A bipolar semiconductor (bipolar intersection semiconductor: BJT) consists of two junctions formed by three semiconductor regions. Two types of structures exist: pnp and npn. There are products available with npns of up to 800 V and pnps of up to -600 V. What’s more, there are additionally predisposition resistor worked in semiconductors (BRTs). Additionally, we offer radio-frequency (RF) transistors that are suitable for oscillator and low-noise amplifier (LNA) applications.
BJT voltages and currents
A basic explanation of how a BJT works when it is in the forward active mode was provided in the previous tutorial. Although this is a good place to start, if you want to analyze or design BJT-based circuits, you will need to know a lot more. After examining some plots that will assist us in comprehending the relationships that exist between BJT currents and voltages, we will begin with the three operational modes that are the most prevalent.
Cutoff and Active Mode
Assume we are working with the straightforward BJT circuit depicted below.
The base-to-emitter (BE) junction is not forward-biased if VIN is connected to ground. There isn’t much current flowing from the collector to the emitter because there isn’t much current flowing from the collector to the base. As a result, the transistor is in cutoff, a nonfunctional mode that it will remain in until the base voltage reaches a level high enough to forward bias the BE junction.
The BE junction begins to conduct when VIN is approximately 0.6 V. The collector current is determined by the base current IB, which is limited by the base resistor RB: IC = βIB. Because the supply voltage connected to the collector through RC is significantly higher than VIN, the base-to-collector (BC) junction is guaranteed to be reverse-biased, which is why the BJT is operating in forward active mode.
Although the voltage at the collector is lower than 5 V as a result of a voltage drop across the resistor RC, we will assume that the collector resistance and initial collector current are small enough to maintain reverse-bias conditions for the BC junction.
Saturation Mode As VIN rises, so does the base current and, consequently, the collector current. The BC junction will begin to enter the forward-bias region as a result of the voltage drop caused by the collector resistor RC eventually occurring.
The transistor is in saturation mode when both the BE junction and the BC junction are forward-biased. When the voltage from the base to the collector is approximately 0.5 V, the BC junction enters the forward bias region. since the base-to-producer voltage stays close to 0.7 V and the producer is grounded, a touch of math lets us know that the voltage at the gatherer will be 0.2 V.
This is known as the gatherer to-producer (VCE) immersion voltage: VCE(sat) equals 0.2 V.
Characteristics of the Current and Voltage
In cutoff mode, the transistor is inactive; The collector cannot receive current. According to the equation IC = IB, as the base current rises in forward active mode, so does the collector current. However, the collector current basically levels out when the transistor reaches saturation: We can keep increasing VIN, but the collector current will only go up very slightly.
We’ve covered BJT working modes, and presently we’re prepared to concentrate on some BJT applications. We’ll look at a BJT-based amplifier and a circuit that uses a BJT as a high-current switch in the following tutorial.