Practical Circuits

Part of The Transistor

Practical transistor circuits — amplifiers, switches, oscillators, and multivibrators — are the building blocks of every electronic system, assembled from a small set of repeating topologies.

Why This Matters

Understanding transistor physics is necessary but not sufficient — you need to know how to arrange transistors, resistors, and capacitors into circuits that actually do something useful. Every electronic function — amplifying audio, switching a relay, generating a clock signal, driving an LED — has a standard circuit topology optimized for that purpose.

A collection of a dozen core circuits covers the vast majority of analog and digital electronics. Master these, and you can analyze any circuit you encounter in salvaged equipment and design new circuits for specific purposes. This is practical knowledge for rebuilding radio communications, instrumentation, and computing from components.

Common-Emitter Amplifier

The most widely used single-transistor amplifier stage:

Circuit:

• NPN transistor
• R1 (47 kΩ) and R2 (10 kΩ) from VCC to ground, center tap to base (voltage divider bias)
• R_C (4.7 kΩ) from VCC to collector (collector load)
• R_E (1 kΩ) from emitter to ground (stability)
• C_E (100 µF) bypasses R_E for AC (restores gain)
• C_in and C_out (10 µF) coupling capacitors block DC at input and output

Performance with 12V supply, BC547:

• Voltage gain: ~100–200 (40–46 dB)
• Input impedance: ~1–5 kΩ
• Output impedance: ~R_C = 4.7 kΩ
• Bandwidth: ~100 kHz

Design procedure:

1. Choose quiescent collector current I_C = 1–5 mA (2 mA is a good starting point)
2. Set V_E ≈ 10% of VCC = 1.2 V → R_E = V_E / I_C = 1.2/0.002 = 600 Ω (use 680 Ω)
3. Set V_C ≈ 60% of VCC = 7.2 V → voltage across R_C = 12 − 7.2 = 4.8 V → R_C = 4.8/0.002 = 2.4 kΩ (use 2.2 kΩ)
4. V_B = V_E + 0.6 = 1.8 V; set divider: R2/(R1+R2) = 1.8/12; current through divider ~10× I_B
5. Calculate R1, R2; choose nearest standard values

Emitter Follower (Common-Collector)

Used for impedance transformation — high input impedance, low output impedance:

Circuit:

• NPN transistor
• Resistive voltage divider bias at base
• No collector resistor (collector connects to VCC)
• R_E (1 kΩ) from emitter to ground
• Output taken from emitter

Properties:

• Voltage gain ≈ 1 (unity — no voltage amplification)
• Input impedance: β × R_E = 100 × 1000 = 100 kΩ
• Output impedance: R_E ∥ (R_source / β) ≈ 10–100 Ω
• No phase inversion

Applications:

• Buffering: connect a high-impedance sensor to a low-impedance load without signal loss
• Power amplifier output driver
• Between stages where impedance mismatch would reduce gain

Transistor Switch

Simplest digital building block — transistor driven between cutoff (OFF) and saturation (ON):

Circuit:

• NPN transistor
• Base resistor R_B from input signal to base
• Collector resistor R_C from VCC to collector, OR load (relay, LED) in collector circuit
• Emitter to ground

Design rules:

1. For hard saturation, drive base with enough current to ensure β × I_B > I_C(max)
2. Choose R_B: I_B = (V_in − V_BE) / R_B; ensure I_B × β > I_C(load)
3. Example: V_in = 5V, load draws 100 mA, β_min = 50
   • Required I_B = 100/50 = 2 mA; use safety factor 5×: I_B = 10 mA
   • R_B = (5 − 0.7) / 0.010 = 430 Ω → use 390 Ω

Timing:

• Turn-on delay: ~nanoseconds to microseconds (transistor and circuit capacitances)
• Stored charge in saturation slows turn-off: add Schottky diode clamp from collector to base to prevent deep saturation and speed up switching

