TIP122NPN Darlington power transistor
Two transistors. One high-gain load-control stage.
TIP122 is an NPN Darlington power transistor: the first internal transistor drives the base of a second, producing high overall current gain. Both collectors share one terminal, and built-in base-emitter resistors help discharge the internal junctions. The reviewed onsemi TIP122G comes in TO-220AB. Its front-facing leads are base, collector and emitter; the exposed metal tab is electrically connected to collector. This is a low-speed switching and amplifier device, with a larger on-state voltage drop than a single transistor or a suitably driven MOSFET. The headline 5 A and 65 W values are absolute limits under specific conditions. Load current, base drive, safe operating area and cooling must be checked together. The example uses a modest resistive load and clearly labelled voltage assumptions.
Base / controlCollector / loadEmitter / return
TIP122GTO-220ABFront view · B1, C2, E3 · tab = C
Click a lead or use the arrow keys. Front view · B1, C2, E3 · tab = C
PIN 1 · Base / control
BaseB
Function
Input of the first transistor in the Darlington pair.
How to connect
Use a current-limiting base resistor and a defined off-state return.
Electrical note
Two base-emitter junctions give a higher on voltage than a single transistor.
PIN 2 · Collector / load
CollectorC
Function
Shared collector of the two internal NPN transistors.
How to connect
Connect to the low side of the controlled load.
Electrical note
The metal tab, shown as contact 4 in the drawing, is also collector. Insulate it from a grounded heatsink where necessary.
PIN 3 · Emitter / return
EmitterE
Function
Output emitter and circuit return.
How to connect
Connect to the common control and load ground.
Electrical note
Front view with leads down: B1, C2, E3.
At a glance
NPN Darlington power transistor
The Darlington connection multiplies current gain, but its two junctions and collector path need more voltage headroom. Check dissipation rather than treating it as an ideal closed switch.
100 V · absolute VCEOCollector limit
5 A · absoluteDC collector limit
1000 At specified test pointsDC gain minimum
150 °C · maximumJunction limit
Operating profile
TIP122G
Device and package
Key parameters and their test conditions
- Selected device
- onsemi TIP122GConditions / notes: Pb-free TO-220AB; 50 units per rail
- Electrical reference
- TIP120/D Rev. 10Conditions / notes: June 2024
- Package drawing
- CASE 221A, style 1Conditions / notes: 2.54 mm lead pitch
- Front pinout
- 1 B · 2 C · 3 EConditions / notes: Tab/contact 4 is collector
- Construction
- Monolithic NPN DarlingtonConditions / notes: Built-in base-emitter shunt resistors
- Complementary device
- TIP127Conditions / notes: PNP; separate polarity and reference checks apply
Package details
Follows the selected physical package
- Manufacturer
- onsemi
- Part number
- TIP122G
- Package
- TO-220AB
- Connections
- 3 pins
- Mounting
- Through-hole
- Lead pitch
- 2.54 mm
- Package code
- 221A style 1
- Thermal reference
- 65 W at case 25°C; 2 W at ambient 25°C
Connections by function
Select a pin to open its function, electrical notes and wiring guidance.
Base / control1 pins
Collector / load1 pins
Emitter / return1 pins
Drive the first base
The external base current enters the first NPN stage.
Multiply current drive
Its emitter drives the second stage while both collectors remain common.
Check voltage and heat
A Darlington needs voltage headroom and appropriate cooling.
The illustrative 12 V circuit uses a 100 Ω load and assumes 1 V collector-emitter drop. It draws about 110 mA; the transistor loses about 0.115 W including the base contribution.
Pin reference
Select a pin to explore its function and connection.
3 pins
| Pin | Signal | Type | Function |
|---|---|---|---|
| B | Base | Input of the first transistor in the Darlington pair. | |
| C | Collector | Shared collector of the two internal NPN transistors. | |
| E | Emitter | Output emitter and circuit return. |
No pins match. Try a signal name or choose All pins.
Device and package
A reviewed manufacturer reference
Manufacturer specifications 6 parameters
| Parameter | Value | Conditions / notes |
|---|---|---|
| Selected device | onsemi TIP122G | Pb-free TO-220AB; 50 units per rail |
| Electrical reference | TIP120/D Rev. 10 | June 2024 |
| Package drawing | CASE 221A, style 1 | 2.54 mm lead pitch |
| Front pinout | 1 B · 2 C · 3 E | Tab/contact 4 is collector |
| Construction | Monolithic NPN Darlington | Built-in base-emitter shunt resistors |
| Complementary device | TIP127 | PNP; separate polarity and reference checks apply |
No specifications match. Try a parameter name or a unit.
TIP120 and TIP121 have lower collector-voltage ratings. Do not substitute their limits for TIP122.
