Amplify with one transistor: a schematic puzzle
Four resistors set the bias, three capacitors let the signal through: the classic common-emitter stage.
First the resting point. R1 and R2 divide 9 V down to about 1.6 V at the base; the emitter follows 0.6 V lower, at 1.0 V, so 1 mA flows through the 1 kΩ emitter resistor. Nearly all of it continues through the 4.7 kΩ collector resistor, dropping 4.7 V and leaving the collector at about 4.5 V, right in the middle, with room to swing both ways. Now the signal: the input capacitor lets the wiggle in without disturbing the bias, the wiggle moves the emitter current, and the collector resistor turns that into a bigger voltage wiggle, upside down. Gain is roughly 4.7 kΩ ÷ 1 kΩ ≈ 5 on its own; with the bypass capacitor shorting the emitter resistor for signals, it jumps past 100. The output capacitor passes the amplified wiggle on and keeps the 4.5 V to itself. Like a heavy door on a weak spring: a fingertip at the base moves the whole slab at the collector.
R1, 47 kΩ: Top of the bias divider. With R2 it parks the base at 9 V × 10 ÷ 57 ≈ 1.6 V, and the pair carries about 0.16 mA, roughly ten times the base current so the base cannot drag it around.
Collector resistor, 4.7 kΩ: Turns current into voltage. 1 mA through 4.7 kΩ drops 4.7 V, parking the collector at 4.5 V, and any change in current becomes a change in output voltage, inverted.
Input capacitor, 1 µF: Lets the signal in and keeps the DC out. Whatever is connected to IN cannot disturb the 1.6 V bias, and the bias cannot leak into the source.
2N2222 transistor: The heavy door. A tiny change in base current lets a change roughly 150 times larger flow from collector to emitter, and the collector resistor turns that into the output voltage.
Output capacitor, 1 µF: Passes the amplified wiggle to OUT and holds back the 4.5 V resting level, so the next stage sees a signal centred on zero.
R2, 10 kΩ: Bottom of the bias divider. Its share of the 9 V is what the base sees. Make it bigger and the transistor conducts harder; smaller and it starves.
Emitter resistor, 1 kΩ: The stabiliser. If the transistor tries to conduct more, the emitter voltage rises, the base-emitter voltage shrinks and the current settles back. The bias no longer depends on the transistor’s gain, which varies two to one between samples.
Bypass capacitor, 10 µF: Gives the signal a shortcut around the emitter resistor while leaving the DC bias untouched. The stabiliser still works for slow drift, but for audio the emitter looks grounded and the gain climbs from about 5 to over 100. Polarised: plus at the emitter, which sits at 1 V.