01 Multivibrator circuit
I. Introduction
This is a very common oscillator circuit, it often appears in many electronic circuits. A question suddenly came to mind, that is, can a Darlington transistor be used to replace the current NPN transistor to generate an oscillating signal? · Let's test it out.
2. NPN circuit
The circuit is built on the breadboard, and the parameters of the components used are the same as those marked in the previous circuit diagram. Next, use an oscilloscope to measure the base and collector signal waveforms of one of the transistors.
Since the circuit is symmetrical, the signal waveforms of the base and collector of the two transistors are the same. This is the waveform after the circuit stabilizes and oscillates. The charging process corresponding to the collector is very fast, and the charging process of the base is charged by the base resistor on the coupling capacitor, and the charging speed is relatively slow, so the circuit oscillation frequency is mainly determined by this charging process. According to the circuit parameters, it can be seen that the theoretically calculated oscillation frequency is roughly the same as the oscillation frequency measured by the oscilloscope. This verifies the oscillator function of the circuit. ·Then directly use BC517, that is, NPN type Darlington tube to replace BC547, - see if the circuit can still work.
3. Darlington tube
Use the transistor assistant to measure the basic parameters of BC517, and see that its corresponding current amplification factor is as high as 94000, and select two BC517 with similar parameters to replace BC547 on the breadboard. After replacement, retest that the circuit works.
After power on, it can be seen that the circuit works normally. The only difference is that the collector saturation voltage of the Darlington triode is larger than that of the NPN, and the oscillation frequency is 192Hz, which is slightly higher than that of the BC547.
※ Summary ※
This article discusses the use of Darlington transistors to form a multivibrator circuit. After direct replacement, the circuit works almost exactly the same. The huge current magnification did not bring about any changes in the circuit. In some respects, we need to re-understand the characteristics of triode amplifier circuits.
