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What is the maximum voltage that can be applied to the base of a bias resistor built-in transistor (BRT)? (How many watts is the allowable power dissipation of the built-in resistors?)

Figure 1 Basic BRT circuit
Figure 1 Basic BRT circuit

You do not need to consider the maximum allowable base voltage for typical applications (in which input voltage (VI) is lower than 10 V), but care is required when VI is higher than 10 V.
The maximum input voltage (VI) is mainly determined by the following factors:
1.    Value of the series base resistor (R1) (i.e., the power dissipated by R1)
2.    Maximum collector current (IC)
The maximum input voltage varies with the usage conditions of a BRT (e.g., hFE), but it is generally determined by the collector current when the R1 value is small and by the R1 value when it is large.

1. Relationship between the value of the series base resistor (R1) and the input voltage (VI)
The allowable power dissipation of the built-in resistors of a BRT is 1/8 W. Let the base-emitter voltage of the internal transistor in the “on” state be Vbe. Then, the current flowing through R1 (IB) is expressed as follows. For the sake of simplicity, let Vbe = 0.7 V.
IB = ( VI – Vbe ) / R1 = ( VI – 0.7 ) / R1
The power dissipation of R1 due to IB must not exceed 1/8 W. Hence:
    1/8 W > R1 * { ( VI – 0.7 ) / R1 }^2
              > ( VI – 0.7 ) ^2 / R1    
    VI < √( R1 / 8 ) + 0.7

2. Relationship between the maximum collector current (IC(max)) and the input voltage (VI)
The maximum collector current (IC(max)) is specified as an absolute maximum rating, which must not be exceeded even instantaneously.
Since the BRT operates in the saturation region, its hFE is in the range of 10 to 20. The current flowing to the base (b) of the internal transistor (Ib) can be calculated by subtracting the current flowing to R2 (IR2) from the IB value calculated above:
    Ib = IB – IR2 = ( VI – Vbe ) / R1 – Vbe / R2 = ( VI – 0.7 ) / R1 – 0.7 / R2
Since the collector current (IC) is equal to Ib times hFE, the following equation must be satisfied:
    IC (max) > IC = hFE * IB = hFE * {( VI – 0.7 ) / R1 – 0.7 / R2 }
    VI < R1 * IC(max) / hFE +( R1 + R2 ) * 0.7 / R2
The lower of the VI values calculated based on the above two factors is the maximum allowable base voltage.
1. Value of the series base resistor (R1)
    VI < √( R1 / 8 ) + 0.7
2. Maximum collector current (IC(max))
    VI < R1 * IC (max) / hFE +( R1 + R2 ) * 0.7 / R2
Figure 2 and Figure 3 show the relationships between input voltage and collector current at different resistor values.
Although the maximum input voltage and its limiting factors depend on the hFE (IC/IB) of the BRT, the following are true:

  • When the R1 value is small: The maximum input voltage is determined by the absolute maximum collector current specified in the datasheet.
  • When the R1 value is large: The maximum input voltage is determined by the power dissipation of R1.
Figure 2 Input voltage vs. collector current (when hFE = 10)
Figure 2 Input voltage vs. collector current (when hFE = 10)
Figure 3 Input voltage vs. collector current (when hFE = 20)
Figure 3 Input voltage vs. collector current (when hFE = 20)
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