Transistors & tubes

Transistors and vacuum tubes do one job: a small signal at the control terminal steers a much larger current. Learn the families, how to bias them, and the circuits they power.

Quick reference

The facts you reach for most — polarity, gain, thresholds, and operating regions — at a glance. Full explanations live in the numbered sections below.

NPN vs PNP

Same rules, opposite polarity. The emitter arrow shows conventional current direction.

NPN: base +0.7 V above emitter → ON
     Ic flows C → E  (current sink)
PNP: base −0.7 V below emitter → ON
     Ic flows E → C  (current source)

Gain

BJT is current gain (β); FETs and tubes are voltage gain via transconductance.

Ic = hFE · Ib      β ≈ 100–300
gm  = ΔId / ΔVgs    # transconductance
Av  ≈ −gm · Rd      # common source / emitter

BJT states

Three regions decide switch vs amplifier.

cutoff:     Vbe < 0.7 V  → Ic ≈ 0
active:     Ic = hFE · Ib  → amplify
saturation: Vce ≈ 0.2 V  → closed switch

MOSFET gate & threshold

Voltage-controlled; the gate is a capacitor, not a resistor.

N-ch ON: Vgs > Vth   # Vth ≈ 1–4 V
P-ch ON: Vgs < Vth
gate draws ~0 DC; 10k pull to a defined level

Saturation vs cutoff

Switch = slam between the two; amplifier = stay in the middle.

saturation: fully ON,  Vce ≈ 0.2 V / Vds ≈ 0
cutoff:     fully OFF, Ic ≈ 0
linear:     in between → amplifier

Triode vs pentode

Extra grids around the basic triode trade feedback for gain.

triode:  cathode · grid · anode   μ ≈ 20–100
tetrode: + screen grid  (less Miller feedback)
pentode: + suppressor grid (no secondary emission)

Common-emitter amp

The go-to voltage gain stage; it inverts the signal.

Av ≈ −Rc / Re    # BJT, inverting
Av ≈ −gm · Rd     # FET common source
bias mid-active region; AC-couple in/out

Bias point

Resistors set the DC operating point the signal swings around.

Vb = Vcc · R2 / (R1 + R2)
Ve = Vb − 0.7 V
Ic ≈ Ie = Ve / Re

One device, two jobs

Every active device is a controlled valve: a weak input signal throttles a strong current.

Input (small signal) → Control (base · gate · grid) → Output (large current / voltage)

1. Switch

Drive it between cutoff and saturation: Vgs >> Vth, or base current large enough to saturate.

2. Amplify

Bias the device mid-way in its linear region, then swing a small signal around that point.

3. Current vs voltage

BJT is current-controlled (base). FET and tube are voltage-controlled (gate / grid).

4. Bias

Resistors set the quiescent point — the DC operating point the signal swings around.

• BJT on = base +0.7 V — Current-controlled; limit base current.
• MOSFET on = Vgs > Vth — Voltage-controlled; charge the gate.
• Tube on = grid > cutoff — Grid voltage steers plate current.
• switch = cutoff ↔ saturation — Amplifier = biased mid-region.

KEY: Switch or amplify? Driven between fully-off and fully-on, any active device is a switch; biased into the middle of its linear region, it’s an amplifier. The only real difference is where you set the operating point.

Bipolar junction transistors

Current-controlled three-terminal device: a small base current sets a much larger collector current.

• Cutoff — Vbe < 0.7 V — both junctions off, Ic ≈ 0. Open switch.
• Active — B-E forward, B-C reverse — Ic = hFE · Ib. Amplifier.
• Saturation — Both junctions forward — Vce ≈ 0.2 V. Closed switch.
• Reverse — E and C swapped — low β, rarely used.

Current gain (hFE)

Collector current is base current times beta.

Ic = hFE · Ib        # β = hFE, ~100–300
Vbe ≈ 0.7 V          # silicon (0.3 V Ge)
Ie = Ib + Ic = (β + 1) · Ib

NPN vs PNP

Same rules, opposite polarity. The emitter arrow shows conventional current direction.

NPN: turn on with base +0.7 V above emitter
     current flows C → E
PNP: turn on with base −0.7 V below emitter
     current flows E → C

Voltage-divider bias

Two resistors set a stable base voltage; the emitter resistor sets the current.

