Home > Circuit Bricks > Current Amplifier
UPDATED:.21:03 21 October 2013

FUNCTION BLOCK
DESCRIPTION
  • A simple transistor as an amplifier to boost current drive.
CIRCUIT DETAILS

The maximum output current capability can be worked out as:

Ie = Ib x hfe where Ib = (Vin - 0.7) / R1

In most cases, the circuit shown should supply up to 200mA.

The output voltage will be 0.7v less than Vin

DARLINGTON PAIR

The maximum output current capability can be worked out as:

Ie = Ib x hfe1 x hfe2 where Ib = (Vin - 1.4) / R1

In most cases, the circuit shown should supply up to 3A - but you will need to use a heatsink for Q2 - it will get hot!

The output voltage will be 1.4v less than Vin

COMPONENT TYPES

By changing the transistor types you can increase the current capability of the drivers.

The Darlington Pair can be combined into one device. Some type available are:

  • TIP122 - 5A
  • TIP141 - 10A
  • TIP142 - 8A
Transistor Case Styles
TO92
(BC549B)
TO220
(TIP41A / TIP122)
TO218
(TIP141/2)

If you are driving high current devices you may find the transistor gets very hot. You can attach a heatsink to the tab of the transistor to help cool it.

For more transistor options, go to the
NPN Transistor Specifications page

Heat transfer can be aided by the use of a 'thermal transfer compund' paste and can improve the efficiency of the heatsink.

TO220 heatsinkThermal Transfer Paste
Written by Phil Townshend - 2008
www.edutek.ltd.uk - Working Electronics For Students & Teachers