Basic Amplifier
Part of Semiconductors
Using a transistor to increase signal power with a simple resistor network.
Why This Matters
A transistor’s core capability is amplification: a small current into the base controls a much larger current from collector to emitter. Building a basic amplifier circuit is the first practical demonstration of this capability and the foundation for radio receivers, audio systems, sensor interfaces, and all analog signal processing.
For a civilization rebuilding electronics, the amplifier circuit is an early milestone. A working amplifier circuit proves your transistors are functional, your power supply is adequate, and your understanding of biasing is correct. It enables you to hear weak radio signals, measure small voltages from thermocouples or strain gauges, and drive loudspeakers from microphone-level signals.
The common-emitter amplifier is the standard starting point: moderate input impedance, high voltage gain, inverted output. Mastering this single configuration provides a mental model that extends to more complex circuits — cascaded amplifiers, feedback networks, oscillators.
Biasing Fundamentals
An amplifier must be biased — set to a DC operating point (quiescent point, or Q-point) in the middle of the transistor’s active region before any signal is applied. Without proper bias, the transistor either cuts off on negative signal swings or saturates on positive ones, clipping the waveform and destroying fidelity.
For a silicon NPN transistor in common-emitter configuration, the Q-point requires:
- Base-emitter voltage ~0.7V (forward biasing the base-emitter junction)
- Collector current at roughly half the maximum usable range (for maximum symmetric swing)
- Collector-emitter voltage at roughly half the supply voltage
The simplest bias circuit is a single base resistor from supply to base. Calculate RB = (Vcc - 0.7) / IB, where IB = IC / hFE (current gain). This works but is extremely sensitive to transistor gain variation — transistors of the same type can have hFE ranging from 50 to 300. A circuit biased for hFE=100 will clip badly with an hFE=200 transistor.
Voltage divider bias is far more stable. Two resistors (R1 and R2) form a divider from supply to ground, setting the base voltage independent of transistor gain. An emitter resistor RE provides negative feedback that further stabilizes the operating point against gain variation and temperature drift. This is the standard configuration for any practical amplifier.
Design procedure for voltage divider bias:
- Choose IC (collector current). Start with 1 mA for general purpose.
- Choose VCE = Vcc/2 for maximum swing.
- VE = 0.1 × Vcc (10% of supply for stability).
- RE = VE / IC.
- VB = VE + 0.7V.
- Choose divider current 10× IB: Idiv = 10 × (IC/hFE).
- R2 = VB / Idiv; R1 = (Vcc - VB) / Idiv.
- RC = (Vcc - VCE - VE) / IC.
The Common-Emitter Circuit
With components calculated and biased, the complete common-emitter amplifier adds coupling capacitors and a bypass capacitor:
Input coupling capacitor (Cin): Blocks DC from the signal source, allowing only the AC signal to reach the base. Prevents the bias network from being disturbed by source resistance. Value: large enough that its impedance at the lowest signal frequency is negligible compared to input resistance. For audio (20 Hz), Xc should be well below 1 kΩ — use 10 µF electrolytic.
Output coupling capacitor (Cout): Blocks DC from appearing at the load, preventing DC current through speakers or subsequent stages. Same sizing logic as Cin.
Emitter bypass capacitor (CE): The emitter resistor RE stabilizes the Q-point but reduces AC gain because it appears as negative feedback for AC signals. CE shorts RE for AC signals while leaving DC bias intact. Without CE: gain ≈ -RC / (RE + re) ≈ low. With CE: gain ≈ -RC / re, where re = 26mV / IC. For IC = 1 mA, re = 26Ω, and with RC = 4.7 kΩ, voltage gain ≈ -180. The bypass capacitor is essential for useful gain.
Voltage gain formula: Av = -RC / re (with CE in place). The negative sign indicates phase inversion — the output is 180° out of phase with the input. This matters when cascading stages.
Practical Construction on a Prototype Board
Breadboard or perfboard construction for the common-emitter amplifier:
Components required: NPN transistor (2N2222, BC547, or equivalent), R1/R2 for bias divider, RC collector resistor, RE emitter resistor, CE bypass capacitor (10-100 µF), Cin and Cout coupling capacitors (1-10 µF), power supply (6-12V), signal source (audio generator or microphone), load (8Ω speaker with output transformer, or high-impedance headphones via coupling cap).
Layout considerations: Keep signal path short. Separate input and output traces to prevent feedback. Bypass the power supply at the board with a 100 µF capacitor — even a good supply has impedance, and without local bypassing, high-gain stages can oscillate.
Testing sequence:
- Power up with no input. Measure VB, VE, VC with a voltmeter. Compare to calculated values. VB should be ~1V (for Vcc=9V with 10% emitter), VE ~0.3V, VC ~4.5V. If VC is near Vcc, transistor is cut off — check base resistors. If VC is near ground, transistor is saturated — check collector resistor value.
- Apply a small AC signal (100 mV peak at 1 kHz). Measure output across RC. Should see ~10-100× the input voltage, inverted.
- Increase input until output clips. Clipping marks the limits of the linear range.
Common failures: No gain with CE installed but gain without — bypass capacitor is open (check polarity of electrolytic). Oscillation (output signal without input) — layout issue, supply bypassing needed, or too much gain in one stage. Low gain but correct Q-point — CE not correctly installed, or transistor’s actual hFE much lower than assumed.
Cascading and Audio Applications
A single stage of 100-180× voltage gain amplifies a 1 mV microphone signal to ~100-180 mV — enough for headphones with a coupling transformer, but not enough to drive a loudspeaker directly. Two cascaded stages provide 10,000-30,000× voltage gain, sufficient for most audio applications.
Cascade connection: output of stage 1 through coupling capacitor to input of stage 2. The second stage’s input resistance loads the first stage’s collector resistor, reducing gain. Account for this in design by ensuring stage 1’s RC is much less than stage 2’s input resistance.
Phase: two common-emitter stages invert twice, producing non-inverted output. This matters for feedback and for certain applications requiring phase coherence.
For a speaker output stage, the final transistor must handle the required current. An 8Ω speaker at 0.5W requires ~250 mA peak. A small-signal transistor like BC547 (rated 100 mA) would be destroyed. Use a medium-power device (2N2219, TIP31) or a complementary push-pull pair for efficiency. Include a transformer between collector and speaker for impedance matching — the transistor’s output impedance (hundreds of ohms) is far higher than 8Ω; without transformation, most power is wasted.
A working audio amplifier chain — microphone, two common-emitter voltage-amplifier stages, push-pull output stage, speaker — is a milestone achievement for a rebuilding civilization. It proves semiconductor fabrication is successful and enables radio reception, intercom systems, and acoustic monitoring.