Cascade triode
01Foreword _
If two transistors are cascaded as Darlington tubes, what will be the corresponding current amplification factor? Let's test it with the transistors at hand, and select two BC547s for cascading. Measure the current magnification after cascading.
1. Two transistors
Take two BC547 transistors and first measure the current amplification factor of each of them. Take measurements using the Transistor Measurement Assistant module. One transistor has beta equal to 247 and the other transistor has beta equal to 256. ·There is not much difference between them. Connect them in the form of a Darlington tube, with the collectors of the two connected in parallel, and the emitter of the previous stage connected to the base of the next stage. -Eventually forms a two-stage Darlington tube.
Next use the same Transistor Measurement Assistant to measure cascaded Darlingtons. Let’s take a look at the measurement results. The corresponding current amplification factor is approximately 48900. At this time, the current flowing through the corresponding transistor is 5.3mA. Let's compare the relationship between this measurement result and the current amplification factor of the two transistors. The product of the current amplification factors of the two transistors is equal to 63232. After cascading, the corresponding current amplification factor is 48900, which is approximately equal to about 76% of the product of the current amplification factors of the two transistors. Since the actual current flowing through the front-stage transistor is relatively small, its corresponding current amplification factor is reduced overall.
Two or three transistors
Let's test the corresponding current amplification factor when three transistors are cascaded into a Darlington tube. The selected transistor current amplification factor is 329. Build them directly into a three-level cascade. Direct spatial connection can reduce the impact of leakage current. Connect them to Transistor Assistant via cables. The test current amplification factor is about 330000. The equivalent emitter current is 4.9mA, and the equivalent base voltage is 1.6V. Three transistors are cascaded, and the measured current amplification factor is 325000. Comparing this current amplification factor with the product of the current amplification factor of the three transistors, we can see that the equivalent Darlington tube current amplification factor is only the current amplification factor of three triodes. 1.56% of the product of multiples.
※ Summary ※
This article measured the current amplification factor of transistors cascaded into Darlington tubes. After two-stage cascades, the current amplification factor is close to the product of the maximum multiple of their respective currents. However, after three-stage cascades, the current amplification factor is only three. 1.5% of the product of the transistor current amplification times. At this time, the base voltage and saturation voltage of the corresponding equivalent triode will increase. This also explains why common Darlington transistors have only two stages of cascade.