Absolute maximum ratings
Individual stress limits
Manufacturer specifications 7 parameters
| Parameter | Value | Conditions / notes |
|---|---|---|
| Collector-emitter / collector-base | 100 V / 100 V | VCEO / VCB |
| Emitter-base voltage | 5 V | Reverse stress limit |
| Continuous / peak collector current | 5 A / 8 A | Thermal and pulse constraints apply |
| Base current | 120 mA | Absolute maximum |
| Case-cooled dissipation | 65 W at TC = 25°C | Derate 0.52 W/°C above 25°C |
| Ambient dissipation | 2 W at TA = 25°C | Derate 0.016 W/°C above 25°C |
| Junction / storage range | −65 to +150°C | Absolute range |
No specifications match. Try a parameter name or a unit.
The 65 W rating requires a controlled case temperature; it is not a free-air rating.
Gain and on-state behavior
Pulsed test points at case 25°C
Manufacturer specifications 6 parameters
| Parameter | Value | Conditions / notes |
|---|---|---|
| hFE at IC = 0.5 A | 1000 minimum | VCE = 3 V |
| hFE at IC = 3 A | 1000 minimum | VCE = 3 V |
| Typical gain headline | 2500 at IC = 4 A | Typical, not a guaranteed minimum |
| VCE(sat), IC 3 A / IB 12 mA | 2 V maximum | Forced ratio 250 |
| VCE(sat), IC 5 A / IB 20 mA | 4 V maximum | Forced ratio 250 |
| VBE(on) | 2.5 V maximum | IC = 3 A; VCE = 3 V |
No specifications match. Try a parameter name or a unit.
Pulsed tests use pulse width ≤ 300 µs and duty cycle ≤ 2%; do not assume these are thermally steady conditions.
Off-state and dynamic data
TIP122 conditions from the selected reference
Manufacturer specifications 6 parameters
| Parameter | Value | Conditions / notes |
|---|---|---|
| ICEO | 0.5 mA maximum | VCE = 50 V; IB = 0 |
| ICBO | 0.2 mA maximum | VCB = 100 V; IE = 0 |
| IEBO | 2 mA maximum | VBE = 5 V; IC = 0 |
| Small-signal current gain | 4 minimum | IC 3 A; VCE 4 V; f 1 MHz |
| Output capacitance | 200 pF maximum | VCB 10 V; IE 0; f 0.1 MHz |
| Switching scope | General-purpose low-speed switching | Storage time depends on base drive and turn-off circuit |
No specifications match. Try a parameter name or a unit.
Leakage and stored charge matter when driving small loads or switching quickly.
Thermal and safe operating area
Current and voltage must be checked together
Manufacturer specifications 6 parameters
| Parameter | Value | Conditions / notes |
|---|---|---|
| Junction-to-case resistance | 1.92°C/W maximum | RθJC |
| Junction-to-ambient resistance | 62.5°C/W maximum | RθJA |
| Active-region SOA | Use the manufacturer’s curves | Second breakdown can limit operation before the power rating |
| Pulse operation | Use transient thermal impedance | Pulse width, duty cycle and starting temperature matter |
| Collector tab | Electrically live | Add insulation if the heatsink must be at another potential |
| Flyback clamp | Required for inductive switching | Do not rely on the internal Darlington resistors as a load clamp |
No specifications match. Try a parameter name or a unit.
The example uses a resistor so the circuit can be understood without an inductive switching transient.
12 V resistor-load switch
Explicit control and load values
Manufacturer specifications 6 parameters
| Parameter | Value | Conditions / notes |
|---|---|---|
| Load | 100 Ω, at least 2 W | From +12 V to collector pin 2 |
| Base resistor | 1 kΩ from 5 V buffered control | Into base pin 1 |
| Pull-down | 10 kΩ from base to emitter | Defines the external off-state path |
| Emitter pin 3 | Common ground | Control and load return |
| Illustrative junction assumptions | VBE = 1.4 V; VCE = 1.0 V | Not guaranteed manufacturer test points |
| Control-source requirement | Approximately 3.6 mA | Verify the selected controller or buffer |
No specifications match. Try a parameter name or a unit.
Check the actual operating point and thermal margins before choosing a real load.
Example calculations
Nominal illustrative arithmetic
Manufacturer specifications 7 parameters
| Parameter | Value | Conditions / notes |
|---|---|---|
| Drive-resistor current | 3.60 mA | (5 − 1.4) / 1 kΩ |
| External pull-down current | 0.14 mA | 1.4 / 10 kΩ |
| Device base-terminal current | 3.46 mA | 3.60 − 0.14 mA; internal paths are part of the device |
| Collector current | 110 mA | (12 − 1.0) / 100 Ω |
| Forced current ratio | Approximately 31.8 | 110 / 3.46 |
| Device power | Approximately 0.115 W | 1.0 × 0.110 + 1.4 × 0.00346 |
| Load-resistor loss | 1.21 W | (0.110 A)² × 100 Ω |
No specifications match. Try a parameter name or a unit.
These calculations are teaching values, not measured readings or a timed simulation.
Low-speed switches
Control moderate DC loads with a suitable current and cooling budget.
Manufacturer application guideDriver stages
Use the high gain where the voltage drop is acceptable.
Manufacturer application guidePower amplification
Operate inside the active-region safe operating area.
Manufacturer application guideTake it into ProTech
Find TIP122 in the Components Library to place its schematic symbol and connect its pins.
Reference page · circuit simulation model not yet availableOpen ProTech