Loading...
1. The transistor is inverted
This is an NPN transistor, and structurally, it's a sandwich structure composed of N-type and P-type semiconductors. It has the function of current amplification. Then now there is a problem. If you are in the circuit, you accidentally mix the emitter and the collector. Do you find any differences in the same circuit? Let's take a look at the differences between the analog amplification and switching application circuits.
2. Switching circuit
An inverted transistor is used to build a switching circuit that drives a light-emitting diode. A 1Hz square wave signal is applied to the base to see how the LED lights up. For comparison, two identical circuits were built on a breadboard. It's just that one of the transistors is set normally, and the other is reversed. You can see that in this circuit, they all work fine. There is no difference from the outside. At this time, you can't tell which one is a normal circuit and which one is an inverted triode circuit.
▲ Figure 1.2.1 Driver LED flashing
If the transistor operating voltage is increased to 12V, you can see the difference between the two circuits. The normally placed triode still drives the LED to flash, but the inverted triode is already in a solid state. It turns out that after the transistor is inverted, the PN junction between BE is reversed, and its breakdown voltage is much smaller than that of the PN between BC. In the case of a working power supply of 12V, in fact, the PN junction between the BE of the triode has broken down, and the transistor is turned on. The LEDs cannot be turned off. On the 8050 triode data sheet, the reverse breakdown voltage of the PN junction between BEs is given, with a minimum of 5V. Compared to the breakdown voltage of 40V for the PN junction between BCs, 5V is indeed much smaller. From this, we can see that one of the biggest troubles of the inverted transistor is that its working voltage is limited to a relatively low level.
In the experiment, if the working voltage does not exceed 10V, the inverted 8050 can still work normally.
3. Amplification circuit
Next, a triode signal co-emission amplification circuit is built, the power supply voltage is 9V, at this voltage, when the triode is inverted, the BE junction will not be broken. This test circuit is set up on a breadboard and the signal source is capacitively coupled to the base of the amplification circuit. The signal has a peak-to-peak value of 20mV. The oscilloscope's yellow waveform is the input signal, and the cyan color is the amplified output signal. First of all, if the transistor is placed in a normal state, you can see that its collector is amplified signal. Contrary to the input signal, the magnification ratio is relatively high, and the bottom of the output signal is saturated and distorted. Turn the transistor upside down. This inverted transistor signal amplification circuit is formed. You can see that the amplified signal still appears at the collector, but the amplitude is reduced, and the signal is not really saturated at the bottom. This means that the magnification of the circuit is reduced.
▲ Figure 1.3.1 Input and output voltage waveforms
※Summary※
In this paper, we test the function of inverting a transistor in a switching circuit and an amplifier circuit. It can be seen that the transistor can still work after being inverted, but there are two main changes. One is that the working voltage of the circuit has decreased, and the other is that the current amplification of the transistor has decreased. In some less demanding occasions, the inverted transistor can still work.