What is the inverted state of a transistor and its function

The working principle and characteristics of a transistor are essential knowledge that electronic technicians must master. There are three commonly used working states for a transistor: cutoff, amplification, and saturation, each with different conditions and characteristics. In fact, there is a fourth working state of the transistor in the circuit, although this state is not commonly used, it is also something we must understand.

What is the inverted state of a transistor?

The collector junction is positively biased and the emitter junction is negatively biased, in an inverted state; The collector junction is positively biased and the emitter junction is positively biased, in a saturated state; The collector junction is reverse biased, and the emitter junction is reverse biased, resulting in an inverted cutoff state; The collector junction is reverse biased and the emitter junction is forward biased, in an amplified state;

In daily power development and design, engineers generally understand that the description of the inverted state of a transistor is actually the forward bias of the collector junction and the reverse bias of the emitter junction, which is an inverted state; The collector junction is positively biased and the emitter junction is positively biased, in a saturated state; The collector junction is reverse biased, and the emitter junction is reverse biased, resulting in an inverted cutoff state; The collector junction is reverse biased and the emitter junction is forward biased, in an amplified state. It can be said that in circuit design, when the potential relationship between the three electrodes of an NPN transistor is UE>UB>UC, the state of the two PN junctions inside the transistor is reverse biased at the be junction and forward biased at the bc junction. At this point, the transistor is operating in an "inverted" state. The working principle of an inverted transistor is similar to that of an amplifying state. When the BC junction is forward biased, the collector region emits electrons, and some free electrons combine with holes in the base region to form a base current. The other part of the electrons is "collected" by the reverse biased emitter junction to form an emitter current. During inversion, due to the low doping concentration in the collector region of the transistor, fewer electrons are emitted, and due to the small area of the emitter region, fewer electrons are ultimately collected, resulting in a small current. Therefore, the transistor has no amplifying ability. The inverted transistor has a current of less than 1. When the base current of the inverted transistor is increased, the inverted transistor can also enter a saturation state, but at this time, the base current is larger. At the same time, the conduction pressure drop of the pipe is much smaller than when it is directly connected.

Common working states of transistors

A transistor is a semiconductor device composed of two PN codes. When different voltages are applied to the three electrodes, the states of the two PN codes are also different, which determines that the transistor operates in different states. In most analog electronic technology materials, the common working states of transistors are cutoff, amplification, and saturation, and their corresponding working conditions and characteristics are shown in the table below:

Analysis of Inverted State of Transistor

In fact, when the potential relationship between the three electrodes of an NPN transistor is UE>UB>UC, the state of the two PN junctions inside the transistor is that the be junction is reverse biased and the bc junction is forward biased. At this point, the transistor is operating in an inverted state. The working principle of a transistor in an inverted state is similar to that of an amplifying state. When the BC junction is positively biased, electrons are emitted from the collector region, and a portion of the free electrons recombine with holes in the base region to form a base current, while another portion of the electrons are "collected" by the negatively biased emitter junction to form an emitter current. When inverted, due to the low doping concentration in the collector region of the transistor, fewer electrons are emitted. At the same time, due to the small area of the emitter region, fewer electrons are ultimately collected, resulting in a small current. Therefore, the transistor does not have amplification capability. The inverted transistor β is less than 1. When the base current of the inverted transistor is increased, the inverted transistor can also enter saturation state, but at this time the base current is larger, and the conduction voltage drop of the transistor is much smaller than that of the positive connection.

Understanding of Inverted Amplification of Transistors

① When a transistor operates in an inverted state, it is equivalent to swapping the emitter and collector (i.e. using the collector as the emitter and the emitter as the collector), and the inverted transistor also has three operating states. However, the ratio of the equivalent collector current (IE) to the base current, denoted as β, is much smaller than when connected in the positive direction. Therefore, in order to make the inverted transistor enter the saturation region, the required base driving current is much larger than when connected in the positive direction, but the voltage drop of the inverted transistor is smaller than when connected in the positive direction.

Application of inverted state of transistor

① The multi emitter transistor used as a signal input in TTL digital integrated circuits is an inverted transistor when the input is at a high level of 1. When a transistor is used in an inverted state, the bias of its two PN junctions is opposite to that when operating in an amplifying state: the emitter junction is reverse biased and the collector junction is forward biased. Therefore, the collector junction may burn out, while the emitter junction may break down. However, due to the small voltage amplification factor β of the transistor operating in an inverted state, such as the β value of a planar transistor used in an inverted state being about 0.1-0.5, there is generally no risk of burnout. Currently, it is rare to use transistors in an inverted state.

② When using a multimeter to detect and determine the three electrodes of a transistor, the "three inversion" method can be used to find the base and determine the transistor type, while the determination of the collector and emitter requires the inverted state of the transistor. Taking the NPN transistor as an example, select the ohm range of R × 100 or R × 1K for the multimeter. As shown in Figure 1, pinch the base and unknown electrode of the transistor with your fingers, connect the black probe of the multimeter to the unknown electrode Y and the red probe to the X, and observe the deflection angle of the probe. Connect again as shown in Figure 2 and observe the deflection angle of the pointer. Compare the two pointer deflection angles, and the one with the larger deflection has the black probe connected to the collector.

The two wiring methods of this judgment method correspond to the two states of the transistor: amplification state and inversion state. The time when the pointer deflection is small, connect the black probe (positive pole of the DC power supply in the multimeter) to the emitter of the transistor. At this point, the potential relationship between the three electrodes of the transistor is UE>UB>UC. The transistor is operating in an inverted state, and the current passing through the deflection of the multimeter pointer is the emitter current. Because this current is small, the pointer deflection is also small. Another wiring method corresponds to the amplification state of the transistor, where the current through the pointer is the collector current, which is relatively large, and the pointer deflection of the multimeter is also relatively large.

There are not many applications of inverted state transistors, but understanding the working principle of inverted state transistors can help beginners correctly use transistors and analyze circuit faults.