This document discusses two ways of connecting an NPN transistor to act as a driver or amplifier. The transistor driver connects the load between the positive voltage and collector, causing the transistor to sink current into its collector and fully turn the load on when the input rises above 0.7V. The transistor amplifier connects the load between the emitter and ground, allowing the output voltage and brightness to be varied as the emitter voltage is 0.7V less than the input. Both configurations amplify current using the transistor's current gain, but the driver has a fully on/off output while the amplifier's output can be varied.

1. TRANSISTOR DRIVERS & AMPLIFIERS
Below are two ways of connecting an NPN transistor to act as a Driver or an Amplifier. Each has its own merits and the one you choose depends on what you want
it to do. In both cases they amplify current using Ic = Ib x hfe
NPN TRANSISTOR DRIVER NPN TRANSISTOR AMPLIFIER
9V 9V
R2
BL2
1K
V2
4.5V
R1 Q2
1K
V1
4.5V
Q1
BL1
0V 0V
l Load wired between +V and the collector. l Load wired between the emitter and 0v.
l The transistor sinks current into its collector. l The transistor sources current from its emitter.
l When the input voltage rises above 0.7v, the l The voltage at the emitter is 0.7v less than the input
transistor saturates and turns the bulb on fully. voltage thus the brightness can be altered.
Advantages: Advantages:
Low voltage to activate, > 0.7v Output voltage can be varied with input,
Current amplifier Current amplifier
No voltage loss
Disadvantages
Disadvantages 0.7v loss in output
Output device fully on or off - not variable Inefficient at low input voltage levels.
Phil Townshend c. 2003