance. It is not the ideal input stage for signal integrity but the best for simplicity and fast high gain.
- The 1MΩ R1 resistor from the input to ground is an anti-pop/bleeder resistor, it will avoid abrupt pop sounds
when the effect is engaged.
- The 22nF C1 capacitor is a film capacitor used to couple the input of the incoming guitar signal and the rest of
the circuit.
- The Q1 transistor just needs to be a low-noise/high-gain transistor (β = 90-700).
- The 100KΩ R3 resistor is a simple pull up resistor for the Q1 transistor.
4.1 Input Impedance.
Is equal to the input impedance of a common emitter stage. It can be calculated as:
Zin = Qin
+
(1 / ((1 / R
1
) + (1 / (β
⋅
re ))))
Assuming the β (gain) of the Q1 transistor is 100, which are typical of the 2N5088 in this circuit and that the emitter leg
signal resistance is 25mV / 1mA or 25Ω. VT is the thermal voltage of a transistor, at room temperature the value is
approximately 25mV. The minimum impedance for the 2N5088, per the datasheet, at 1kHz is 1,000.
Zin = 1,000
+
(1 / ((1 / 1,000,000) + (1 / (100
⋅
25))))
Zin = 1,000
+
(1 / ((1 / 1,000,000) + (1 / 2,500)))
Zin = 1,000
+
(1 / (0. 0.0004))
Zin = 1,000
+
(1 / 0.000401)
Zin = 1,000
+
2,494
Zin = 3,493 Ω (3.5 K ) @ 1 kHz
For this math calculation the feedback network is ignored but in practice, it will lower the input impedance closer to
3.4KΩ. The Oil Tanker Fuzz has a very low input impedance that will change with the position of the R
FUZZ
potentiometer.
So the feedback network has a big impact on this parameter.