Transistor Circuits Configuration: Current Amplifier, Limiter, Oscillator, Latch


You may build a bullock cart using wheels, but to build a motor car you will surely need much more than that. Similarly, it is impossible to complete complex electronic circuits without involving semiconductor parts like transistors and ICs. In fact transistors are the fundamental building blocks of modern electronics and even the much complicated ICs are made up of millions of built-in transistors.

I have already discussed regarding the working principle of transistors in one of my previous articles.

We’ll continue from there and move on to study regarding some simple transistor circuits.

Simple Transistor Configurations

There can be many different ways to wire a transistor. Let’s study a few basic configurations of transistors and see how they perform important functions in electronic circuits through the following illustrations:

Transistor Circuits

Current Amplifier: Using just a couple of NPN transistors a simple current amplifier circuit can be built. As shown in the diagram, two transistors are connected in a Darlington configuration. The current amplified by the first transistor is further amplified by the second transistor. Thus the pair produces a very hi-gain and double current amplification. It can be useful in circuits where it becomes necessary to sense and amplify minute input signals. The circuit is so sensitive that the LED connected to its output can be made to glow, just by touching base of the transistor by your finger.

Transistor Circuits, Current Limiter, circuit image

Current Limiter: According to the given circuit diagram, two transistors and resistors can be connected together to form a versatile current limiter circuit. Here any load that is connected to the output can be safe guarded from high currents. For examples if the current through the LEDs at the output crosses the set limit, the base of T1 is immediately grounded by T2 so that the dangerous excess current through the LEDs cannot flow. The circuit is well used as electronic power limiters, fuses or short circuit protectors.

Transistor Circuits, Oscillator, Circuit Image

Oscillator: When two transistors and a couple of passive components are connected as shown in the figure, the circuit starts to oscillate. The frequency of oscillation can be altered by changing the values of either the resistor or the capacitor. By increasing its frequency to a suitably high level, it can be used to drive a speaker or a buzzer to produce an audio alarm note. By sufficiently reducing its frequency, the circuit may be used to flash a LED as a warning indicator.

Transistor Circuits, Electronic Latch, Circuit Image

Latch: Another simple yet important configuration of transistors in the form of an electronic latch is shown in the diagram alongside. Here when a positive trigger is applied to the base of T1, T2 receives the required biasing voltage and instantly conducts. A positive feedback is also immediately sent back from the collector of T2, to the base of T1, so that now even if the trigger is removed, the circuit will sustain the position and remain latched. It’s used in circuits where it is required to switch ON the output load in response to an input pulse or trigger. The load is held permanently in that position or is latched unless switched OFF or acted upon externally to break the latch.

The transistor circuits discussed above may look quite basic but nonetheless does find its applications in various electronic circuits to perform some crucial operations.