Astable Multivibrator (Free-Running Oscillator)

Two transistors in cross-coupled configuration generate a square wave:

Circuit (equal values for 50% duty cycle):

• Q1 and Q2: two NPN transistors
• R_C1 = R_C2 = 10 kΩ (collector resistors to VCC)
• R_B1 = R_B2 = 100 kΩ (base resistors)
• C1 = C2 = 10 µF (coupling capacitors, cross-coupled: Q1 collector to Q2 base, Q2 collector to Q1 base)

Frequency: f ≈ 1 / (1.4 × R_B × C)

• With R_B = 100 kΩ, C = 10 µF: f ≈ 0.7 Hz (visible LED blink)
• With R_B = 10 kΩ, C = 100 nF: f ≈ 714 Hz (audible tone)
• With R_B = 1 kΩ, C = 10 nF: f ≈ 7 kHz (radio-frequency oscillation begins)

Operation:

1. Initially one transistor conducts, pulling its collector low
2. This pulls the other transistor’s base negative through the coupling capacitor
3. The blocked transistor’s collector rises to VCC
4. The coupling capacitor on the other side charges through R_B
5. When the cap charges enough, the blocked transistor turns on — cycle reverses
6. The process repeats indefinitely

Monostable Multivibrator (One-Shot)

Produces a single timed output pulse when triggered:

Circuit:

• Q1 normally ON, Q2 normally OFF
• Trigger: negative pulse to Q1 base briefly turns it off
• This lets Q2 base rise through R_B, turning Q2 on
• Q2 turns on, its collector falls, holding Q1 off through C
• After time T = 0.7 × R_B × C, C charges through R_B and Q1 turns back on
• System returns to stable state

Pulse width: T = 0.7 × R_B × C

• R_B = 100 kΩ, C = 10 µF → T ≈ 0.7 seconds
• R_B = 10 kΩ, C = 1 µF → T ≈ 7 ms

Two-Stage Audio Amplifier

A practical voice-amplification circuit:

Stage 1 — Common-emitter (for voltage gain):

• R1 = 47 kΩ, R2 = 10 kΩ (bias divider)
• R_C = 4.7 kΩ, R_E = 1 kΩ
• C_E = 100 µF bypass, C_coupling = 10 µF

Stage 2 — Emitter follower (for current drive):

• Drives speaker directly
• R1 = 100 kΩ, R2 = 22 kΩ (bias divider)
• R_E = 47 Ω (speaker impedance ~8 Ω loads this)
• No R_C — emitter is output

Expected performance:

• Overall gain: ~50–100 for voice frequencies
• Input from electret microphone (with 10 kΩ load resistor)
• Output: small speaker or earphone (8 Ω)
• Power from 9V battery

Component Selection Guide

Circuit	Transistor	Key Consideration
Audio amplifier	BC547, 2N3904	Low noise, β > 100
Power switch (>500 mA)	TIP31, 2N3055	High I_C and V_CE ratings
RF amplifier	BF494, 2N2222	High f_T (>100 MHz)
Astable oscillator	Any NPN, β > 50	Matched pair for 50% duty cycle
Crystal oscillator	BF199, 2N918	High f_T for crystal frequency

Summary

Practical Circuits — At a Glance

• Common-emitter: the standard amplifier stage, voltage gain 100–200, use voltage divider bias with emitter resistor
• Emitter follower: impedance buffer, gain ≈ 1, input impedance β×R_E
• Transistor switch: drive base with I_B = I_C(load) / β × (safety factor 5×)
• Astable multivibrator: cross-coupled two-transistor oscillator, f ≈ 1/(1.4 × R_B × C)
• Monostable: one-shot timer, pulse width T = 0.7 × R_B × C
• Two-stage audio amp: common-emitter into emitter follower = high gain + speaker drive
• All circuits above work with any general-purpose NPN transistor (BC547, 2N3904, 2N2222)