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Figure 1 shows a basic switch circuit using an NPN BRT.
When the NPN BRT is on in the saturation region, its collector voltage drops to the GND level because of an external resistor (RL) and the collector current (IC). In practice, however, there is a voltage level called collector-emitter saturation voltage (VCE(sat)) between the collector and GND (emitter) potentials. VCE(sat) can be reduced by increasing the base current (IB).
(Because VCE(sat) << VCC when the circuit of Figure 1 is in the “on” state, IC = VCC / RL, which is almost constant. Therefore, increasing IB causes hFE (= IC / IB) to decrease. This means that the BRT goes into deeper saturation, reducing VCE(sat).)
Usually, only a limited range of voltage can be applied to the base. So, let’s consider how to pass more base current (Ib) at a given base voltage.
The internal base current (Ib) is expressed as follows:
Ib ＝ IB – IR2 = ( VI – Vbe ) / R1 – Vbe / R2
where VI is the input voltage and Vbe is the base-emitter voltage of the internal transistor, which can be considered constant at roughly 0.7 V *.
The Ib equation indicates that Ib can be increased by using a BRT with:
1) Small R1 value
2) Large R2 value
This does not mean a BRT with a small resistor ratio (R1/R2), but a BRT with a small R1 value. Such BRTs pass a greater current to to the base at a given input voltage (VI).
However, the drawback of such BRTs is that they consume more power and enter deeper saturation, causing the switching speed to decrease. Also, the saturation voltage increases as temperature increases, as shown in Figure 4.
Take these factors into consideration to achieve the optimum design.
* : Figure 3 shows the VBE(sat) – IC curves of the general-purpose 2SC2712 transistor that is equivalent to the transistor used in the BRT.) VBE varies only by a few hundreds of millivolts in the IC range in which the BRT is used. This VBE variation is negligible relative to the input voltage.