Designing a BJT Circuit as a Current Amplifier

Basic Understanding of BJT

A Bipolar Junction Transistor (BJT), is a three-layer, two pn-junction device.
It can work as a digital switch or as an amplifier.
There are two types: NPN and PNP, with different polarities.
The three layers are known as the emitter, base, and collector.

BJT as a Current Amplifier

BJTs can function as current amplifiers since they produce a larger output current at the collector than the input current at the base.
The interaction between the base-emitter junction and base-collector junction is what allows a BJT to act as an amplifier.
To operate a BJT as an amplifier, it needs to be in the active region, whereby the base-emitter junction is forward-biased and the base-collector junction is reverse-biased.
The amplification factor, known as current gain or beta (β), is the ratio of the output current (Ic) to the input current (Ib).
For most transistors, the beta factor ranges between 20 and 1000.

Designing a BJT Circuit as a Current Amplifier

Required materials when designing a BJT current amplifier include a suitable NPN transistor (or PNP), resistors, capacitors, and a power source.
Consider the base current, which is usually quite small, as a minor change in this current can result in greater changes in the output current.
Use an appropriately valued base resistor (Rb) to provide necessary base current and ensure the BJT function in the active region.
To facilitate AC coupling and block DC components, use capacitors. This prevents DC offset and distortion in the amplified signal.
Emitter resistor (Re) is used for negative feedback to stabilise the current gain and improve linearity of the circuit. Bypass capacitors can be used in parallel to Re to increase gain at certain frequencies.
Use a collector resistor (Rc) for converting the output current into a voltage, for further processing in the circuit.
Power supply values should be chosen considering the transistor’s power rating and desired output.
In designing this circuit, be aware of thermal runaway, which is a risk when the transistor starts heating and the collector current increases. Some form of temperature control or heat degradation may be necessary.

Practical Application and Troubleshooting

The load line analysis can help understand and troubleshoot an amplifier’s operation.
Keep in mind the temperature dependencies of a BJT. As temperature increases, so does the collector current, which can lead to potential damage.
Main areas to check for faults include the base-emitter junction, output impedance, and possible overloading of later stages.
To tune performance, consider tweaking component values, particularly collector and base resistors. Changing the transistor itself can also vary performance.

Signature characteristics of a functioning amplifier include an output that is a copy of the input but larger, and an output that is 180 degrees out of phase with the input.