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How does a bias resistor built-in transistor (BRT) operate?

The following describes how a BRT operates when voltage (VI) is applied to the B terminal in the “off” state to turn on the BRT in the saturation region.
The VI voltage is gradually increased.

1. The internal transistor (Q) is off.
When VI is applied to the B terminal when Q is off, current flows only through the built-in bias resistors, R1 and R2.
Therefore, a voltage divided by R1 and R2 is applied to the base (b) of Q (Vb = R2 / ( R1 + R2 )).

2. Q turns on.
When Vb reaches roughly 0.5 V, base current (Ib) begins to flow, turning on Q, as indicated by the IB - VBE curve (Figure 2). This causes collector current, which is Ib times hFE, to flow. At this point in time, Q is still in the active region, not in the saturation region. Therefore, its hFE is 120 to 700, as is the case with the hFE of general-purpose transistors (such as the 2SC2712).
As VI further increases, Vbe increases, causing IC to increase exponentially as shown in Figure 2.
In practice, however, the base voltage (Vb) hardly increases with VI. VBE varies only between 0.6 V and 1.0 V at an ambient temperature of 25°C as shown in Figure 3. Therefore, the base voltage is often treated as being constant in a simplified calculation. Here, let’s assume that VBE = 0.7 V.
Ib is expressed as:
    Ib = IB – IR2 = ( VI – Vbe ) / R1 – Vbe / R2
As Ib increases, the collector current (IC) increases proportionally.
Let the collector-emitter (C-E) voltage in the basic BRT circuit of Figure 1 be VCE. Then:
    VCE = VCC – RL * IC
As IC increases further , VCE decreases, causing the BRT to enter the saturation region.

3. Q enters the saturation region.
As VCE decreases, the transistor (Q) enters the saturation region.
(There is no clear definition of the saturation region. Toshiba regards the region in which hFE ≤ 20 as the saturation region*.)
An increase in Ib causes IC to increase, causing a voltage drop across RL to increase and therefore VCE to approach the GND level.
That is, Q traces the IC – VCE curve (Figure 4) from #2 to #3. Eventually, VCE becomes nearly zero.
Figure 5 shows the VCE(sat) – IC curves of an actual BRT.

Let the collector-emitter voltage in the saturation region be VCE(sat). Then,
    IC = ( VCC – VCE(sat) ) / RL = constant
As VI increases, IB increases, but IC remains almost constant. Therefore, hFE decreases as VI increases.

* The saturation region is sometimes defined as a region where the sign of VBC is opposite to that in the active region.

Figure 1 Basic BRT circuit
Figure 1 Basic BRT circuit
Figure 2 Example of IB – VBE curves (2SC2712)
Figure 2 Example of IB – VBE curves (2SC2712)
Figure 3 Example of VBE – IC curves (2SC2712)
Figure 3 Example of VBE – IC curves (2SC2712)
Figure 4 Example of the operating regions of a bipolar transistor (2SC2712 IC-VCE curves)
Figure 4 Example of the operating regions of a bipolar transistor (2SC2712 IC-VCE curves)
Figure 5 VCE(sat) - IC
Figure 5 VCE(sat) - IC
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