Vb = Vcc · R2 / (R1 + R2)
Ve = Vb − 0.7 V
Ic ≈ Ie = Ve / Re

⚠: Never drive a base without a resistor. The base-emitter junction is a diode: once Vbe passes ~0.7 V it looks like a short. Pick Rb = (Vin − Vbe) / Ib to limit base current.

2N3904  NPN, 200 mA
2N3906  PNP, 200 mA
2N2222  NPN, 800 mA
BC547   NPN / BC557 PNP

2N3055  NPN, 15 A
TIP120  NPN Darlington, 5 A
BD139   NPN, 1.5 A
TIP41C  NPN, 6 A

Metal-oxide-semiconductor FETs

Voltage-controlled, high-impedance gate; the workhorse of switching and power.

N-channel P-channel enhancement depletion logic-level

Threshold & on-resistance

Drive the gate well past threshold to reach a low on-resistance.

Vgs(th)  ≈ 1–4 V    # logic-level 1–2 V
Rds(on)  ≈ mΩ–Ω     # lower = less heat
fully ON: Vgs ≫ Vth, Vds ≈ Id · Rds(on)

Gate drive

The gate is a capacitor, not a resistor: it draws current only while charging.

Qg (gate charge) ≈ few–100 nC
Ciss = Cgs + Cgd   # input capacitance
hard drive: 10–12 V, low-impedance driver
floating gate → add 10k pull-down

Logic-level vs standard

Logic-level parts turn fully on at 3.3–5 V; standard parts need ~10 V gate drive.

logic-level: Vgs(th) ~1 V, spec'd at 4.5 V
standard:    Vgs(th) ~2–4 V, spec'd at 10 V
check Rds(on) at YOUR gate voltage

ESD: The gate oxide is thin and fragile. A floating gate charges to an unpredictable voltage (device half-on and hot), and static discharge can puncture the oxide outright. Tie the gate to a defined level and mind ESD.

Vds(max)   drain-source voltage
Id(max)    continuous drain current
Vgs(max)   gate voltage (±20 V typ)

Vgs(th)  threshold
Rds(on)  on-resistance @ Vgs
Qg       total gate charge
Ciss     input capacitance

JFET, IGBT, and the rest

Close cousins with different strengths — pick by voltage, speed, and noise.

JFET

Normally on; the gate is a reverse-biased junction, so it draws almost no current and adds little noise.

N-ch: Id flows drain → source at Vgs = 0
off:  Vgs < Vgs(off)   # pinch-off, negative
use:  low-noise preamp, RF, current source

IGBT

A MOS gate driving a bipolar output: easy to drive, yet handles big volts and amps.

ratings: 600 V / 1200 V, 10s–100s of amps
turn-on: Vge > Vge(th) (~4–6 V)
slower than MOSFET, has tail current
use: VFD, inverter, welder, EV traction

⌁: Pick by the job. Low-noise, high-impedance small-signal → JFET. Fast logic-level switching → MOSFET. High voltage + high current at moderate speed → IGBT. Very high voltage or RF power → tube.

Vacuum tubes

Thermionic valves: a heated cathode, a control grid, and hundreds of volts.

Cathode (heated, emits electrons) → Grid (controls flow) → Anode (collects electrons)

Grid bias

The grid must sit negative relative to the cathode to set the idle plate current.

cathode bias: Rk develops Vk = Ik · Rk
              grid referenced to 0 V → Vgk = −Vk
fixed bias:   negative supply on the grid

Plate curves & load line

Plate current vs plate voltage for stepped grid voltages; the load line picks the operating point.

μ  = amplification factor (triode ~20–100)
gm = transconductance (mA/V)
Rp = plate resistance (kΩ)
load line: Vp = B+ − Ip · Ra

HV: Plate supply is lethal. B+ is typically 200–500 V, and filter capacitors hold that charge long after power-off. Drain the caps before touching anything, and keep one hand in your pocket.

12AX7  high-μ dual triode
12AU7  medium-μ dual triode
6SN7   octal dual triode
EF86   small-signal pentode

EL84    small pentode (15 W)
6L6     beam power (25 W)
EL34    power pentode (25 W)
5AR4    full-wave rectifier

Amplifier circuits

Three basic configurations cover almost every amplifier you’ll build.

