Understanding a Basic Multivibrator Circuit
In classic electronics literature, there’s mention of constructing a multivibrator using a neon lamp and a capacitor. Remarkably, the straightforward multivibrator depicted in Figure 1 operates on the same principle, substituting the neon lamp with an NPN transistor and operating at significantly lower voltages. This fundamental concept can be easily verified, prompting the question: why explore this seemingly basic function further? The circuit’s operation unfolds in this manner: during reverse operation (when the emitter is positive concerning the collector), the NPN transistor exhibits a distinctive negative characteristic, a phenomenon easily observable. At approximately 9 V, the base-emitter diode undergoes the familiar avalanche effect. This event triggers a rapid release of charge carriers in the junction (barrier) layer due to their increased density and speed.
Avalanche Effect in Transistor: Understanding the Process
As the number of charge carriers increases, akin to an avalanche, the current also grows. This phenomenon mirrors the behavior observed in a 9 V Zener diode. Notably, the internal resistance of the Zener diode remains positive throughout. Now, when the transistor operates in reverse mode, it contributes to this effect. Although the roles of the emitter and collector switch, the symmetrical construction of the transistor ensures its equal functionality in reverse operation. A slight current gain, ranging from approximately 3 to 10, can be measured.
This occurs because the charge carriers traverse the thin base layer to reach the junction barrier, allowing the transistor to continue functioning. Here lies the crux of the matter: the avalanche effect occurs precisely within this barrier layer. With an increasing number of charge carriers, this effect intensifies, leading to what can be described as an avalanche squared. Once this initial avalanche is triggered, a weaker voltage is adequate to sustain the effect. Consequently, the collector current amplifies the avalanche effect, ensuring the presence of the negative characteristic.
The strength of the discharge current is sufficient to drive an LED (see Figure 2). For this, we need nevertheless a voltage greater than 9 V. The circuit functions adequately with two almost dead (discharged) 9 V batteries. The LED will still flash for a long time, right until the very last drop of energy in the batteries. The flashing frequency will slow down as the battery runs down.
For mechanical reasons and to simplify construction, the charge resistor is fitted between the batteries.