# Transistor as current amplifier

Let's say I want to put transistor in a circuit; a transistor that amplifies the current. So after amplification components are able to "take" more current for themselves if needed. But the question is: if I amplify the current from 1 Amp to 5 Amps, does the transformer, which I used to power the circuit has to be capable of working at 5 Amps too? Have to all components be able to work at 5 Amps? Or is this required only for components after the amplification part of circuit?

I know this question is not clear but this thing bothers me all the time when learning about transistors. Please reply.

Yes, generally the power supply has to be capable of supplying all the current. There are exceptions we'll look at later.

When we say "a transistor amplifies current" what we mean is that we can use a transistor to allow a small current to control the flow of a larger current. This is analogous to, for example, power steering where a weak turning effort on the wheel of the car or truck is amplified into a high force at the wheels. The power obviously has to come from somewhere and in our electronic amplifier it comes from the battery or power supply.

The exceptions I mentioned earlier include cases where a transistor is used as an interface between a low power circuit and another high power circuit. In this case the amplified current is coming from a different power supply.

simulate this circuit – Schematic created using CircuitLab

Figure 1. A common emmiter transistor switch. A couple of milliamps flows from the 3.3 V micro into the base of the transistor which turns on and allows about 500 mA to flow through a 6 W lamp powered by another supply. Note the common ground connection between the two circuits.

The equivalent source impedance of the XFMR output must be lower than the load, otherwise the voltage will drop.

This means the supply must be able to deliver the load current at rated voltage.