Emitter / source follower

Gain is ~1, but it turns a weak, high-impedance source into a stiff, low-impedance one.

Av ≈ 1 (no inversion)
Zin high, Zout low
CE  → CC  → load   # buffer the output

Class & push-pull

Two devices each handle half the waveform; class controls how much they overlap.

A  : conducts 360°, clean, ~25–30% efficient
B  : 180° each, crossover distortion
AB : slight overlap, no crossover, ~50–60%
D  : PWM switching, ~90%+

1. Bias — Set the quiescent point in the active region so the device never clips the signal.
2. Gain stage — Common emitter / source (or a tube) multiplies the small signal.
3. Follower — An emitter / source follower buffers the output to drive a real load.
4. Output — AC-couple through a capacitor to remove the DC bias.

Av ≈ −Rc / Re   (BJT)
Av ≈ −gm · Rd    (FET)

Av ≈ 1
Zin = β · Re (BJT)

Switching & power

Drive the gate, switch the load, and keep the heat under control.

H-bridge

Four switches steer current both ways through a motor; PWM sets speed, diagonals set direction.

forward:  Q1 + Q4 on
reverse:  Q2 + Q3 on
brake:    short the motor (low sides on)
add dead time → no shoot-through

Linear vs switching

A linear regulator burns the difference as heat; a switcher chops it for efficiency.

linear:    P = (Vin − Vout) · I   # ~50% typ
switching: ~85–95%, needs inductor
low noise → linear; efficiency → switching

Heat

Every dissipated watt raises the junction temperature; keep it under the limit.

Pconduction = Id² · Rds(on)
Tj = Ta + P · Rθja
heat sink → lower Rθ; Tj(max) ≈ 150 °C

⌁: Inductive loads need a flyback diode. When the switch opens, a motor or relay coil forces its current to keep flowing, generating a voltage spike that can destroy the transistor. Put a diode across the coil, cathode to V+.

Vds(max) > supply + spikes
Id(max)  > load current
Vgs(th)  < your drive voltage

Rds(on)  → less conduction heat
Qg       → faster switching, less loss
Rθja     → cooler junction

Pitfalls

The failure modes that quietly kill transistor and tube circuits.

Forgot the base resistor?

A BJT base-emitter junction is a diode; once it hits ~0.7 V it passes huge current.

Rb = (Vin − Vbe) / Ib
Ib = Ic / hFE
no Rb → dead transistor

Floating MOSFET gate

A gate left unconnected charges to an unpredictable voltage, so the transistor turns half-on and overheats.

gate pull-down (N) / pull-up (P): 10k
ESD can puncture the thin oxide
solder/discharge to GND when handling

Thermal runaway (BJT)

As the junction heats, β and Ic rise, which heats it more — a positive feedback loop.

cause: Ic ↑ → heat ↑ → β ↑ → Ic ↑ …
fix: emitter resistor Re (degeneration)
     heatsink + keep Vce low in saturation

Tube high voltage

B+ and the filter capacitors stay charged after power-off and can deliver a lethal shock.

B+ = 200–500 V+
discharge caps before probing
one hand in pocket; never work live

Bad bias

Bias too cold → crossover/clipping; too hot → the device runs hot and wastes power.

too low:  signal clips (cutoff)
too high: saturation, wasted heat
check the quiescent current (Ic / Id / Ip)

No heat sink

Even a few watts can push a TO-220 past its 150 °C junction limit.

P = V · I  (or Id² · Rds(on))
Rθja without sink ≈ 60 °C/W
add sink → Rθja drops to ~10 °C/W

Gate ringing / oscillation

Long or un-damped gate traces can ring, spiking Vgs past its limit and making the device switch erratically.

fix: series gate resistor (10–100 Ω)
     short, wide traces; local bypass cap
     keep Vgs within ±20 V

Exceeding the SOA

A device can fail even inside its max V and I ratings if it sees high voltage and high current at the same time.

secondary breakdown → local hotspot
linear pass: P = (Vin − Vout) · I
check the SOA curve; keep Tj < 150 °C

!: When in doubt, measure. Bias is set by resistors and voltages — not vibes. Check Vbe (~0.6–0.7 V), Vgs vs Vth, and the idle current before you trust a circuit.