92 Nicolas Collins
- The CMOS Inverter amplification stage. As with our oscillator circuits,
the six sections of the 4049 chip are interchangeable. - The input resistor, RI, generally around 10kOhms.
- The feedback resistor, RF, larger than RI, can be a pot for variable gain.
- The input capacitor, CI, generally around 0.1uf.
- The feedback capacitor, CF, usually omitted or very small (10-100pF.)
- The output capacitor, CO, always around 10uF.
4049
The gain -- how much the circuit amplifies the incoming signal -- is determined
by the ration of RF/RI. So if RI=10kOhms and RF=100kOhm, the gain is 10,
which means that any signal you plug into the circuit comes out 10 times louder.
If RI=10kOhms and RF=10mOhms the gain is 1,000, which makes it much
MUCH louder.
The input and output capacitors (CI & CO) block the DC voltage present in the
circuit from the reaching whatever you’re plugging into. They usually don’t
affect the sound much (see “Tone Control” below) but are necessary for
protection. Likewise, the feedback capacitor (CF) is usually omitted unless one
needs tone control or, when amplifying a lot (> 100x) a small capacitor (c. 10-
100PF) prevents the circuit from oscillating or getting very noisy.
By substituting different values for RF, or using a pot instead of a fixed resistor,
you can make a convenient preamplifier for a contact microphone, coil. electric
guitar, acoustic microphone, etc. To hear the subtler effects of component
substitutions you may need to listen through larger speakers than the mini
amplifier.
Tone Control
The frequency response of this circuit is determined by the feedback capacitor
(CF) and input capacitor (CI ). If CF is not used, the circuit is pretty much flat in
frequency response. If a small capacitor is used, say 100pF, you will notice a
slight rolloff of high frequencies. If you make CF large, around .01uf, you will